142 COMPARATIVE ANATOMY 



c. Muscles of the Appendages. 



The most primitive condition of the muscles of the extremities 

 is met with in Fishes and Dipnoans, in which the musculature of 

 each surface of the fin forms a more or less uniform mass which 

 may become differentiated into layers. Everything goes to prove 

 that all the muscles of the appendages are to be looked upon 

 primarily as derivatives of the lateral muscles of the trunk, i.e., 

 of the myotomes; and although in the Amniota they have 

 apparently an independent origin, this is probably only due to an 

 abbreviation of development. 



Two principal groups of appendicular muscles may always be 

 distinguished : one lying in the region of the pectoral and pelvic 

 arches, dorsally and ventrally, the other in the free extremity. In 

 Fishes and Dipnoans the latter consist essentially of elevators, 

 adductors, and depressors of the fins ; while from the Amphibia 

 onwards, in correspondence with the moi-e highly-differentiated 

 organs of locomotion, considerable complication is seen, and 

 there is a much more marked separation into individual muscles 

 corresponding with the different sections of the extremity. Thus 

 elevators, depressors, rotators, flexors, extensors, and adductors are 

 present in connection with the upper arm and thigh, fore-arm and 

 shank, and hand and foot, and the digits are also moved by a 

 highly-differentiated musculature. The number of muscles gradu- 

 ally increases in passing from the Urodela through the Sauropsida 

 to the Mammalia. 



When, as in the Primates, the anterior extremity is con- 

 verted into a prehensile organ, new groups of muscles appear 

 known as pronators and supinators. The former are derived from 

 flexors, the latter from extensors. 



D. The Eye-Muscles. 



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

 vision.) 



Visceral Muscles. 



Fishes. — Considerable differences exist in the visceral mus- 

 culature of Fishes.^ In Elasmobranchs, Filrbringer classifies these 

 muscles as follows : — 



A. Cranial muscles (consisting originally of transverse or 

 circular fibres) supplied by the V'^ VII*!^, IX">, and X"' 

 cerebral nerves. 



^ In Cydostomes there is a remarkable transformation of the cranio-viaceral 

 musculature in correspondence with their peculiar cranial skeleton (suctorial 

 apparatus) and branchial basket. 



