MUSCULAR SYSTEM 101 



Kuge has called attention to a phylogenetic shifting of the navel. 

 This occurs during the shortening of the thoraco-lumbar portion 

 of the trunk (in relation to the segments of the rectus abdominis), 

 and is accompanied by a gradual elimination of the posterior 

 segments of the rectus. This process may not be yet finished ; 

 if, as has already been argued in dealing with the vertebral column 

 (ante, p. 43), a progressive abbreviation of the thoracic region of 

 the trunk is still taking place. 



In front (ventrad) of the point of origin of the rectus 

 abdominis, at the upper edge of the pelvis, there lies, in Man, 

 the inconstant pyramidalis abdominis muscle. This is sometimes 

 developed only on one side, and sometimes unrepresented, in 

 which case it may be replaced by a tract of fibrous tissue. On 

 the other hand, either one or both halves of this muscle may be 

 double ; and there are variations no less remarkable in its form 

 and size. The pyramidalis usually runs either about half-way from 

 the symphysis pubis to the navel, or only a third of that distance ; 

 it may sometimes, however, reach as far as the navel. In young 

 children it is relatively larger than in adults. These facts rnay 

 all be taken as evidence that the pyramidalis in Man (and the 

 same applies to many Mammals, e.g. the Anthropoids) possesses 

 all the peculiarities of an organ which has long been in a state 

 of degeneration. It claims our attention principally as a striking 

 example of the tenacity with which certain structures remain 

 in the organism and are handed on, through inheritance, long 

 after they have lost their specific significance. The reason for 

 such continuance can only be that, in the course of phylo- 

 genetic development, the muscle has undergone a change of 

 function, and has become associated with or subordinated to 

 other muscles or groups of muscles. In this case the pyramidalis 

 has been overmastered by the rectus abdominis. 



In the non-placental Mammals (Monotremata andMarsupialia) 

 the pyramidalis is powerfully developed in connection with 

 the epipubes (so-called marsupial bones) ; and even in some 

 Placentalia, such as the Insectivora (e.g. Myogale pyrenaica), it may 

 almost reach the ensiform process of the sternum, thus playing 

 an important part in strengthening the abdominal wall. The 

 pyramidalis is undoubtedly an old muscle dating far back to 

 pre-Mammalian times. 



Both the abdominal oblique muscles may be considered as 

 continuations of the intercostal muscles into the abdominal region, 

 and, anteriorly, the scaleni muscles of the neck may be looked 



