OCR Text |
Show 406 MR. G. E. DOBSON ON THE [Mar. 1, most superficial of the four in Gymnura, while the least superficial of the three is much smaller than the others. This smaller number of tendons, and evident tendency towards still further reduction, appears to be related to the greater depth of the symphysis pubis, and consequently greater union of the bones of opposite sides, as well as to the lessened proclivity to their separation, owing to the greater transverse diameter of the pelvic opening, which is probably quite sufficient to freely permit of the passage of the foetus during parturition. Quadratus lumborum is much larger and more fleshy than in Gymnura, extending without interruption from the last intercostal space and base of last rib to the crest of the ilium, inserted successively by small tendons into the extremity of each lumbar transverse process. Psoas magnus and psoas parvus are large ; but the latter is inserted into the anterior sacro-iliac ligament and into the margin of the pelvis. The pectoralis major has no separate anterior sternal part, as in Gymnura; nor has its deep lamina any insertion into the clavicle. The coraco-brachialis, which is altogether wanting in that genus, is well developed here, and consists of two parts:-the superficial, a long slender muscle inserted into the lower third of the humerus; the deep (coraco-brachialis brevis), short, and inserted below the lesser tuberosity. There is a small teres minor, but, as in Gymnura, no trace of the supinator longus. The palmaris longus is large, with a distinct tendon, which, passing downwards internal to that of the flexor carpi ulnaris, becomes superficial to it, and glides over the prominent pisiform bone (to which the latter is attached) into the hand, becoming there connected with the palmar fascia, and giving origin wholly or in part to the palmaris brevis, from which is derived the flexor perforatus for the pollex and outer finger, the flexor sublimis digitorum supplying the three middle toes only. Lumbricales, when present, are rudimentary, and connected with the third and fourth digits only. In the hind limbs the sartorius, so feebly developed in Gymnura, is represented by two strong muscles:--one from the fascia covering the iliacus and from the margin of the pelvis in front of the acetabulum ; the other, as large, from the margin of the pelvis lower down, midway between the acetabulum and the symphysis pubis. Both are inserted into the tibia, the first above the second, below the attachment of the internal lateral ligament of the knee-joint. These two well-developed muscles are evidently the differentiated representatives of the muscular aponeurosis covering the adductors, and of the small muscle which joins it from the pectineal line in Gymnura. The semitendinosus arises simply from the tuber ischii, and is inserted into the tibia behind the tendon of the gracilis; it has no connexion with the biceps flexor cruris. Extensor digitorum longus, as in Gymnura, arises from the external condyle of the femur, and supplies tendons to the four outer toes; these vary considerably in number, arrangement, and connections with other tendons. The |