416 DAVIDSON BLACK 



Though the peripheral relations of the motor fibers distributed 

 to the trapezius are subject to considerable variation among 

 amphibians, the essential features of innervation remain the 

 same in both urodeles and anurans, so that there can be no doubt 

 that the branch of the X nerve supplying the amphibian cucu- 

 laris is the true homologue of the so-called external ramus of 

 the N. accessorius of mammalian anatomy. From this it must 

 follow that in amphibians as in sharks, the caudal end of the 

 motor vagus column represents the nucleus accessorius of higher 

 forms. 



In considering the possible significance of the apparent ab- 

 sence of a trapezius muscle in many bony fishes (10), the im- 

 portant influence of the operculum upon the development of 

 the levator musculature of the shoulder gifdle became again 

 emphasised. However, in view of the fact that a levator muscle 

 innervated by the X nerve and quite definitely homologous with 

 the selachian trapezius is developed in some bony fishes but not 

 in others, the presence of a bony operculum cannot be the sole 

 factor determining the absence of such a muscle in some teleosts, 

 as Gegenbaur originally thought. The extensive development 

 of a bony operculum such as obtains in many modern teleosts 

 must certainly place a restricting influence upon the growth of 

 the levator musculature of the shoulder, so that if a trapezius 

 muscle innervated by the caudal X roots be retained in any of 

 these animals, a proportionate reduction or suppression of 

 muscular levator elements of somatic origin may be looked for, 

 while if all the latter components of this complex be retained, 

 a complete suppression of visceral trapezius elements might well 

 result. 



The cucularis muscle of sauropsidans and mammals is char- 

 acteristically innervated from two sources, and the elements 

 thus innervated are inextricably mixed within the fascial sheath 

 of the muscle in question. In fishes such an intermixture of 

 elements within the substance of the trapezius is not known, so 

 that either somatic (shoulder levator) elements and visceral 

 (trapezius) elements are both present as anatomically indepen- 

 dent units, or one or other of them may be suppressed. Thus 



