Human Anatomy Chapter 11 The Muscular System

Consider the biceps brachia flexing the forearm at the elbow as an object is lifted. When the forearm is raised, the elbow is the fulcrum. The weight of the forearm plus the weight of the object in the hand is the load. The force of contraction of the biceps brachia pulling the forearm up is the effort.

A *lever* is a rigid structure that can move around a fixed point called a *fulcrum*. A lever is acted on at two different points by two different forces: the *effort* (E), which causes movement, and the *load* or *resistance*, which opposes movement. The effort is the force exerted by muscular contraction; the load is typically the weight of the body part that is moved or some resistance that the moving body part is trying to overcome (such as the weight of a book you might be picking up). Motion occurs when the effort applied to the bone at the insertion exceeds the load.

As you study the deltoid, note that the muscle has three sets of fibers (anterior, lateral, and posterior) that enable it to function as three distinct muscles used in flexion, abduction, rotation, and extension of the humerus. The points of origin on the clavicle, acromion, and spine of the scapula are near the same three points as the insertions of the trapezius.

Among the scapular muscles, the *deltoid* is a thick, powerful shoulder muscle that covers the shoulder joint and forms the rounded contour of the shoulder. This muscle is a frequent site of intramuscular injections.

In a /reverse muscle action/ (RMA), when the hyoid is stabilized, the digastric depresses the mandible and is therefore synergistic to the lateral pterygoid in the opening of the mandible. The *stylohyoid* muscle elevates and draws the hyoid bone posteriorly, thus elongating the floor of the oral cavity during swallowing. The *mylohyoid* muscle elevates the hyoid bone and helps press the tongue against the roof of the oral cavity during swallowing to move food from the oral cavity into the throat. The *geniohyoid* muscle elevates and draws the hyoid bone anteriorly to shorten the floor of the oral cavity and to widen the throat to receive food that is being swallowed. It also depresses the mandible.

As a group, the suprahyoid muscles elevate the hyoid bone, floor of the oral cavity, and tongue during /deglutition/ (swallowing). As its name suggests, the *digastric* muscle has two bellies, anterior and posterior, united by an intermediate tendon that is held in position by a fibrous loop. Developmentally, each belly arises from a different pharyngeal arch, which accounts for its dual innervation. This muscle elevates the hyoid bone and larynx (voice box) during swallowing and speech.

Therefore, a limb muscle compartment is a group of skeletal muscles that arose from a common developmental origin. As the muscles of a compartment develop, the nerves and blood vessels develop along with them. Because of this, the muscles of a compartment share common blood and nerve supply. Also, because the muscles of a compartment are grouped on the same side of joints, the anterior compartment muscles are typically flexors of the joints they cross, and posterior compartment muscles are typically extensors of the joints they cross.

As developing muscle tissue migrates into the embryonic free limbs, it forms two principal masses, an anterior mass of muscle and a posterior mass of muscle. These developing masses of muscle are separated by the developing bones and connective tissue of the free limb. As joints form between the developing bones of the free limb, the muscle masses differentiate into multiple muscles that are enveloped in fascia and separated by the bones, creating anterior and posterior compartments of muscle in the different regions of the free limb.

Unlike the recti and oblique muscles, it does not move the eyeballs as its tendon passes the eyeball to insert into the upper eyelid. Rather, it raises the upper eyelids, that is, opens the eyes. It is therefore an antagonist to the orbicularis oculi, which closes the eyes.

Developmentally related to the extrinsic eye muscle is the *levator palpebrae superioris*. This muscle splits off of the superior rectus during development.

Bipennate

Fascicles are arranged on both sides of centrally positioned tendons. Example: Rectus femoris muscle

Unipennate

Fascicles are arranged on only one side of tendon. Example: Extensor digitorum longs muscle.

Multipennate

Fascicles attach obliquely from many directions to several tendons. Example: Deltoid muscle

CIRCULAR

Fascicles in concentric circular arrangements form sphincter muscles that enclose an orifice (opening). Example: Orbicularis oculi muscle

FUSIFORM

Fascicles nearly parallel to longitudinal axis of muscle. Terminate in flat tendons. Muscle tapers toward tendons, where diameter is less than at belly. Example: Digastric muscle

PARALLEL

Fascicles parallel to longitudinal axis of muscle. Terminate either end in flat tendons. Example: Sternohyoid muscle

TRIANGULAR

Fascicles spread over broad area converge at thick central tendon. Gives muscle a triangular appearance. Example: Pectoralis major muscle

Fascicular arrangement often represents a compromise between power and range of motion. Pennate muscles, for instance, have a large number of short fibered fascicles distributed over their tendons, giving them greater power but a smaller range of motion. In contrast, parallel muscles have comparatively fewer fascicles, but they have long fibers that extend the length of the muscle, so they have a greater range of motion but less power.

Fascicular arrangement affects a muscle's power and range of motion. As a muscle fiber contracts, it shortens to about 70 percent of its resting length. Thus, the longer the fibers in a muscle, the greater the range of motion it can produce. However, the power of a muscle depends not on length but on its total cross-sectional area. Therefore, because a short fiber can contract as forcefully as a long one, the more fibers per unit of cross-sectional area a muscle has, the more power it can produce.

The tendon of insertion slides across the superior aspect of the shoulder joint beneath the acromion; inflammation in this tunnel-like region can cause swelling and accompanying pain. The *infraspinatus* is a triangular muscle, also named for its location in the infraspinous fossa of the scapula. A portion of the muscle is superficial and other portions are deep to the trapezius and deltoid. The *teres minor* is a cylindrical, elongated muscle, located between the trees major and infraspinatus muscles. Its belly lies parallel to the inferior edge of the infraspinatus and is sometimes indistinguishable from it. The *coracobrachialis* is an elongated, narrow muscle in the arm, located in the lateral wall of the axilla along with the biceps brachii.

Four deep muscles of the shoulder—subscapularis, supraspinatus, infraspinatus, and trees minor—strengthen and stabilize the shoulder joint. These muscles join the scapula to the humerus. Their flat tendons fuse together to form the *rotator (musculotendinous) cuff*, a nearly complete circle of tendons around the shoulder joint, like the cuff on a shirtsleeve. These four muscles are often referred to as the "SITS" muscles and this could serve as a mnemonic for remembering their names. The *subscapularis* is a large triangular muscle that fills the sub scapular fossa of the scapula and forms a small part in the apex of the posterior wall of the axilla. The *supraspinatus*, a rounded muscle named for its location in the supraspinous fossa of the scapula, lies deep to the trapezius.

In the arm, the biceps brachii, brachial is, and coracobrachialis muscles comprise the *anterior (flexor) compartment*. The triceps brachii muscle forms the *posterior (extensor) compartment*.

In the limbs, functionally related skeletal muscles and their associated blood vessels and nerves are grouped together by fascia into regions called *compartments*.

1) The fulcrum is between the effort and the load in *first-class levers*. 2) The load is between the fulcrum and the effort in *second-class levers*. 3) The effort is between the fulcrum and the load in *third-class levers*.

Levers are categorized into three types according to the positions of the fulcrum, the effort, and the load:

The biceps brachii, brachial is, and brachioradialis muscles are the flexor muscles. The extensor muscles are the triceps brachii and the anconeus.

Most of the muscles that move the radius and ulna (forearm bones) cause flexion and extension at the elbow, which is a hinge joint.

• *Elevation*: superior movement of the scapula, such as shrugging the shoulders or lifting a weight over the head. • *Depression*: inferior movement of the scapula, as in pulling down on a rope attached to a pulley. • *Abduction* (protraction): movement of the scapula laterally and anteriorly, as in doing a push-up or punching. • *Adduction* (retraction): movement of the scapula medially and posteriorly as in pulling the oars in a rowboat. • *Upward rotation*: movement of the inferior angle of the scapula laterally so that the glenoid cavity is moved upward. This movement is required to move the humerus past the horizontal as in raising the arms in a "jumping jack." • *Downward rotation*: movement of the inferior angle of the scapula medially so that the glenoid cavity is moved downward. This movement is seen when a gymnast on parallel bars supports the weight of the body on the hands.

Movements of Scapula

This mechanism is further assisted when you take a deep breath and close the rims glottidis (the space between the vocal folds). The trapped air in the respiratory system prevents the diaphragm from elevating. The increase in intra-abdominal pressure also helps support the vertebral column and helps prevent flexion during weightlifting. This greatly assists the back muscles in lifting a heavy weight.

Movements of the diaphragm also help return venous blood passing through abdominal veins to the heart. Together with the anterolateral abdominal muscles, the diaphragm helps to increase intra-abdominal pressure to evacuate the pelvic contents during defecation, urination, and childbirth.

In the process of flexing the forearm at the elbow, for instance, the biceps brachii is the prime mover, and the triceps brachii is the antagonist. The antagonist and prime mover are usually located on opposite sides of the bone or joint, as is the case in this example.

Movements often are the result of several skeletal muscles acting as a group. Most skeletal muscles are arranged in opposing (antagonistic) pairs at joints—that is, flexors-extensors, abductors-adductors, and so on. Within opposing pairs, one muscle, called the *prime mover* or *agonist* (=leader), contracts to cause an action while the other muscle, the *antagonist*, stretches and yields to the effects of the prime mover.

The three regions of the body—the free limbs, the trunk, and the head each have distinct patterns of muscle development.

Muscles arise from common masses of muscle tissue in the developing embryo and fetus.

Because of their insertions, the muscles of facial expression move the skin rather than a joint when they contract.

Muscles of facial expression lie within the subcutaneous layer. They originate from the fascia or bones of the skull and insert into the skin.

The *posterior (extensor) compartment* muscles of the forearm share a common origin on the lateral epicondyle of the humerus (8 of the 12 muscles), insert on the metacarpals and phalanges, and function as extensors. Within each compartment, the muscles are grouped as superficial or deep. The *superficial anterior compartment* muscles are arranged in the following order from lateral to medial: *pronator teres*, *flexor carpi radialis*, *palmaris longus*, and *flexor carpi ulnaris* (the ulnar nerve and artery are just lateral to the tendon of this muscle at the wrist). The *flexor digitorum superficial* is muscle is deep to the other three muscles and is the largest superficial muscle in the forearm. These muscles make up the fleshy mass that is deep to the hairless skin of the anterior forearm. The common origin is on the medial epicondyle. The palmaris longs muscle inserts into the thickened palmar aponeurosis and lies superficial to the flexor retinaculum.

Muscles of the forearm that move the wrist, hand, and digits are many and varied. Those in this group that act on the digits are known as *extrinsic muscles of the hand* (exoutside) because they originate outside the hand and insert within it. As you will see, the names for the muscles that move the wrist, hand, and digits give some indication of their origin, insertion, or action. Based on location and function, the muscles of the forearm are divided into two groups: (1) anterior compartment muscles and (2) posterior compartment muscles. The *anterior (flexor)* compartment muscles of the forearm share a common origin on the medial epicondyle of the humerus (5 of the 8 muscles), typically insert on the carpals, metacarpals, and phalanges, and function primarily as flexors. The bellies of these muscles form the bulk of the forearm. -One of the muscles in the superficial anterior compartment, the palmaris longus muscle, is missing in about 20 percent of individuals (usually in the left forearm) and is commonly used for tendon repair.

Each arch is supplied by a unique cranial nerve; therefore, all the muscles of an arch, or functional group, are innervated by one nerve. For example, the fifth cranial nerve (the trigeminal nerve) innervates all the muscles of mastication (first pharyngeal arch muscles) and the seventh cranial nerve (the facial nerve) innervates all the muscles of facial expression (second pharyngeal arch muscles).

Muscles of the head also arise as functional groups from the embryonic pharyngeal arches and some of the cranial somites. For example, the muscles of mastication arise from the muscle tissue of the first pharyngeal arch. The muscles of facial expression arise from the second pharyngeal arch.

Four muscles, the pectoralis major, deltoid, trapezius, and latissimus dorsi muscles, are not only superficial but also have a large surface area and dominate the superficial musculature of the shoulder, chest, and upper back.

Muscles of the proximal upper limb are arranged in diverse groups; muscles are superficial, deep, or very deep and grouped either as stabilizers of the pectoral or shoulder girdle (clavicle and scapula) or muscles that act on the shoulder joint.

Although one of the functions of the biceps brachia muscle is to move the forearm, the belly of the muscle lies over the humerus, not over the forearm.

Muscles that move a body part often do not cover the moving part.

Three pairs of extrinsic eye muscles control movements of the eyeballs (1) superior and inferior recti (2) lateral and medial recti (3) superior and inferior obliques The four recti muscles (superior, inferior, lateral, and medial) arise from a tendinous ring in the posterior orbit and insert into the sclera of the eye. The *superior* and *inferior recti* move the eyeballs superiorly and inferiorly. The *lateral* and *medial recti* move the eyeballs laterally and medially, respectively.

Muscles that move the eyeballs are called *extrinsic eye muscles* because they originate outside the eyeballs (in the orbit) and insert on the outer surface of the sclera ("white of the eye"). The extrinsic eye muscles are some of the fastest contracting and most precisely controlled skeletal muscles in the body.

The subclavius is a small, cylindrical muscle under the clavicle that extends from the clavicle to the first rib. It steadies the clavicle during movements of the pectoral girdle.

Muscles that move the pectoral girdle can be classified into two groups based on their location in the thorax: *anterior* and *posterior thoracic muscles*. The anterior thoracic muscles are the *subclavius*, pectorals minor, and serratus anterior.

The two muscular portions are held together by a strong *aponeurosis* (sheetlike tendon), the *epicranial aponeurosis*, also called the *galea aponeurotica*, that covers the superior and lateral surfaces of the skull. The *buccinator* muscle forms the major muscular portion of the cheek. The duct of the parotid gland (a salivary gland) passes through the buccinator muscle to reach the oral cavity. The buccinator muscle is so named because it compresses the cheeks (bucc = cheek) during blowing. For example, when a musician plays a brass instrument such as a trumpet. It functions in whistling, blowing, and sucking and assists in chewing.

Muscles that surround the orifices (openings) of the head are the eyes, nose, and mouth. These muscles function as /sphincters/, which close the orifices. /Dilators/ dilate or open the orifices. The *oribicularis oculi* muscle closes the eye, and the legator palpebrae superioris muscle opens it. The *occipitofrontalis* is an unusual muscle in this group because it is made up of two parts: (1) anterior part called the *frontal belly* (*frontalis*), which is superficial to the frontal bone (2) posterior part called the *occipital belly* (*occipitals*), which is superficial to the occipital bone

Name: Meaning: Example:

Name: Meaning: Example:

Name: Extensor Meaning: Increases a joint angle Example: Extensor carpi ulnaris

Name: Abductor Meaning: Moves a bone away from the midline Example: Abductor pollicis longus

Name: Vastus Meaning: Huge Example: Vastus lateralis

Name: Deltoid Meaning: Triangular Example: Deltoid

Name: Gracilis Meaning: Slender Example: Gracilis

Name: Flexor Meaning: Decreases a joint angle Example: Flexor carpi radialis

Name: Brevis Meaning: Short Example: Adductor brevis

Name: Latissimus Meaning: Widest Example: Latissimus dorsi

Name: Adductor Meaning: Moves a bone closer to the midline Example: Adductor longus

Name: Levator Meaning: Raises or elevates a body part Example: Levator scapulae

Name: Minimus Meaning: Smallest Example: Gluteus minimus

Name: Longus Meaning: Long Example: Adductor longus

Name: Longissimus Meaning: Longest Example: Longissimus capitis

Name: Magnus Meaning: Large Example: Adductor magnus

Name: Oblique Meaning: Diagonal to midline Example: External oblique

Name: Maximus Meaning: Largest Example: Gluteus maximus

Name: Major Meaning: Larger Example: Pectoralis major

Name: Minor Meaning: Smaller Example: Pectoralis minor

Name: Rhomboid Meaning: Diamond-shaped Example: Rhomboid major

Name: Orbicularis Meaning: Circular Example: Orbicularis oculi

Name: Pectinate Meaning: Comblike Example: Pectineus

Name: Piriformis Meaning: Pear-shaped Example: Piriformis

Name: Platys Meaning: Flat Example: Platysma

Name: Quadratus Meaning: Square, four-sided Example: Quadratus femoris

Name: Tensor Meaning: Makes a body part rigid Example: Tensor fasciae latae

Name: Rotator Meaning: Rotates a bone around its longitudinal axis Example: Rotatore

Name: Trapezius Meaning: Trapezoid Example: Trapezius

Name: Serratus Meaning: Saw-toothed Example: Serratus anterior

Name: Pronator Meaning: Turns palm posteriorly Example: Pronator teres

Name: Sphincter Meaning: Decreases the size of an opening Example: External anal sphincter

Name: Depressor Meaning: Lowers or depresses a body part Example: Depressor laabii inferioris

Name: Supinator Meaning: Turns palm anteriorly Example: Supinator

Name: Rectus Meaning: Parallel to midline Example: Rectus abdominis

Name: Transverse Meaning: Perpendicular to midline Example: Transversus abdominis

The pectorals major and latissimus dorsi thus are called *axial muscles*, because they originate on the axial skeleton. The remaining seven muscles, the *scapular muscles*, arise from the scapula.

Of the nine muscles that cross the shoulder joint, all except the pectoralis major and latissimus dorsi originate on the scapula (shoulder blade).

It has two origins: a smaller clavicular head and a larger sternocostal head. The superior clavicular head attaches more distally on the humerus than its inferior counterpart, the sternal head, giving the tendon a twisted appearance when the arm is in the anatomical position. This improves the mechanical advantage of the muscle.

Of the two axial muscles that move the humerus (arm bone), the *pectorals major* is a large, thick, fan-shaped muscle that covers the superior part of the thorax and forms the anterior fold of the axilla. When this muscle and the latissimus dorsi are well developed, the axilla is deepened.

The 11 pairs of *internal intercostal* muscles occupy the intermediate layer of the intercostal spaces. The fibers of these muscles run at right angles to the external intercostals, in an oblique direction inferiorly and posteriorly from the inferior border of the rib above to the superior border of the rib below. They draw adjacent ribs together during forced exhalation to help decrease the size of the thoracic cavity. The deepest muscle layer is made up of the paired *innermost intercostal* muscles. These poorly developed muscles extend in the same direction as the internal intercostals and may have the same role.

Other muscles involved in breathing, called *intercostal* muscles, span the intercostal spaces, the spaces between ribs. These muscles are arranged in three layers. The 11 pairs of *external intercostal* muscles occupy the superficial layer, and their fibers run in an oblique direction inferiorly and anteriorly from the rib above to the rib below. They elevate the ribs during inhalation to help expand the thoracic cavity.

PENNATE

Short fascicles in relation to total muscle length. Tendon extends nearly entire length of muscle.

In abduction of the arm, the deltoid muscle serves as the prime mover, and fixators (pectorals minor, trapezius, subclavius, serratus anterior muscles, and others) hold the scapula firmly against the back of the chest. The insertion of the deltoid muscle pulls on the humerus to abduct the arm. Under different conditions—that is, for different movements—and at different times, many muscles may act as prime movers, antagonists, synergists, or fixators.

Some muscles in a group also act as *fixators*, stabilizing the origin of the prime mover so that the prime mover can act more efficiently. Fixators steady the proximal end of a limb while movements occur at the distal end. For example, the scapula is a freely movable bone that serves as the origin for several muscles that move the arm. When the arm muscles contract, the scapula must be held steady.

The *supinator* of the forearm is aptly named the supinator muscle and it assists the biceps brachii in producing this action. You use the powerful action of the supinator when you twist a corkscrew or turn a screw with a screwdriver.

Some muscles that move the radius and ulna are involved in pronation and supination at the radioulnar joints. The pronators, as suggested by their names, are the *pronator teres* and *pronator quadrates* muscles.

If movement at these intermediate joints was unrestrained, you would not be able to flex your fingers without flexing the wrist at the same time. (Try to make a strong fist while also flexing the wrist. It is hard to do, isn't it?) Synergistic contraction of the wrist extensor muscles stabilizes the wrist joint and prevents unwanted movement, while the flexor muscles of the fingers contract to bring about the primary action, efficient flexion of the fingers. Synergists are usually located close to the prime mover.

Sometimes a prime mover crosses other joints before it reaches the joint at which its primary action occurs. The biceps brachii, for example, spans both the shoulder and elbow joints, with primary action on the forearm. To prevent unwanted movements at intermediate joints or to otherwise aid the movement of the prime mover, muscles called *synergists* contract and stabilize the intermediate joints. As an example, muscles that flex the fingers (prime movers) cross the intercarpal and radiocarpal joints (intermediate joints).

An unpaired /submental triangle/ is formed by the upper part of the combined right and left anterior triangles. The anterior triangle contains sub mental, submandibular, and deep cervical lymph nodes; the submandibular salivary gland and a portion of the parotid salivary gland; the facial artery and vein; carotid arteries and internal jugular vein; the thyroid gland and infra hyoid muscles; and the following cranial nerves: glossopharyngeal (IX), vagus (X), accessory (XI), and hypoglossal (XII).

The *anterior triangle* is bordered superiorly by the mandible, medially by the cervical midline, and laterally by the anterior border of the sternocleidomastoid muscle. It has its apex at the sternum. The anterior triangle is subdivided into three paired triangles: /submandibular, carotid, and muscular/.

At its distal attachment in the forearm, a thin flat tendon, the bicipital aponeurosis, splits away from the rest of the tendon. This flat tendon descends medially across the brachial artery and vein and fuses with the fascia over the forearm flexor muscles. It helps protect the median nerve and brachial vessels.

The *biceps brachii* is the large muscle located on the anterior surface of the arm. As indicated by its name, it has two heads of origin (long and short), both from the scapula. The muscle spans both the shoulder and elbow joints. In addition to its role in flexing the forearm at the elbow joint, it also supinates the forearm at the radioulnar joints and flexes the arm at the shoulder joint.

For this reason, it is called the "workhorse" of the elbow flexors. Its thick belly is wider than that of the biceps brachii. It is visible and easily palpated on the lateral aspect of the arm where it is sandwiched between the biceps brachii and triceps brachii muscles.

The *brachialis* is deep to the biceps brachii muscle. It is the most powerful flexor of the forearm at the elbow joint.

The tendons of the flexor digitorum profundus, along with the more superficial flexor digitorum superficial is, pass with the flexor pollicis longs through the carpal tunnel. Also in this group is the pronator quadratus at the distal ends of the radius and ulna.

The *deep anterior compartment* muscles are arranged in the following order from lateral to medial: *flexor pollicis longus* (the only flexor of the distal phalanx of the thumb) and flexor *digitorum profundus* (ends in four tendons that insert into the distal phalanges of the fingers).

*abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor indicis*.

The *deep posterior compartment* muscles are arranged in the following order from lateral to medial:

The *iliocostalis group* consists of three muscles: the *iliocostalis cervicis* (cervical region), *iliocostalis thoraces* (thoracic region), and *iliocostalis lumborum* (lumbar region). The *longissimus group* resembles a herringbone and consists of three muscles: the *longissimus capitis* (head region), *longissimus cervicis* (cervical region), and *longissimus thoracis* (thoracic region). The *spinalis group* also consists of three muscles: the *spinalis capitis*, *spinalis cervicis*, and *spinalis thoracis*.

The *erector spinae* is the largest muscle mass of the back, forming a prominent bulge on either side of the vertebral column. It is the chief extensor of the vertebral column. It is also important in controlling flexion, lateral flexion, and rotation of the vertebral column and in maintaining the lumbar curve. As noted above, it consists of three groups: iliocostalis (laterally placed), longissimus (intermediately placed), and spinals (medially placed). These groups in turn consist of a series of overlapping muscles, and the muscles within the groups are named according to the regions of the body with which they are associated.

Together, the external oblique, internal oblique, and transverses abdomens form three layers of muscle around the abdomen. In each layer, the muscle fascicles extend in a different direction. This is a structural arrangement that affords considerable protection to the abdominal viscera, especially when the muscles have good tone.

The *external oblique* is the superficial muscle. Its fascicles extend inferiorly and medially. The *internal oblique* is the intermediate flat muscle. Its fascicles extend at right angles to those of the external oblique. The *transversus abdominis* is the deep muscle, with most of its fascicles directed transversely around the abdominal wall.

The *superior constrictor* is quadrilateral and thinner than the other constrictors. As a group, the constrictor muscles constrict the pharynx during deglutition (swallowing). The sequential contraction of these muscles moves food and drink from the mouth into the esophagus.

The *inferior constrictor* muscle is the thickest of the constrictor muscles. Its inferior fibers are continuous with the musculature of the esophagus, whereas its superior fibers overlap the middle constrictor. The *middle constrictor* is fan-shaped and smaller than the inferior constrictor, and it overlaps the superior constrictor.

Like the pectorals major and levator scapulae muscles, the latissimus dorsi has a twist in it near the insertion that increases its mechanical advantage. Sharing a similar insertion with the pectoralis major and latissimus dorsi is the teres major. The *teres major* is a thick, rounded muscle in cross-section (therefore its name) that also helps form part of the posterior wall of the axilla.

The *latissimus dorsi* is a broad, triangular muscle located on the interior part of the back that forms most of the posterior wall of the axilla. The reverse muscle action (RMA) of the latissimus dorsi enables the vertebral column and torso to be elevated, as in doing a pull-up. It is commonly called the "swimmer's muscle" because its many actions are used while swimming; consequently, many competitive swimmers have well-developed "lats."

This muscle contains a twist in the belly. The twist inverts the superior and inferior fibers as they approach the insertion and ensures that the muscle will elevate the scapula rather than rotating it. Its reverse muscle action (RMA), when the origin and insertion are switched, is to extend the neck.

The *levator scapulae* is a narrow, elongated muscle in the posterior portion of the neck. It is deep to the sternocleidomastoid and trapezius muscles. As its name suggests, one of its actions is to elevate the scapula.

The scapulae of persons who spend a lot of time with their arms in front of them, such as pianists, factory workers, and those who use computers, may develop chronically contracted pectorals minor muscles. Contracted muscles become shorter and wider and since the brachial plexus (the major nerve network to the upper limb) runs between the pectorals minor and the rib cage, chronic contraction can compress nerves. The compressed nerves emulate symptoms of carpal tunnel syndrome.

The *pectorals minor* is a thin, flat, triangular muscle that is deep to the pectorals major. This muscle causes, among other actions, abduction of the scapula.

A transverse line drawn between the ischial tuberosities divides the perineum into an anterior urogenital triangle that contains the external genitals and a posterior anal triangle that contains the anus. Several perineal muscles insert into the perineal body of the perineum. Clinically, the perineum is very important to physicians who care for women during pregnancy and treat disorders related to the female genital tract, urogenital organs, and the anorectal region.

The *perineum* is the region of the trunk inferior to the pelvic diaphragm. It is a diamond-shaped area that extends from the pubic symphysis anteriorly, to the coccyx posteriorly, and to the ischial tuberosities laterally.

The posterior triangle is subdivided into two triangles, /occipital/ and /supraclavicular/ (/omoclavicular/), by the inferior belly of the omohyoid muscle. The posterior triangle contains part of the subclavian artery, external jugular vein, cervical lymph nodes, brachial plexus, and the accessory (XI) nerve.

The *posterior triangle* is bordered inferiorly by the clavicle, anteriorly by the posterior border of the sternocleidomastoid muscle, and posteriorly by the anterior border of the trapezius muscle.

These tendinous intersections are fused with the anterior wall of the rectus sheath but have no connections to the posterior wall of the sheath. Muscular persons may possess easily demonstrated intersections as the result of exercise and the ensuing hypertrophy of the rectus muscle. Hypertrophy of the muscle tissue, of course, has no effect on the connective tissue of the intersections. Body builders focus on the development of the "*six-pack*" effect of the abdomen. Small percentages of the population have a variant of the intersections and are able to develop an "eight-pack."

The *rectus abdominis* muscle is a long muscle that extends the entire length of the anterior abdominal wall, originating at the pubic crest and pubic symphysis and inserting on the cartilages of ribs 5-7 and the xiphoid process of the sternum. The anterior surface of the muscle is interrupted by three transverse fibrous bands of tissue called *tendinous intersections*, believed to be remnants of septa that separated myotomes during embryological development. There are usually three tendinous intersections, one at the level of the umbilicus, one near the xiphoid process, and one midway between the other two. A fourth intersection is sometimes found below the level of the umbilicus.

The rhomboid major is about two times wider than the rhomboid minor. The two muscles are often identified by their attachments. The muscles lie deep to the trapezius and superficial to the erector spine. The rhomboids and the trapezius are functionally the only muscles holding the upper limb to the posterior axial skeleton. Both muscles are used when forcibly lowering the raised upper limbs, as in driving a stake with a sledgehammer.

The *rhomboid major* and *rhomboid minor* lie deep to the trapezius and are not always distinct from each other. They appear as parallel bands that pass inferiorly and laterally from the vertebrae to the scapula. Their names are based on their shape—that is, a rhomboid (an oblique parallelogram).

This muscle can be highly developed in body builders and athletes. It is an antagonist of the rhomboids and is responsible for abduction of the scapula. A large portion of the belly is deep to the anterior scapula. The muscle is thus riding over the rib cage. The lateral and inferior portion of the breast lies superficial to the serratus anterior muscle.

The *serratus anterior* is a large, flat, fan-shaped muscle between the ribs and scapula. It is so named because of the sawtoothed appearance of its origins on the ribs.

All three muscles of the longitudinal layer elevate the pharynx and larynx during deglutition and speech. Elevation of the pharynx widens it to receive food and liquids, and elevation of the larynx causes a structure called the epiglottis to close over the rims glottides (space between the vocal folds) and seal the respiratory passageway. Food and drink are further kept out of the respiratory tract by the suprahyoid muscles of the larynx, which elevate the hyoid bone. Additionally, the respiratory passageway is sealed by the action of the intrinsic muscles of the larynx, which bring the vocal folds together to close off the rims glottidis. After deglutition, the infrahyoid muscles of the larynx depress the hyoid bone and larynx.

The *stylopharyngeus* muscle is a long, thin muscle that, as its name suggests, arises from the styloid process and enters the pharynx between the middle and superior constrictors. Its fibers blend into the constrictors and some insert along with the palatopharyngeus on the thyroid cartilage. The *salpingopharyngeus* muscle is a thin muscle that descends in the lateral wall of the pharynx and also inserts along with the palatopharyngeus muscle. The *palatopharyngeus* muscle also descends in the lateral wall of the pharynx and inserts along with the preceding muscles on the posterior border of the thyroid cartilage.

*extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum* (occupies most of the posterior surface of the forearm and divides into four tendons that insert into the middle and distal phalanges of the fingers), *extensor digit minimi* (a slender muscle usually connected to the extensor digitorum), and the extensor carpi ulnaris.

The *superficial posterior compartment* muscles are arranged in the following order from lateral to medial:

It extends and laterally flexes the vertebral column. This muscle is large and thick in the lumbar region and is important in maintaining the lumbar curve. The *rotators* muscles of this group are short and are found along the entire length of the vertebral column. These small muscles contribute little to vertebral movement, but play important roles in monitoring the position of the vertebral column and providing proprioceptive feedback to the stronger vertebral muscles.

The *transversospinales* are so named because their fibers run from the transverse processes to the spinous processes of the vertebrae. The semispinalis muscles in this group are also named according to the region of the body with which they are associated: *semispinalis capitis* (head region), *semispinalis cervicis* (cervical region), and *semispinalis thoraces* (thoracic region). These muscles extend the vertebral column and rotate the head. The *multifidus* muscle in this group, as its name implies, is segmented into several bundles.

The weight of the head (about 12 pounds) is functionally doubled with each inch that the head flexes from the neutral position (directly over the atlas). For example, for the person who has a head-forward posture of 2 in. from neutral, the trapezius and smaller muscles of the posterior neck need to contract as though the head weighed 36 pounds. The trapezius is thus overworked in such persons and becomes painful. The head-forward position is usually a habit that the patient can correct with practice.

The *trapezius* is a large, flat, triangular sheet of muscle extending from the skull and vertebral column medially to the pectoral girdle laterally. It is the most superficial back muscle and covers the posterior neck region and superior portion of the trunk. The two trapezius muscles form a trapezoid (diamond-shaped quadrangle)—hence its name. The three sets of fibers (superior, middle, and inferior) enable this muscle to cause multiple actions. The superior fibers extend the head and neck.

As its name implies, it has three heads of origin, one from the scapula (long head) and two from the humerus (lateral and medial heads). The long head crosses the shoulder joint; the other heads do not. The anconeus is a small muscle located on the lateral part of the posterior aspect of the elbow that assists the triceps brachii in extending the forearm at the elbow joint.

The *triceps brachii* is the large muscle located on the posterior surface of the arm. It is the more powerful of the extensors of the forearm at the elbow joint.

The superior oblique muscle moves the eyeballs inferiorly and laterally. The *inferior oblique* muscle originates on the maxilla at the anteromedial aspect of the floor of the orbit. It then passes posteriorly and laterally and inserts on the posterolateral aspect of the eyeball. Because of this arrangement, the inferior oblique muscle moves the eyeballs superiorly and laterally.

The actions of the oblique muscles cannot be deduced from their names. The *superior oblique* muscle originates posteriorly near the tendinous ring, then passes anteriorly superior to the medial rectus, and ends in a round tendon. The tendon extends through a pulley like loop of fibrocartilaginous tissue called the /trochlea/ (=pulley) on the anterior and medial part of the rood of the orbit. Finally, the tendon turns and expands into a broad flat sheet that inserts on the posterolateral aspect of the eyeball.

The first three muscles named are arranged from superficial to deep.

The anterolateral abdominal wall is composed of skin, fascia, and four pairs of muscles: the external oblique, internal oblique, transverses abdominis, and rectus abdominis.

The inferior free border of the external oblique aponeurosis forms the *inguinal ligament*, which runs from the anterior superior iliac spine to the pubic tubercle. Just superior to the medial end of the inguinal ligament is a triangular slit in the aponeurosis referred to as the *superficial inguinal ring*, the outer opening of the inguinal canal. The *inguinal canal* contains the spermatic cord and ilioinguinal nerve in males, and the round ligament of the uterus and ilioinguinal nerve in females.

The aponeuroses (sheath like tendons) of the external oblique, internal oblique, and transverses abdomens muscles form the *rectus sheaths*, which enclose the rectus abdominis muscles. The sheaths meet at the midline to form the *linea alba* (white line), a tough, fibrous band that extends from the xiphoid process of the sternum to the pubic symphysis. In the latter stages of pregnancy, the linea alba stretches to increase the distance between the rectus abdomens muscles.

The two muscles in this group are named on the basis of their superior attachments (insertions): *splenius capitis* (head region) and *splenius cervicis* (cervical region). They extend the head and laterally flex and rotate the head.

The bandage-like *splenius* muscles are attached to the sides and back of the neck.

Imagine the following situation. You pick up a book from a table by flexing the elbow joint to lift the book. You then hold the book steady in front of you as you look at it. Next, you slowly lower the book back to the table by extending the elbow joint. The biceps brachia muscle controls this full range of activity. Isotonic concentric contraction of the biceps brachii overcomes the weight of the book and raises it off the table as the elbow joint is flexed. Isometric contraction of the biceps brachii holds the book steady in front of you with a flexed (bent) elbow as you look at it. Finally, isotonic eccentric contraction of the biceps brachii slowly gives way to the load and the book is lowered back down to the table as the elbow joint extends.

The connective tissues surrounding the contractile components within the muscle belly emerge from either end of the muscle as the tendons, to blend with the periosteum and attach to the bone. When these contractile components work, they generate tension within the muscle as the sliding filaments of the sarcomere attempt to shorten This results in two potential types of isotonic contraction—concentric and eccentric. An /isotonic contraction/ is one in which enough muscle fibers are contracting to shorten the muscle against the load. An isometric contraction is when the number of fibers contracting and generating a force are equal to the opposite force of the load, so the muscle does not change length. During a *concentric contraction*, the muscle shortens as it produces a constant tension and overcomes the load it is moving. In an *eccentric contraction*, the muscle produces a constant tension but lengthens as it gives in to the load it is moving. As a result, the same muscle is the active controller of two opposite movements at a joint.

These structures include the aorta, along with the thoracic duct and azygos vein, which pass through the *aortic hiatus*; the esophagus with accompanying vagus (X) nerves, which pass through the *esophageal hiatus*; and the inferior vena cava, which passes through the *caval opening (foramen for the vena cava)*. In a condition called a hiatus hernia, the stomach protrudes superiorly through the esophageal hiatus.

The diaphragm has three major openings through which various structures pass between the thorax and abdomen.

The *peripheral muscular portion* of the diaphragm originates on the xiphoid process of the sternum, the inferior six ribs and their costal cartilages, and the lumbar vertebrae and their intervertebral discs and the twelfth rib. From their various origins, the fibers of the muscular portion converge and insert into the *central tendon*, a strong aponeurosis located near the center of the muscle. The central tendon fuses with the inferior surface of the pericardium (covering of the heart) and the pleurae (coverings of the lungs).

The dome-shaped *diaphragm* is the most important muscle that powers breathing. It also separates the thoracic and abdominal cavities. The diaphragm has a convex superior surface that forms the floor of the thoracic cavity and concave, inferior surface that forms the roof of the abdominal cavity.

In the body, this arrangement favors speed and range of motion over force. The elbow joint, the biceps brachii muscle, and the bones of the arm and forearm are one example of a third-class lever. In flexing the forearm at the elbow, the elbow joint is the fulcrum , the contraction of the biceps brachii muscle provides the effort (E), and the weight of the hand and forearm is the load L .

The effort is between the fulcrum and the load in *third-class levers*. These levers operate like a pair of forceps and are the most common levers in the body. Third-class levers always produce a mechanical disadvantage because the effort is always closer to the fulcrum than the load.

Those muscles include the *lateral cricoarytenoid*, which brings the vocal folds together (adduction), thus closing the rims glottides, and the *posterior cricoarytenoid*, which moves the vocal folds apart (abduction), thus opening the rims glottides. The *transverse arytenoid* closes the posterior portion of the rims glottides. The last intrinsic muscle functions as a sphincter to control the size of the inlet of the larynx, which is the opening anteriorly from the pharynx (throat) into the larynx. This muscle is the *oblique arytenoid*.

The extrinsic muscles move the larynx as a whole; the intrinsic muscles of the larynx move only parts of the larynx. Based on their actions, the intrinsic muscles may be grouped into three functional sets. The first set includes the *cricothyroid* and *thyroarytenoid* muscles, which regulate the tension of the vocal folds (true vocal cords). The second set varies the size of the rims glottides (space between the vocal folds) which adjusts the tension of the vocal folds.

There are few first-class levers in the body. One example is the lever formed by the head resting on the vertebral column. When the head is raised, the contraction of the posterior neck muscles provides the effort (E), the joint between the atlas and the occipital bone (atlantooccipital joint) forms the fulcrum , and the weight of the anterior portion of the skull is the load .

The fulcrum is between the effort and the load in *first-class levers*. Scissors and seesaws are examples of first-class levers. A first-class lever can produce either a mechanical advantage or mechanical disadvantage depending on whether the effort or the load is closer to the fulcrum. (Think of an adult and a child on a seesaw.) If the effort (child) is farther from the fulcrum than the load (adult), a heavy load can be moved, but not very far or fast. If the effort is closer to the fulcrum than the load, only a lighter load can be moved, but it moves far and fast.

The two bellies insert as the *sternal head* and the *clavicular head* of the SCM. The bellies also function differently; muscular spasm in the two bellies cause somewhat different symptoms. The large *trapezius* muscle, which has an important functional role in moving and stabilizing the scapula extends the head. It is assisted by the bilateral contraction of the *semispinalis capitis, splenius capitis, and longissimus capitis* muscles, which also extend the head. However, when these same muscles contract unilaterally, their actions are quite different, involving primarily rotation of the head.

The head is attached to the vertebral column at the atlantooccipital joints formed by the atlas and occipital bone. Balance and movement of the head on the vertebral column involves the action of several neck muscles. For example, acting together (bilaterally), contraction of the two *sternocleidomastoid (SCM)* muscles flexes the cervical portion of the vertebral column and flexes the head. Acting singly (unilaterally), each sternocleidomastioid muscle laterally flexes and rotates the head. Each SCM consists of two bellies; they are more evident near the anterior attachments. The separation of the two bellies is variable and thus more evident in some persons than in others.

In this case, however, the two bellies are referred to as /superior/ and /inferior/, rather than anterior and posterior. Together, the omohyoid, *sternohyoid*, and *thyrohyoid* muscles depress the hyoid bone. In addition, the *sternothyroid* muscle depresses the thyroid cartilage (Adam's apple) of the larynx to produce low sounds. The RMA of the thyrohyoid muscle elevates the thyroid cartilage to produce high sounds.

The infrahyoid muscles are sometimes called "strap" muscles because of their ribbonlike appearance. Most of the infrahyoid muscles depress the hyoid bone and some move the larynx during swallowing and speech. The *omohyoid* muscle, like the digastric muscle, is composed of two bellies connected by an intermediate tendon.

This type of lever produces the most force. This class of lever is uncommon in the human body. An example is standing up on your toes. The fulcrum is the ball of the foot. The load is the weight of the body. The effort (E) is the contraction of the muscles of the calf, which raise the heel off the ground.

The load is between the fulcrum and the effort in *second-class levers*. Second-class levers operate like a wheelbarrow. They always produce a mechanical advantage because the load is always closer to the fulcrum than the effort. This arrangement sacrifices speed and range of motion for force.

For example, it would not be possible to raise the arm above the head if the scapula did not move with the humerus. During abduction, the scapula follows the humerus by rotating upward.

The main action of the muscles that move the pectoral girdle is to stabilize the scapula so it can function as a steady origin for most of the muscles that move the humerus. Because scapular movements usually accompany humeral movements in the same direction, the muscles also move the scapula to increase the range of motion of the humerus.

The extrinsic muscles of the larynx, which are associated with the anterior aspect of the neck, are called *infrahyoid muscles* because they lie inferior to the hyoid bone.

The muscles of the larynx (voice box), like those of the eyeballs and tongue, are grouped into *extrinsic muscles of the larynx* and *intrinsic muscles of the larynx*.

This arrangement gives the pelvic diaphragm the appearance of a funnel suspended from its attachments. The pelvic diaphragm separates the pelvic cavity above from the perineum below. The anal canal and urethra pierce the pelvic diaphragm in both sexes, and the vagina also goes through it in females.

The muscles of the pelvic floor are the levator ani and ischiococcygeus. Together with the fascia covering their internal and external surfaces, these muscles are referred to as the *pelvic diaphragm*, which stretches from the pubis anteriorly to the coccyx posteriorly, and from one lateral wall of the pelvis to the other.

The deep muscles of the female perineum are the *compressor urethrae*, *sphincter urethrovaginalis*, and *external urethral sphincter*. The deep muscles of the perineum assist in urination and ejaculation in males and urination and compression of the vagina in females. The *external anal sphincter* closely adheres to the skin around the margin of the anus and keeps the anal canal and anus closed except during defecation.

The muscles of the perineum are arranged in two layers; *superficial* and *deep*. The muscles of the superficial layer are the *superficial transverse perineal* muscle, the *bulbospongiosus*, and the *ischiocavernosus*. The deep muscles of the male perineum are the deep transverse perineal muscle and external urethral sphincter.

Their names are the inferior, middle, and superior constrictor muscles. The *longitudinal layer* is composed of three muscles that descend from the styloid process of the temporal bone, auditory (eustachian) tube, and soft palate. Their names are the stylopharyngeus, salpingopharyngeus, and palatopharyngeus, respectively.

The muscles of the pharynx are arranged in two layers, an outer circular layer and an inner longitudinal layer. The *circular layer* is composed of three constrictor muscles, each overlapping the muscle above it, an arrangement that resembles stacked flowerpots.

Inhalation (breathing in) occurs when the thoracic cavity increases in size, and exhalation (breathing out) occurs when the thoracic cavity decreases in size.

The muscles of the thorax (chest) alter the size of the thoracic cavity so that breathing can occur.

Of these, the masseter is the strongest muscle of mastication. The *medial* and *lateral pterygoid* muscles assist in mastication by moving the mandible from side to side to help grind food. Additionally, the lateral pterygoids protract the mandible (thrust it forward).

The muscles that move the mandible (lower jaw bone) at the /temporomandibular joint/ (TMJ) are known as the muscles of /mastication/ (chewing). Of the four pairs of muscles involved in mastication, three are powerful closers of the jaw and account for the strength of the bite: *masseter*, *temporalis*, and *medial pterygoid*.

The erector spinae muscle group (consisting of the iliocostalis, longissimus, and spinalis muscles) arises either from the midline or more laterally but usually runs almost longitudinally, with neither a significant lateral nor medial direction as it is traced superiorly. The muscles of the transversospinalis group (semispinalis, multifidus, rotators) arise laterally but extend toward the midline as they are traced superiorly. Deep to these three muscle groups are small segmental muscles that extend between spinous processes or transverse processes of vertebrae. Note that the rectus abdomens, external oblique, internal oblique, and quadrates lumborum muscles also play a role in moving the vertebral column.

The muscles that move the vertebral column (backbone) are quite complex because they have multiple origins and insertions and there is considerable overlap among them. One way to group the muscles is on the basis of the general direction of the muscle bundles and their approximate lengths. For example, the splenius muscles arise from the midline and extend laterally and superiorly to their insertions.

It is a common chamber for the respiratory and digestive systems, opening anteriorly into the larynx and posteriorly into the esophagus.

The pharynx (throat) is a somewhat funnel-shaped tube posterior to the nasal and oral cavities.

The anterolateral abdominal wall can contract and distend; the posterior abdominal wall is bulky and stable by comparison.

The posterior abdominal wall is formed by the lumbar vertebrae, parts of the ilia of the hip bones, psoas major and iliac us muscles and quadrates lumborum muscle.

If, instead, the load is farther from the fulcrum and the effort is applied closer to the fulcrum, then a relatively large effort is required to move a small load (but at greater speed). This is called a *mechanical disadvantage* Compare chewing something hard (the load) with your front teeth and the teeth in the back of your mouth. It is much easier to crush the hard food item with the back teeth because they are closer to the fulcrum (the jaw or temporomandibular joint) than are the front teeth.

The relative distance between the fulcrum and load and the point at which the effort is applied determine whether a given lever operates at a mechanical advantage or a mechanical disadvantage. For example, if the load is closer to the fulcrum and the effort farther from the fulcrum, then only a relatively small effort is required to move a large load over a small distance. This is called a *mechanical advantage*.

The fascicles, however, may form one of five patterns with respect to the tendons: parallel, fusiform (spindle-shaped, narrow toward the ends and wide in the middle), circular, triangular, or pennate (shaped like a feather).

The skeletal muscle fibers (cells) within a muscle are arranged in bundles known as *fascicles*. Within a fascicle, all muscle fibers are parallel to one another.

The tongue's mobility is greatly aided by its attachment to the mandible, styloid process of the temporal bone, and hyoid bone.

The tongue is a highly mobile structure that is vital to digestive functions such as /mastication/ (chewing), detection of taste, and /deglutition/ (swallowing). It is also important in speech.

*Extrinsic tongue muscles* originate outside the tongue and insert into it. They move the entire tongue in various directions, such as anteriorly, posteriorly, and laterally. *Intrinsic tongue muscles* originate and insert within the tongue. These muscles alter the shape of the tongue rather than moving the entire tongue. The extrinsic and intrinsic muscles of the tongue insert into both lateral halves of the tongue.

The tongue is divided into lateral halves by a median fibrous septum. The septum extends throughout the length of the tongue. Inferiorly, the septum attaches to the hyoid bone. Muscles of the tongue are of two principal types: extrinsic and intrinsic.

It supports the pelvic viscera and resists the inferior thrust that accompanies increases in intra-abdominal pressure during functions such as forced exhalation, coughing, vomiting, urination, and defecation. The muscle also functions as a sphincter at the anorectal junction, urethra, and vagina. In addition to assisting the levator ahi, the ischiococcygeus pulls the coccyx anteriorly after it has been pushed posteriorly during defecation or childbirth.

The two components of the *levator ani* muscle are the *pubococcygeus* and *iliococcygeus*. The levator ani is the largest and most important muscle of the pelvic floor.

Most muscles cross at least one joint and are usually attached to articulating bones that form the joint.

Those skeletal muscles that produce movements do so by exerting force on tendons, which in turn pull on bones or other structures (such as skin).

Both groups of muscles stabilize the hyoid bone, allowing it to serve as a firm base on which the tongue can move.

Two groups of muscles are associated with the anterior aspect of the neck: (1) the *suprahyoid muscles*, so called because they are located superior to the hyoid bone (2) the *infrahyoid muscles*, named for their position inferior to the hyoid bone

1) Unipennate 2) Bipennate 3) Multipennate

What are the three pennate fascicles?

The origin is usually proximal and the insertion distal, especially in the limbs; the insertion is usually pulled toward the origin. The fleshy portion of the muscle between the tendons is called the *belly* (body). The actions of a muscle are the main movements that occur when the muscle contracts. Certain muscles are also capable of *reverse muscle action (RMA)*. During specific movements of the body the actions are reversed and therefore the positions of the origin and insertion of a specific muscle are switched.

When a skeletal muscle contracts, it pulls one of the articulating bones toward the other. The two articulating bones usually do not move equally in response to contraction. One bone remains stationary or near its original position, either because other muscles stabilize that bone by contracting and pulling it in the opposite direction or because its structure makes it less movable. Ordinarily, the attachment of a muscle's tendon to the stationary bone is called the *origin*. The attachment of the muscle's other tendon to the movable bone is called the *insertion*.

For example, the *genioglossus* (origin: the mandible) pulls the tongue downward and forward, the *styloglossus* (origin: the styloid process) pulls the tongue upward and backward, the *hyoglossus* (origin: the hyoid bone) pulls the tongue downward and flattens it, and the *palatoglossus* (origin: the soft palate) raises the back portion of the tongue.

When you study the extrinsic tongue muscles, you will notice that all of their names end in /glosses/, meaning tongue. You will also notice that the actions of the muscles are obvious, considering the positions of the mandible, styloid process, hyoid bone, and soft palate, which serve as origins for these muscles.

For example, while extending the forearm at the elbow against resistance (such as pushing against some resistance with the back of the hand as you straighten the elbow), the triceps brachii becomes the prime mover, and the biceps brachii is the antagonist. If a prime mover and its antagonist contract at the same time with equal force, there will be no movement.

With an opposing pair of muscles, the roles of the prime mover and antagonist can switch for different movements.

These muscles flex, laterally flex, and rotate the head and assist in deep inhalation.

Within the *scalene group*, the *anterior scalene* muscle is anterior to the middle scalene muscle, the *middle scalene* muscle is intermediate in placement and is the longest and largest of the scalene muscles, and the *posterior scalene* muscle is posterior to the middle scalene muscle and is the smallest of the scalene muscles.

They function primarily in stabilizing the vertebral column during its movements, and providing proprioceptive feedback.

Within the segmental muscle group, the *interspinales* and *intertransversarii* muscles unite the spinous and transverse processes of consecutive vertebrae.