MUSCULAR SYSTEM 101 
Ruge 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 cr 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 may 
all be taken as evidence that the pyramidalis in Man (and the 
same apphes 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 and Marsupialia) 
the pyramidalis is powerfully developed in connection with 
the epipubes (so-called marsupial bones); and even in some 
Placentaha, such as the Insectivora (¢.g. Wyogale 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 
