MUSCULAR SYSTEM 137 



over the face so as to become grouped around the eyes, nose, 

 mouth, and ears. They are supplied by the facial nerve, and attain 

 their greatest development in the Primates, in which certaia 

 other facial muscles are derived from the deeper-lying sphincter 

 colli. 



Parietal Muscles. 



A, Mtischs of the Tnmk. 



In Amphioxus (Fig. 219) the body muscles are made up of a 

 series (60or more) of lateral muscular segments or ?;?2/otowz(;s separated 

 by > shaped connective-tissue septa or myocommata, between 

 which the fibres run longitudinally. The myotomes have an 

 alternating arrangement on the two sides. On the ventral region 

 of the anterior two-thirds of the body there is a thin transverse 

 sheet of fibres. 



In Fishes and Dipnoans the myotomes and myocommata are 

 arranged in pairs and consist, on either side of the body, of two 

 portions, a dorsal and a ventral, separated from one another by a 

 connective-tissue septum extending from the axial skeleton to the 

 integument (comp. Fig. 116).^ The myotomes meet together in 

 the mid-dorsal and mid- ventral lines. 



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 vertebrae and pairs of nerves corre- 

 sponding primitively to that of the myotomes. 



The lateral muscles largely retain their primitive relations in 

 Fishes and Dipnoans, but on the ventral side of the trunk, 

 where they enclose the body-cavity (comp. Amphioxus), 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 the more primitive relations. 



Amphibia. — In Urodeles (Figs. 116 and 117) primary ami 

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

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

 group consists of internal and external obliqid and recti. The 

 secondary muscles arise by delamination from the primary, and 

 give rise to a superficial external oUique, a superficial rectus, a 

 transversalis, and a subvertebralis. These, however, only attain 

 importance in caducibranchiate forms, in which they become 

 marked during metamorphosis, and the primary musculature then 



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

 Lepidosteus posteriorly to the gills. 



