116 COMPARATIVE ANATOMY. 



Thus, even in many Fishes, differentiations occur on the ventral 

 side which bring about the formation of straight and oblique 

 abdominal muscles (rectus et obliqui abdominis). 



This differentiation is carried further in certain Dipnoi, and 

 is still more marked in tailed Amphibians. In the latter the 

 ventral muscles of the trunk become split into four layers, and in 

 the higher types, such as the sexually-mature Salamander and 

 Triton, a rectus abdominis lying right and left of the median 

 line is plainly differentiated (Fig. 99, Re, He). 



The outermost layer of the lateral muscles of the abdomen 

 does not appear to be retained in the higher types ; the other three 

 layers however remain, and are distinguished from without in- 

 wards, according to the direction of their fibres, as external and 

 internal oblique, and transversalis (Figs. 98, 99, 0, Ob}. 



The external and internal obliques extend from the visceral skeleton, 

 that is, from the floor of the mouth, to the pelvic arch, the former even 

 being directly continuous with the musculature of the tail (Fig. 98) ; the 

 transversalis ceases in the region of the heart, and stands in the closest 

 relation with the fascia transversalis and the peritoneum, on the outer side of 

 which it lies. A similar arrangement is seen in all Vertebrates from the 

 Urodeles onwards. 



The muscular system of the trunk of Anura shows a negative condition 

 as compared with that of Urodeles as above described : the lateral muscles of 

 the abdomen consist of two layers only, and their metameric arrangement seen 

 in the larva becomes later more and more obliterated. The rectus abdominis 

 is always well differentiated, and possesses a varying number of myocommata. 



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 form 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 inter- 

 nal intercostal muscles are now differentiated. In the lumbar 

 region the ribs become gradually withdrawn from the muscles lying 

 between them ; the muscles thus lose their intercostal charac- 

 ter, and form connected sheets, extending between the last pair 

 of ribs and the pelvic arch (e.g. the quadratus lumborurn, 

 which lies close against the vertebral column, and the obliqui). 



The rectus abdominis, which is always well developed, be- 

 comes divided into three portions, a ventral, an internal (a 

 subdivision of the latter), and a lateral. 



While no important differentiation is noticeable in the dorsal 

 portion of the lateral body-muscles in Urodeles, a great sub- 

 division of these muscles is seen in Reptiles. In them may be 

 distinguished a longissimus, an ileocostalis, interspinales, 

 semispinales, multifidi, splenii, and levatores costarum, 



