96 Basic Structure of Vertebrates 



action as being extensors, flexors, abductors, adductors, retrac- 

 tors, protractors, rotators, pronators (acting to put the palm or 

 sole down on the ground in quadrupedal locomotion), or supinators 

 (rotating hand or foot so that the palm or sole is up). 



BRANCHIOMERIC MUSCLES 



The head and the branchial region of a fish are occupied by systems 

 of muscles which are specialized in relation to the alimentary and re- 

 spiratory functions of the regions. The most anterior of these muscles 

 are attached to the jaws, serving to open and close them. Behind the 

 jaws are complex sets of muscles attached to the skeletal visceral 

 arches which encircle the pharyngeal region of the digestive tube. 

 These arches strengthen the wall surrounding the pharynx and support 

 the gills. The muscles attached to the arches effect the respiratory 

 movements which operate to maintain a flow of water entering by the 

 mouth and passing out through the branchial chambers (Figs. 93, 95). 

 Sheets of superficial muscles whose fibers extend transversely act as 

 constrictors of the pharynx. Small muscles extending between ad- 

 jacent arches serve to shorten the pharyngeal region. Others compress 

 it dorsoventrally. Dorsal levators pull the arches upward. Ventral de- 

 pressors pull them down. In gill-breathing amphibians these respira- 

 tory muscles are reduced and simplified in connection with the reduc- 

 tion in the number of visceral arches and gills. 



These muscles of the jaws and visceral arches have long been called 

 "visceral muscles" because they are intimately related to the wall of 

 the pharynx and are concerned with the visceral functions of alimenta- 

 tion and respiration. Also, a peculiarity of their embryonic origin sets 

 them apart from the ordinary body-muscles of the trunk. The body- 

 muscles are derived from segmented mesoderm, the dorsal myo- 

 tomes. These "visceral muscles" develop from the unsegmented 

 ventrolateral mesoderm which, in the region of the trunk, produces 

 the peritoneum — both parietal and visceral — and the layers of non- 

 striated "involuntary" muscle in the wall of the digestive tube (Fig. 

 90). However, they do secondarily acquire a segmented arrangement 

 corresponding to that of the visceral arches and gill-chambers, but this 

 secondary segmentation probably does not correspond to the segmen- 

 tation of the somatic myomeres. The fibers of "visceral" muscles are 

 striated and "voluntary" and are histologically like body-muscles. 

 Their innervation, however, comes entirely from cranial nerves while, 

 with very few exceptions, body-muscles are innervated by spinal nerves. 

 All things considered, the term "visceral," as applied to the muscles of 

 the jaws and branchial region, is misleading because it seems to ally 

 them with the truly visceral nonstriated muscles of the wall of the di- 



