452 THE MUSCULATURE 



Variations. — The muscle in the male is much larger than in the female. Some of the more 

 anterior fibre-bundles may extend to the dorsal surface of the penis (clitoris) and form a pubo- 

 cavernosus or levator penis muscle. 



The transversus perinei superficialis (figs. 392, 394, 405) arises from the inferior ischial 

 ramus. The fibre-bundles extend in front of the rectum superficial to the deep transverse mus- 

 cle and are inserted into the central tendon of the perineum. Some cross to the opposite side. 

 Some of the fibre-bundles are continuous with those of the external sphincter or of the pubo- 

 rectalis of the opposite side. 



Action. — It acts with the deep transverse muscle in fixing the central part of the perineum. 



Nerve-supply. — By a branch from the perineal division of the pudic. 



Variations. — It is frequently absent or poorly developed. 



VI. THE MUSCULATURE OF THE LOWER LIMB 



The lower limbs are used chiefly for the support and propulsion of the body. 

 Variety of movement is subordinated to strength and precision. In contrast with 

 the upper limbs, which perform a vast variety of complex movements under 

 conscious control, the lower limbs are called upon to perform chiefly the relatively 

 simple movements which are used in walking or running, without our paying much 

 attention to them. 



The contrast between the two extremities is best marked in the girdles, the relations of 

 which to the trunk have already been described, p. 444. The shoulder girdle is constantly 

 called upon for movements in various directions which increase the freedom of action of the whole 

 extremity. The sterno-clavicular and acromio-clavicular joints are movable so that the scapula 

 can be carried in various directions over the thorax. The bones of the hip-girdle on each side, 

 on the other hand, are ossified into a single hip-bone (os innominatum). The two hip-bones are 

 almost immovably united to one another in front by the symphysis pubis and behind each is 

 united to the sacrum by a joint which, although a diarthrosis, likewise permits but slight move- 

 ment. The sacrum in turn is composed of vertebrae firmly ossified together. The pelvis, 

 composed of the two hip-bones and the sacrum forms a strong support for the trunk. Such 

 movements as it makes are due chiefly to the lumbo-sacral joint and to a less extent to the joints 

 between the lumbar vertebrae. These joints permit the pelvis, in a limited manner, to be flexed 

 and extended, abducted, adducted, and rotated. Flexion is produced by the rectus and the 

 oblique muscles of the abdomen (fig. 387) and by the psoas muscles (fig. 406), extension is pro- 

 duced by the quadratus lumborum (fig. 406) and the sacrospinalis (fig. 381). Rotation and 

 abduction are produced when these muscles act on one side only. The weight of the body in 

 the sitting posture is transmitted through the sacrum and hip-bones to the ischial tuberosities. 

 In this position the pelvis is flexed. The weight of the body in the standing position is trans- 

 mitted to the femora through the acetabulum on each side. In this position the pelvis is ex- 

 tended. In walking the pelvis is rotated forward toward the limb that is being advanced. 



The hip-joint is a true ball-and-socket joint, but freedom of movement is greatly limited 

 by the powerful musculature which surrounds it, as well as by the ligaments. Movements 

 here, however, are freer than at the shoulder-joint, if the shoulder girdle be left out of considera- 

 tion. At the hip-joint the most frequent and most free movements are those of flexion and 

 extension, the main movements in'walking or running; but abduction, adduction, circumduction, 

 and rotation are of the greatest importance in balancing the body. 



At the knee-joint the main movements are also those of flexion and extension and the mus- 

 culature is so arranged that the chief flexors of the knee which lie at the back of the thigh are 

 extensors of the hip (fig. 408) while the extensor musculature of the knee which lies at the front 

 of the thigh flexes the hip (fig. 411). Flexion of the hip, however, through the action of gravity 

 on the foot passively brings about flexion at the knee, while flexion of the knee likewise passively 

 brings about flexion of the hip, since the flexed knee tends to swing forward. These passive 

 movements, due to gravity, are of importance in walking. The gastrocnemius (fig. 413), the 

 most powerful extensor of the ankle-joint, is also a powerful flexor of the knee-joint. At the 

 knee-joint, in addition to flexion and extension, some rotation is possible, best marked when 

 the knee is flexed. This rotation is of value in walking over rough ground in that it helps 

 to accommodate the foot to the ground. It is also of value in sitting on a flat surface. While 

 there is thus .some rotation at the knee-joint not found at the elbow-joint, the free movement 

 of the radius about the ulna which accompanies pronation and supination in the forearm, is 

 unrepresented in the leg where the fibula is firmly united to the tibia at each end. 



The joint between the bones of the leg and the tarsus permits merely of flexion and exten- 

 sion in contrast to the wrist-joint which also permits of adduction and abduction. Flexion 

 and extension are also more limited at the ankle than at the wrist. On the other hand, the 

 movements of inversion and eversion which take place in the intertarsal joints are not needed 

 in the wrist l)ecauHo of the pronation and supination of the forearm. Inversion and eversion 

 of the foot are of value; in walking on rough ground. 



The movc!mcnts of the toes resemble those of the fingers except that they are, in most 

 individuals, far more rcstricitcd. The greatest restriction is seen at the joint between the 

 metatarsal of the big toe and the tarsus, as compared with that between the metatarsal of the 

 thumb and the carpus. 



The musculature of the inferior extremity, like that of the superior, can be 

 divided according to its development and innervation into two great subdivisions 



