182 COMPARATIVE ANATOMY 



The ventral muscles of Reptiles represent the primary as well 

 as. the secondary muscles of Amphibians, though differing in their 

 further development, in consequence of which and of the course 

 taken by the nerves, relations of the parts are seen which lead up 

 to the condition occurring in Mammals. The primitive segmen- 

 tation may be retained or more or less completely lost, in which 

 latter case the muscles in question run together to form broad 

 plates. 



The distinction between thoracic and abdominal regions becomes 

 gradually more plainly marked, and, in addition to the four 

 muscular layers present in Amphibians, well-marked external and 

 internal intercostal muscles are present : these are homologous with 

 the primary abdominal muscles of the last-named Order, as are also 

 the obliquus profundus (belonging to the system of the internal 

 intercostals) and the median, deep rectus abdominis. A transvcrsus 

 is present except in Snakes. A subvertebralis extends from rib to 

 rib, but is wanting in the lumbar region. A quadratus lumborum 

 (lumbar portion of the intercostalis) appears first in Reptiles, and 

 from it a psoas major and psoas minor may become differentiated. 



The rectus muscle, which in Amphibia extends anteriorly to 

 the pectoral arch and is in part continuous with the neck muscles, 

 is in Reptiles interrupted at the sternum, so that pre- and post- 

 sternal portions can be distinguished. The rectus abdominis is 

 always well developed, and may consist of a segmented median and 

 of unsegmented lateral portions : it is not strictly comparable to 

 that of Urodeles, and the pyramidalis does not correspond to the 

 like-named muscle of Mammals. 



While no important differentiation is noticeable in the dorsal 

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

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

 distinguished a longissimus, an iliocostalis, interspinalcs, scmispinalcs, 

 iiiidtifidi splenii, and levatores costarum, together with the scaleni, 

 which belong to the last-mentioned group. 



The muscles of the main part of the tail retain primitive 

 relations similar to those seen in Fishes : at the root of the tail 

 and in the cloacal region, however, new muscles become differ- 

 entiated, viz., the ilio-,ischio-, -Andpubo-catula/is and muscles of the 

 anus (already indicated in Anura) and generative organs. 



Birds. In Birds the primitive character of the trunk-muscles 

 has disappeared far more than in Reptiles. This is mainly to be 

 accounted for by the excessive development of the muscles of the 

 anterior extremity the pectoralis major l more particularly and 

 the corresponding backward extension of the breast-bone. 



External and internal oblique muscles are both present in the 



1 The relative size of the pectoralis major does not always correspond to 

 the power of flight. It is very compact in Carinatas, and contains elements 

 corresponding to the pectorales major and minor of Man. 



