MUSCULAR SYSTEM 181 



by a connective tissue septum extending from the axial skeleton 

 to the integument at the region of the " lateral line " x (cf. Fig. 140). 

 The myomeres meet together in the mid-dorsal and mid-ventral 

 lines, and constitute the great lateral muscles of the trunk. 



This primitive metameric arrangement of the lateral muscles of 

 the trunk forms a characteristic feature in Vertebrates, and stands 

 in close relation with the segmentation of the axial skeleton and 

 spinal nerves, the number of vertebral segments and pairs of 

 nerves corresponding primitively to that of the myomeres. 



The lateral muscles largely retain their primitive relations in 

 Fishes, but on the ventral side of the trunk, where they enclose 

 the body-cavity, certain differentiations occur which indicate the 

 formation of the recti and obliqui abdominis of higher types. The 

 dorsal portions of these parietal muscles, as well as the ventral 

 portions in the caudal region, retain a more primitive condition. 



Amphibians. In Urodeles (Figs. 140 and \&\) primary and 

 secondary ventral trunk-muscles can be distinguished, and both of 

 these groups, like the dorsal muscles, are segmented. The former 

 consists of internal obliques, arising directly from the muscle-plate 

 of the somite, and of external obliques developed from the ventral 

 border of the myomeres ; the obliqui towards the ventral middle 

 line are connected with the rectus abdominis. 



The secondary muscles arise by delamination from the primary, 

 and give rise to a superficial external oblique, a superficial rectus, a 

 transversal-is, and a sulvcrtebraiis. These, however, only attain im- 

 portance in caclucibranchiate forms, in which they become marked 

 during metamorphosis, and the primary musculature then under- 

 goes more or less reduction. Thus various conditions of the ventral 

 musculature are found amongst Urodeles. 



In the broad-bodied Anura, on the other hand, both primary 

 and secondary muscles present a marked uniformity and relative 

 simplicity ; in the adult they give rise to a segmented rectus, in 

 part passing into a sternohyoid, a non-segmented obliquus externus, 

 and a transversalis, as well as to a cutaneus abdominis derived 

 from the external oblique. No trace of an internal oblique can be 

 seen in the adult. 



Reptiles. In Reptiles, the lateral muscles of the trunk attain 

 a much higher grade of development. This is to be accounted for 

 by the more perfect condition of the skeleton, more especially of 

 the ribs and pectoral arch. The ribs and intercostal muscles now 

 play an important part in respiration, and changes, necessitated by 

 the higher development of the lungs, are thus brought about. 



1 This septum is not present in Myxinoids, and is absent in Petromyzon and 

 Lepidosteus posteriorly to the gills. 



