186 COMPARATIVE ANATOMY 



Fishes the latter group consist essentially of elevators, adductors, and 

 depressors of the fins, and these again may become differentiated 

 into several layers. From the Amphibia onwards, in correspondence 

 with the more highly differentiated organs of locomotion, consider- 

 able complication is seen, and there is a much more marked separa- 

 tion into individual muscles corresponding with the different sections 

 of the extremity. Thus elevators, depressors, rotators, flexors, extensors, 

 protractors, retractors, abductors, and adductors are present in con- 

 nection with the pectoral and pelvic arches, the upper arm and 

 thigh, forearm and shank, and hand and foot : the digits are 

 also moved by a highly-differentiated musculature. The number 

 of muscles gradually increases in passing from the Urodela through 

 the Sauropsida to the Mammalia, and greatly influences the form 

 of the skeleton. 



The most important muscles of the shoulder, the origin of 

 which from the trunk gradually becomes broader in the higher 

 forms, are the cucullaris, the stcrnocleidomastoideus (belonging 

 morphologically to the cucullaris, and, like it, supplied by the 

 spinal accessory nerve), the rhomboidci, and the Icvator anguli 

 scapulae : these act as rotators, protractors, and retractors of the 

 scapula. 



The muscles connected with the pelvic arch cannot all be 

 looked upon as the serial homologues of those of the more movable 

 shoulder, for in many respects the different mechanical relations 

 of the hind limb have caused modifications in the muscles. Thus, 

 representatives of the Icvator anguli scapukv, rhomboideus, and 

 sc.rratus magnus are not present. 



A much greater similarity especially marked in Urodeles 

 exists between the muscles of the free portions of the fore and 

 hind limbs. In correspondence with the fact that the angle formed 

 by the upper and middle sections points in opposite directions in 

 the pectoral and pelvic limbs, the extensor muscles of the former 

 are on its posterior border, and those of the latter on its anterior 

 border, while the flexors have the converse arrangement. From 

 the latter the pronators have arisen : these are more specialised 

 in the fore limb than in the hind limb. The supinaturs originated 

 from the extensors. 



A very varied differentiation of the individual layers of muscle 

 takes place in different Vertebrates in connection with the shank 

 and foot, as well as the fore-arm and hand. The degree of differ- 

 entiation of the muscles in question in general corresponds to the 

 functional specialisations of the foot and hand, and is most marked 

 in the hand of Primates, more especially of Man. 



D. The Eye-Muscles. 



(These will be dealt with in connection with the organ of 

 vision.) 



