MUSCULAR SYSTEM 183 



abdominal region, but only slightly developed : this is more par- 

 ticularly true of the internal oblique, which appears to be under- 

 going degeneration. No trace of a transversalis can be distinguished 

 in the abdominal region, but, on the other hand, a distinct, paired, 

 unsegmented rectus is present, reduced anteriorly and posteriorly. 

 External and internal intercostals are well developed, and a 

 triang-ularis stcrni (last trace of the transversus) 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 body, 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. 



Mammals. In general, there is a reduction of the ventral 

 musculature in Mammals. Three lateral abdominal muscles are 

 always present, an external and internal oblique and a transversalis. 

 In many cases, more particularly in Tupaia and in Lemurs, the 

 external oblique possesses tendinous intersections, thus indicating- 

 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 abdommis 

 The latter consists of a single band on either side and possesses a 

 varying number of myocommas ; it is no longer connected with the 

 axial muscles of the neck belonging to the same system (sterno- 

 hyoid, sternothyroid, &c.) as is the case in Urodeles, for the sternum 

 is always interposed between them, as in the Sauropsida. It, how- 

 ever, may occasionally (e.g. in lower Primates) reach as far forwards 

 as the region of the first rib : in higher forms it becomes more pi- 

 less shortened, the greatest loss of myomeres being seen in 

 Anthropoids and Man, in connection with the development and 

 relations of the great adductor (pectoralis major) of the fore limb. 



In Monotremes and Marsupials, the strong pyramidaiis muscle 

 lies on the ventral side of the rectus abdominis. It arises from 

 the inner border of the marsupial bones (p. 155) and may 

 extend forwards as far as the sternum. In the higher Mammals, 

 in which marsupial bones are wanting, the pyramidaiis usually 

 becomes greatly reduced or entirely lost. Traces of it are, how- 

 ever, commonly to be met with even in the Primates, arising from 

 the anterior border of the pubis, right and left of the middle line. 1 



The external and internal oblique muscles are represented in 

 the thoracic region in Mammals, as in the Sauropsida, in the form 



1 A sphincter marsupii muscle is developed in connection with the marsupixim 



(Figs. 28 and 138). 



