138 



COMPARATIVE ANATOMY 



undergoes more or less reduction. Thus various conditions of the 

 ventral musculature are found amongst Urodeles. 



In the 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, an obliquus 

 externus, and a transversal is, 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 



FIG. 116. THE MUSCULATURE OF Siredon pisciformis. (From the side.) 



LI, lateral line ; D, dorsal, and V, ventral portion of caudal muscles ; EM, dorsal 

 portion of lateral muscles of the trunk ; O, 0, outer layer of the external 

 oblique muscle, arising from the lateral line, and extending to the fascia, F ; 

 at * a piece of this layer is removed, exposing the inner layer of the muscle 

 (Ob] ; at Re the oblique fibres of the latter pass into longitudinal fibres, 

 indicating the beginning of the differentiation of a rectus abdominis ; at Re 1 

 the rectus-system is seen passing to the visceral skeleton ; Me, fibrous parti- 

 tions between the myotomes of the dorsal portion of the lateral muscles ; T, 

 temporal ; Ma, masseter ; Dg, digastric ; Mh l , mylohyoid (posterior portion) ; 

 Ce, external ceratohyoid muscle ; Lv, levator arcuum branchialium ; ftt , 

 levator branchiarum ; Cph, cervical origin of the constrictor of the pharynx ; 

 Th, thymus ; Lt, latissimus clorsi ; Ds, dorsalis scapulse ; Cu, cucullaris ; 

 SS, suprascapula ; Ph, procoraco-humeralis. 



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 more important development of the lungs, are thus brought 

 about. 



The distinction between thoracic and abdominal regions becomes 

 gradually more plainly marked, and distinct external and internal 

 intercostal muscles are now differentiated. In the lumbar region 

 the ribs become gradually withdrawn from the muscles lying 



