io8 



COMPARATIVE ANATOMY 



fate. The six muscles, consisting of striated fibers, which effect the 

 movements of the eyeball in its orbit are developed from head mesoderm 

 which is probably the equivalent of three somites or epimeres of the trunk. 

 But there is nothing corresponding to the mesomere of trunk mesoderm. 

 The neck region, whether or not differentiated externally, corresponds 

 approximately to that of the embryonic pharyngeal pouches. In this 

 region there is, on each side, a lateral sheet of mesoderm corresponding, 

 except as to its derivatives, to the hypomere of the trunk. Whereas the 

 trunk hypomere forms no muscle other than (indirectly by way of mesen- 

 chyme) the unstriated muscle of the digestive tube and other visceral 

 parts, the pharyngeal hypomeric mesoderm produces striated muscle 



Fig. 8o. — Diagrams showing the relations of the coelomic cavities (black) in fishes 

 (A), amphibians and savxropsida (B), and mammals (C). L, liver; P, lungs; 5, septum 

 transversum; D, diaphragm. In B the lungs lie in the peritoneal (or pleuroperitoneal) 

 cavity; in C they occupy special pleural subdivisions of the coelom. (From Kingsley.) 



which constitutes an elaborate system of muscles related to the skeleton 

 of the jaws and to the visceral skeleton which supports the gill region 

 of the enteron. These muscles effect the movements of the jaws and of 

 the branchial apparatus. They are commonly referred to as "visceral 

 muscles," a designation which is, however, unfortunately misleading, 

 for "visceral" implies a position much deeper than that occupied by 

 certain of these muscles which he quite superficially and just under the 

 skin of a fish. In amniotes various muscles of the neck and facial region, 

 including in man the very deUcate superficial muscles which control 

 facial "expression," can be recognized as derivatives of certain of the 

 "visceral" muscles of a fish — which makes the term the more inappro- 

 priate. To call the muscles of this system "branchiomeric," as urged by 

 H. H. Wilder, would obviate confusion. 



The diaphragm of the mammal is not the exact equivalent of the 

 septum transversum of other vertebrates. The latter arises as merely a 

 fold of peritoneum. The diaphragm is muscular. The muscles are 

 striated. The muscular part of the diaphragm is formed by ingrowth 



