832 



C() M PARATIVE ANATOM Y 



the ventral or hypobranchial half) are derived from the circular group. 

 The most anterior of this circular group (Figs. 467, 468) are those which 

 open (digastric or depressor mandibulae) or close (adductors) the 

 mouth, and the mylohyoid which extends between the two rami of the 

 lower jaw. There are usually several adductors, known as masseter, 

 temporalis, pterygoideus, accordingly as they have their origin from 

 different parts of the skull. 



The longitudinal muscles are largely confined to small slips which 

 pass from one arch to the next. These muscles undergo considerable 

 variation in the amphibians. In the amniotes there is also much varia- 

 tion but some of them are reduced on account of the loss of branchial 

 respiration with a consequent degeneration of the parts connected 

 with it. The most noticeable visceral muscles, therefore, in the higher 

 groups are those connected with the opening and closing of the mouth. 



Up to this point all muscles mentioned have had a direct connection 

 with the skeletal system. With an increasing degree of development 

 there develops a dermal musculature. Here the muscles are inserted 

 directly in the skin although being derived from skeletal muscles. 

 Primitive conditions of this kind are found in reptiles and birds and serve 

 to move scales, scutes and feathers. This musculature attains its highest 

 development in many of the four-footed animals, who use it to twitch 

 the skin when insects attack them. In the primates the platysma 

 myoides in the neck and head is the only muscle of the kind. It is in- 

 nervated by a facial nerve which in its primitive condition came from 

 the hyoid region. The platysma divides, giving rise to such muscles as 

 the orbiculares which close the lips and eyelids, and the muscles by 

 which orre lifts the lips, nose, lids, and by which some are able to move 

 the ears. 



semimembranosus 

 gracilis 



semimembranosu'i 

 semitendinosui 



Fig. 469. 



Cross-sections through the thigh of A, rabbit, and B, cat, to show the location 

 of the muscles. Black spots are nerves, small circles, blood vessels, a, greater 

 saphenous nerve, artery, and vein ; b, peroneal nerve ; c, tibial nerve ; d, sciatic 

 vein; e, femoral nerve, artery, and vein; /, sciatic nerve. (From Hyman after 

 Bensley.) 



