140 COMPARATIVE ANATOMY 



The rectus abdominis, which is always well developed, but does 

 not extend anteriorly to the sternum, becomes divided into three 

 portions, — a ventral, an internal, and a lateral. 



While no important differentiation is noticeable in the dorsal por- 

 tion of the lateral body-muscles in Urodeles, a marked subdivision 

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

 a longissimus, an ilecostalis, inter spinales, semispinales, multifidi, 

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

 of which belong to the last-mentioned group, and others to the 

 intercostal muscles, 



The muscles of the main part of the tail retain primitive rela- 

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

 the cloacal region, however, new muscles become differentiated. 



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 pedoralis major more particu- 

 larly, — and the corresponding backward extension of the breast- 

 bone. 



External and internal oblique muscles are present, but only 

 slightly developed : this is more particularly true of the internal, 

 which appears to be undergoing degeneration. No trace of a 

 transversalis can be distinguished ; but, on the other hand, a paired, 

 unsegmented rectus is present. 



External and internal intercostals are well developed, and a 

 triangularis sterni appears for the first time on the inner surface 

 of the sternal ends of the ribs. 



The dorsal portion of the trunk musculature is only slightly 

 developed in the region of the trunk, though very strongly marked 

 in the neck. 



All these modifications in Birds seem to be accounted for by 

 the specialisation of the mechanisms for flight and respiration, to 

 assist which the greatest possible number of muscles are brought 

 into play and thereby influence the whole organism : an essential 

 difference is thus brought about between Birds and Reptiles. 



lyiaminals. — Three lateral abdominal muscles are always 

 present in Mammals, an external and internal oblique and a trans- 

 versalis. In many cases, more particularly in Tupaia and in Lemurs, 

 the external oblique possesses tedinous intersections, thus indicat- 

 ing its primitive segmental character; but in general all these 

 muscles consist of broad uniform sheets. Towards the middle line 

 they pass into strong' aponeuroses, which ensheath the rectus 

 abdominis. The latter consists of a single band on each side and 

 possesses a varying number of myocommata; it is no longer con- 

 nected with the axial muscles of the neck belonging to the same 

 system (sternohyoid, sternothyroid, &c.) as is the case in Urodeles, 



