The study of the muscular system is called [1] myology . A muscle consists of not only muscular tissue but also fibrous connective tissues, including the [2] endomysium , which allows room for nerves and capillaries to reach each muscle fiber within a fascicle; [3] epimysium , which ensheaths the muscle as a whole; and [4] perimysium , which encloses each fascicle and separates them from each other. The [5] superficial fascia separates the muscles from the skin. The 3 grades into a tendon at the end of many muscles, but other muscles have no tendons and are said to show a [6] direct (fleshy) attachment to a bone. When a muscle contracts, one end of it, called the [7] origin , remains relatively stationary, whereas the other end, the [8] insertion , usually moves a bone. The middle part between 7 and 8 is called the [9] belly of a muscle.

Muscles can be classified according to shape, or arrangement of their fascicles. Circular muscles typically act as [10] sphincters that regulate openings such as the eyelids, mouth, and anus. In the straplike [11] bipennate muscles, the fascicles are all more or less parallel. In a [12] answer muscle, fascicles converge from either side on a central tendon in a featherlike arrangement. [13] fusiform muscles are thick in the middle and tapered at each end.

Muscles do not work in isolation, but in groups. The muscle that is primarily responsible for the action of a joint is called the [14] prime mover (agonist) . A muscle that contributes to this action is a [15] synergist , and one that resists movement and stabilizes a bone is called a [16] fixator . Many muscles work in [17] antagonistic pairs on opposite sides of a joint—for example, the biceps brachii and triceps brachii, which flex and extend the elbow, respectively. [18] intrinsic muscles are located entirely within an area of reference—for example, within the hand, eye, or larynx. [19] extrinsic muscles act on that region through their tendons but are located outside it—for example, muscles of the forearm that act on the fingers.

It is important to become familiar with a number of Latin names that describe the muscles. For example, biceps means two [20] heads and quadriceps means four of them; pollicis means "of the [21] thumb " and hallucis means "of the [22] great toe "; latissimus means a muscle is very [23] wide, or broad , and minimus means it is relatively [24] small . Words that describe the shape of a muscle include teres, meaning [25] round , and deltoid, meaning [26] triangular .

The subtle facial expressions of humans result from the action of numerous small facial muscles. For example, the "kissing muscle," or [27] orbicularis oris , encircles the mouth, and a similar muscle, the [28] orbicularis oculi , encircles the eye. A smile results from contraction of the [29] zygomaticus major and minor, which insert at the corners of the mouth. The [30] depressor anguli oris muscles, by contrast, draw the corners of the mouth downward, producing a frown. The [31] frontalis elevates your eyebrows and wrinkles your forehead. The main muscle of the cheek, used in blowing and sucking, is the [32] buccinator .

Other muscles of the head are responsible for the tongue movements used in speech and food manipulation. In the names of these muscles, the root -glossus indicates the tongue and is preceded by a root indicating the origin of the particular muscle; for example, the [33] styloglossus muscle originates from the styloid process of the cranium, and the [34] hyoglossus muscle originates from the hyoid bone. Two powerful chewing muscles are the [35] temporalis , which arises from the temporal bone above the ear, and the [36] masseter , which arises from the zygomatic arch. These insert at different places on the mandible and elevate it when you bite. There are four muscles that form a suprahyoid group, including the [37] mylohyoid , which forms the floor of the mouth, and the two-bellied [38] digastric , which opens the mouth. When you swallow, the [39] thyrohyoid muscle elevates the larynx to prevent choking; this action is felt as the bobbing of the "Adam’s apple" during swallowing.

Head movements are produced by muscles that originate on the spine, thoracic cage, and pectoral girdle and insert on the cranium. The major flexors, which make you look downward, are the three scalenes and the ropy [40] sternocleidomastoid muscle of the neck, named for its origins on the breastbone and collarbone and its insertion on the bony mound behind your ear. Extensors of the neck, which make you look upward, include: a deep muscle with vertical fascicles, the [41] semispinalis capitis ; a more superficial muscle with oblique fascicles, the [42] splenius capitis , nicknamed the "bandage muscle"; and a large triangular muscle of the upper back, the [43] trapezius .

Inhaling is achieved mainly by a downward excursion of the [44] diaphragm and contraction of the external [45] intercostal muscles, which elevate the ribs. Exhaling is normally a passive process, but deep exhalation can be aided by some of the abdominal muscles. The [46] rectus abdominis is a vertical, medial, straplike muscle of the abdomen on each side of the linea alba. The most superficial abdominal muscle lateral to this is the [47] external abdominal oblique . On the back, the prime mover of spinal extension is the [48] erector spinae muscle, which is divided into three columns. Deep to this, muscle 41 continues inferiorly as the semispinalis [49] cervicis in the neck region and semispinalis [50] thoracis in the thoracic region. In the lumbar region, the major deep muscle is the [51] quadratus lumborum .

Because humans have an upright posture, our viscera are supported largely by muscles of the pelvic floor. The area between the thighs, called the [52] perineum , is divided into two triangles—the [53] urogenital triangle anteriorly and the [54] anal triangle posteriorly. In the superficial layer of pelvic floor muscles, the [55] bulbospongiosus (bulbocavernosus) wraps around the bulb of the penis in males and encloses the vagina in females. In the middle muscle layer, the external [56] urethral sphincter and external [57] anal sphincter control the elimination of urine and feces, respectively. The deep layer of pelvic muscles includes the coccygeus and the [58] levator ani . The latter is named for the fact that it elevates the anus during defecation.

The "boxer’s muscle" that thrusts your arm forward when you punch or throw something is called the [59] serratus anterior ; it extends from the first nine ribs to the medial border of the scapula. The large [60] trapezius muscle of the upper back has three groups of fascicles that can tilt, retract, or depress the scapula in addition to extending the neck, as mentioned at 43. The [61] rhomboideus major and minor extend from the vertebrae to the scapula and retract the scapula.

The thick superficial muscle of the mammary region, called the [62] pectoralis major , flexes the shoulder and adducts the humerus. The broad [63] latissimus dorsi muscle of the lower back produces strong downward movements of the humerus, as in hammering or swimming. The thick, triangular [64] deltoid muscle caps the shoulder and abducts the humerus; injections are often given here. On the dorsal side of the scapula, the [65] supraspinatus muscle is named for the fact that it occupies the space superior to the spine, and the [66] infraspinatus muscle is named for the fact that it occupies most of the scapular surface inferior to the spine. The anterior surface of the scapula is occupied by the [67] subscapularis . The tendons of these three muscles and the teres minor form the [68] rotator cuff , a common site of injuries in certain athletes.

The elbow is flexed by two synergistic muscles, the superficial [69] biceps brachii and deeper [70] brachialis . Their antagonist is the three-headed [71] triceps brachii on the posterior side of the humerus. Another synergist of elbow flexion is the [72] brachioradialis , but its belly lies in the forearm beside the radius. Supination of the forearm is achieved, unsurprisingly, by the supinator muscle, but pronation is achieved by two muscles—the [73] pronator teres near the elbow and the [74] pronator quadratus near the wrist.

The fingers are controlled mostly by muscles in the forearm, many of whose tendons pass under a bracelet-like [75] flexor retinaculum on the anterior side of the wrist and [76] extensor retinaculum on the posterior side. These bands of connective tissue prevent the tendons from bowing up when the muscles contract. Two flexors of the wrist have tendons that stand out prominently just proximal to the palm of your hand—the lateral tendon belongs to the [77] flexor carpi radialis , and the medial tendon belongs to the [78] palmaris longus . The four tendons visible on the back of your hand belong to a single muscle that extends the fingers and is appropriately named the [79] extensor digitorum . The "anatomical snuffbox," a little pit on the back of the hand at the base of the thumb, is bordered laterally by the tendons of the extensor pollicis brevis and the [80] abductor pollicis longus , a muscle that draws the thumb away from the palm into a "hitchhiking" position. Medially, the anatomical snuffbox is bordered by the tendon of the [81] extensor pollicis longus .

The fleshy pad of the palm at the base of the thumb is the [82] thenar eminence. The web between the thumb and palm is formed by the [83] adductor pollicis muscle, which as its name implies, is an antagonist of 80. Its action, and hence its name, can be inferred quite easily from its location, as is true of many of the muscles we are considering here. In the midpalmar region, there are four dorsal and three palmar [84] interosseous muscles between the metacarpal bones and a deeper set of four wormlike [85] lumbrical muscles that flex the first knuckle of each digit except the thumb.

The broad anterosuperior surface of the pelvis is occupied by the [86] iliacus muscle, which together with the psoas major flexes the waist, as in taking a bow. When you are walking, the backswing of your stride results from the action of the [87] gluteus maximus muscle of the buttock. Two other muscles with similar names, the [88] gluteus medius and gluteus minimus , shift your body weight slightly to the left when you raise your right foot, or vice versa. The large muscle of the anterior thigh is the [89] quadriceps femoris . It has four heads—the [90] vastus lateralis, medialis, and intermedius and the [91] rectus femoris. The medial compartment of the thigh includes: the [92] adductor longus, brevis, and magnus (all three named for the same action of the thigh); the long [93] gracilis , named for its slender shape; and the [94] pectineus , named for its comblike shape. Muscles of the back of the thigh (the posterior compartment) are called the hamstrings. Their tendons border the pit of the knee (popliteal fossa)—the [95] biceps femoris tendon on the lateral side and the [96] semimembranosus and [97] semitendinosus tendons on the medial side.

Below the knee on the anterior side of the leg, the [98] tibialis anterior lies along the lateral side of the tibia; it dorsiflects the foot. The [99] extensor hallucis longus extends the great toe, and the [100] extensor digitorum longus extends the other toes. The superficial muscles of the posterior compartment include the large, two-headed [101] gastrocnemius and the deeper [102] soleus , which together form the triceps surae and insert on the heel by way of the [103] calcaneal (Achilles) tendon. They produce strong plantar flexion, as in jumping to make a basketball shot. Among the deep muscles are the [104] flexor hallucis longus , which flexes the great toe; the [105] flexor digitorum longus , which flexes the other toes; and the [106] popliteus , which unlocks the knee so it can flex. On the plantar aspect of the foot, the most superficial muscle is the [107] flexor digitorum brevis, a synergist of 105.