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The muscular system consists of muscular cells, the contractile elements with the specialized property of exerting tension during contraction, and associated connective tissues. The three morphologic types of muscles are voluntary muscle, involuntary muscle, and cardiac muscle. The voluntary, striated, or skeletal muscles are involved with general posture and movements of the head, body, and limbs. The involuntary, nonstriated, or smooth muscles are the muscles of the walls of hollow organs of the digestive, circulatory, respiratory, and reproductive systems, and other visceral structures. Cardiac muscle is the intrinsic muscle tissue of the heart. See Muscle
Muscle groups are particularly distinct in elasmobranchs and other primitive fishes, and they are generally defined on the basis of their embryonic origin in these animals. Two major groups of skeletal muscles are recognized, somatic (parietal) muscles, which develop from the myotomes, and branchiomeric muscles, which develop in the pharyngeal wall from lateral plate mesoderm. The somatic musculature is subdivided into axial muscles, which develop directly from the myotomes and lie along the longitudinal axis of the body, and appendicular muscles, which develop within the limb bud from mesoderm derived phylogenetically as buds from the myotomes.
The vertebrate muscular system is the largest of the organ systems, making up 35–40% of the body weight in humans. The movement of vertebrates is accomplished exclusively by muscular action, and muscles play the major role in transporting materials within the body. Muscles also help to tie the bones of the skeleton together and supplement the skeleton in supporting the body against gravity. See Skeletal system
Most of the axial musculature is located along the back and flanks of the body, and this part is referred to as trunk musculature. But anteriorly the axial musculature is modified and assigned to other subgroups. Certain of the occipital and neck myotomes form the hypobranchial muscles, and the most anterior myotomes form the extrinsic ocular muscles.
The hypaxial musculature of tetrapods can be subdivided into three groups: (1) a subvertebral (hyposkeletal) group located ventral to the transverse processes and lateral to the centra of the vertebrae, (2) the flank muscles forming the lateral part of the body wall, and (3) the ventral abdominal muscles located on each side of the midventral line. The subvertebral musculature assists the epaxial muscles in the support and movement of the vertebral column. Most of the flank musculature takes the form of broad, thin sheets of muscle that form much of the body wall and support the viscera. The midventral hypaxial musculature in all tetrapods consists of the rectus abdominis, a longitudinal muscle on each side of the midline that extends from the pelvic region to the anterior part of the trunk.
The hypobranchial musculature extends from the pectoral girdle forward along the ventral surface of the neck and pharynx to the hyoid arch, chin, and into the tongue. It is regarded as a continuation of part of the hypaxial trunk musculature.
Limb muscles are often classified as intrinsic if they lie entirely within the confines of the appendage and girdle, and extrinsic if they extend from the girdle or appendage to other parts of the body. In fishes, movements of the paired fins are not complex or powerful and the appendicular muscles in the strictest sense are morphologically simple. In terrestrial vertebrates, the limbs become the main organs for support and locomotion, and the appendicular muscles become correspondingly powerful and complex. The muscles are too numerous to describe individually, but they can be sorted into dorsal and ventral groups, because tetrapod muscles originate embryonically in piscine fashion from a dorsal and a ventral premuscular mass within the limb bud. In general, the ventral muscles, which also spread onto the anterior surface of the girdle and appendage, act to protract and adduct the limb and to flex its distal segments; the dorsal muscles, which also extend onto the posterior surface of the girdle and appendage, have the opposite effects (retraction, abduction, and extension). The limb muscles also serve as flexible ties or braces that can fix the bones at a joint and support the body.
Flight in birds has entailed a considerable modification of the musculature of the pectoral region. As one example, the ventral adductor muscles are exceedingly large and powerful, and the area from which they arise is increased by the enlargement of the sternum and the evolution of a large sternal keel. Not only does a ventral muscle, the pectoralis, play a major role in the downstroke of the humerus, but a ventral muscle, the supracoracoideus, is active in the upstroke as well.
In a number of terrestrial vertebrates, particularly amniotes, certain of the more superficial skeletal muscles of the body have spread out beneath the skin and inserted into it. These may be described as integumentary muscles. Integumentary muscles are particularly well developed in mammals and include the facial muscles (Fig. 1) and platysma, derived from the hyoid musculature, and often a large cutaneous trunci. The last is derived from the pectoralis and latissimus dorsi and fans out beneath the skin of the trunk. The twitching of the skin of an ungulate is caused by this muscle.
Many of the bones serve as lever arms, and the contractions of muscles are forces acting on these arms (Fig. 2). The joint, of course, is the fulcrum and it is at one end of the lever. The length of the force arm is the perpendicular distance from the fulcrum to the line of action of the muscle; the length of the work arm is the perpendicular distance from the fulcrum to the point of application of the power generated in the lever. Compactness of the body and physiological properties of the muscle necessitates that a muscle attach close to the fulcrum; therefore, the force arm is considerably shorter than the work arm. Most muscles are at a mechanical disadvantage, for they must generate forces greater than the work to be done, but an advantage of this is that a small muscular excursion can induce a much greater movement at the end of the lever.
Slight shifts in the attachments of a muscle that bring it toward or away from the fulcrum, and changes in the length of the work arm, can alter the relationship between force and amount or speed of movement.
In general, the force of a muscle is inversely related to the amount and speed of movement that it can cause. Certain patterns of the skeleton and muscles are adapted for extensive, fast movement at the expense of force, whereas others are adapted for force at the expense of speed. In the limb of a horse, which is adapted for long strides and speed, the muscles that move the limb insert close to the fulcrum and the appendage is long. This provides a short force arm but a very long work arm to the lever system (Fig. 2). In the front leg of a mole, which is adapted for powerful digging, the distance from the fulcrum to the insertion of the muscles is relatively greater and the length of the appendage is less, with the result that the length of the force arm is increased relative to the length of the work arm.
the collection of contractile elements and muscle cells usually combined in animals and man into muscles and joined together by connective tissue.
Unicellular animals, sponges, coelenterates, and some acoelo-mate Turbellaria do not have muscles. Such animal move bycontractions of epithelial cell processes and by oscillations of flagella or cilia. The muscle fibers of most worms are separated from the epithelium and are well developed. Along with the external integuments, they form a skin-muscle sac usually consisting of external, annular fibers and internal, longitudinal fibers. In addition, special muscle fascicles may develop at the the bases of setae, in the septa of the body cavity, around the intestine, and in the walls of the blood vessels. In worms and mollusks almost the entire muscular system consists of smooth muscle. In arthropods the developed muscular system is a complex of striated muscles attached to the external skeleton.
The muscular system is most highly developed in chordates, especially in the vertebrates, on the average comprising one-third to one-half of the body mass. The system performs vital functions in animals, from locomotion and maintenance of the body balance to the transport of substances, for example, food and blood, within the organism. It is in the muscular system that chemical energy is converted into both mechanical and thermal energy, keeping heat exchange with the surrounding environment at an equilibrium.
The muscular system in chordates is organized into two sub-divisions: the visceral and the parietal, or somatic, musculatures. The visceral musculature supports the internal organs and consists mostly of smooth muscle originating in the lateral plates. The parietal musculature consists of striated muscles that originate in the myotomes and enable the organism to interact with its environment. The parietal musculature is divided into axial muscles, which lie along the main axis of the body, and muscles of the extremities, which originate from special gemmae on the myotomes.
The parietal musculature originally controlled horizontal flexures of the body, allowing the animal to move in water. It consisted (as in the modern lancelet Branchiostoma lanceolatum) of two longitudinal, lateral muscles that were divided into the separate myotomes, or myomeres, by transverse, curved connective-tissue septa, or myosepta (myocommas). With the formation of a head and the loss of flexibility in the anterior section of the trunk in vertebrates, the anterior myotomes were reduced or partially took over another function. Thus, the three pro-otic myotomes developed into the four rectus and two obliquus muscles, which control eye movements. In terrestrial vertebrates these myotomes gave rise to the retractor muscle of the eye.
The first few postotic myotomes are wholly preserved only in cyclostomes, forming the suprabranchial and hypobranchial musculatures. The anterior postotic myotomes have completely disappeared in fish and terrestrial vertebrates. The suprabranchial part of the posterior myotomes has also disappeared or become greatly reduced, while the hypobranchial part is well developed and forms, along with the anterior trunk myotomes, the sublingual musculature. In fish the sublingual musculature connects the pectoral girdle to the most anterior visceral arches; in terrestrial vertebrates it connects the pectoral girdle to the sublingual apparatus, and the latter to the chin. The muscles of the tongue are derived from the sublingual musculature.
The trunk musculature proper of cyclostomes consists of several myotomes made up of longitudinal fibers. In a lateral view, each transverse myoseptum forms a wavy line. Fish have a horizontal myoseptum that separates the stronger spinal section from the weaker ventral section of each myotome. The dorsal musculature consists of longitudinally arranged muscle fibers. This longitudinal pattern is preserved along the midline of the abdomen but becomes oblique on the sides, resulting in the differentiation of several layers of muscle. Both the change in mode of locomotion as animals shifted to a terrestrial life and the strengthening of the extremities led to a relative decrease in the mass of the axial musculature, to a loss of the axial musculature’s rigid segmentation, and to a broadening of axial musculature functions.
The dorsal musculature has changed comparatively little in the course of evolution. In amphibians, as in fish, the paired, segmented muscles extending from the head to the tip of the tail were preserved. In reptiles, these muscles differentiated into three groups of longitudinal muscles that flex and stabilize the vertebral column. From the anterior end, muscles developed that control the movements of the head and neck. The tail musculature is at the posterior end. In snakes, which have essentially returned to the original mode of locomotion, the dorsal musculature is well developed and complex. In turtles and birds it has been greatly reduced owing to the limited mobility of the spine. Mammals retain the three groups of dorsal muscles that appeared in reptiles. In addition, mammals developed a powerful muscle in the lumbar region that keeps the spinal column erect.
The abdominal musculature of terrestrial vertebrates functions mostly to support the internal organs, to compress the body cavity, and to aid pulmonary respiration. As a result, it is quite complex. Under the transverse processes and alongside of the vertebrae are short subvertebral muscles that serve as antagonists to the dorsal musculature in the regions of the neck and tail. Along the midline lie the paired rectus abdominis muscles. They perform a special function in mammals as antagonists to the powerful spinal extensor muscles. External and internal abdominal oblique muscles are found within the side walls of the abdomen. Still deeper lies the transversus abdominis.
As the amniotes developed a thoracic cavity and an apparatus suitable for costal respiration, several muscles differentiated in the region of the ribs. In mammals the higher metabolic rate necessitated the development of several additional muscle groups to assist respiration: the serratus superior, the scalenus, and especially the diaphragm, a derivative of the cervical myotomes. Still another series of muscles in terrestrial vertebrates branched out from the external obliquus abdominis in the upper part of the trunk. This so-called secondary musculature of the anterior extremity (including the trapezius of visceral origin) is especially well developed in the higher vertebrates. In mammals this series controls the movements of the shoulder girdle. The secondary musculature also attaches the shoulder girdle to the axial skeleton, since there is no bony connection between them.
The musculature of the extremities has changed the most in the course of vertebrate evolution. In fish the musculature of the paired fins consists solely of a dorsal muscle that raises the fin and draws it away from the midline (abduction) and a ventral muscle that lowers the fin and draws it toward the midline (adduction). Separate muscle fibers may push the fin forward (protraction) or pull it back (retraction). Unpaired fins are controlled by muscle fascicles attached to the dermal rays. The electric organs of fish are derived from muscle tissue. In terrestrial vertebrates the extremities, having been converted into complex levers, became the principal means of locomotion and support. Therefore, the musculature of the extremities differentiated into many muscles that control the complicated movements of the limbs and hold the bones securely in the joints. However, from the study of embryos it becomes apparent that every muscle derives from either the dorsal or ventral musculatures.
In amphibians and primitive terrestrial reptiles, the ventral (abdominal) musculature is especially well developed. These muscles are even more powerful in the anterior extremities of birds. In the course of mammalian evolution, the extremities shifted downward under the body, which drastically changed the mechanisms of movement and support in these animals. In mammals an arched trunk became supported by the anterior and posterior extremities. The means of retracting the posterior extremities, which is the basis of forward motion in the animal, also changed. Whereas in reptiles forward motion is accomplished by the action of muscles of ventral origin that unite the tail and femur, in mammals this function was assumed by the gluteal muscles when the position of the extremities changed and the locomotor function of the tail was lost. The gluteal muscles are derived from the dorsal musculature and extend from the widened iliac bones to the femur. The adductors and other posterior femoral muscles—the biceps, the semitendinosus, and the semimembranosus—have basically become retractors. In higher vertebrates the superficial muscles of the shoulder girdle gave rise to the subcutaneous musculature of the trunk.
REFERENCESShmal’gauzen, I. I. Osnovy sramitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.
Beklemishev, V. N. Osnovy sravnitel’noi anatomii bespozvonochnykh, 3rd ed., vol. 2. Moscow, 1964.
V. B. SUKHANOV