38 THE STRUCTURE OF MAN 
pressure on the ventral, the ribs which in the quadrupeds are 
the more necessary for enclosing and supporting the viscera, 
might degenerate in the abdominal or lumbar region. The 
pressure of the viscera is no longer in the ventral, but in the 
caudal direction (cf. Fig. 23). We find, in consequence, a 
compensating expansion of the iliac fosse of the bones of the 
pelvic girdle. The fact that this change is specially pronounced 
in women is easily explained by functional (sexual) adaptation, 
and it thus tends to confirm the above theory. 
The shifting of the centre of gravity towards the dorsal 
side explains why the vertebral ends of the lowest ribs are so 
firmly attached, and also why the dorsal portion of the thoracic 
bony skeleton is much longer than the ventral. In this con- 
nection we have naturally to take into account the great muscles 
which are statically and mechanically required by the axial 
skeleton, and for which these ribs furnish points of origin and 
insertion. But even supposing that the ribs were not required 
for this purpose, there are other related structures which, to a 
certain extent, favour their persistence. The chief of these is 
the serratus posticus inferior muscle, which is inserted into the 
four lower ribs, and the latissimus dorsi which partly arises from 
the last three. 
It may be remarked, however, that the mere presence of. 
these two muscles, as will be seen later on, is insufficient to 
account for the persistence of the lower ribs. Indeed, the latter 
might well be degenerating so far as the former are concerned, 
for not only is the serratus posticus inferior distinctly rudi- 
mentary, but the parts of the latissimus dorsi attached to 
these ribs are quite insignificant in comparison with the rest 
of the muscle. But, notwithstanding this, the action of the 
serratus to a certain degree favours the retention of these ribs 
(chip aon 
Returning now to the more important factors which deter- 
mine the transformation of the thorax, we must, as Ruge rightly 
points out, take into account the influence of the fore-hmbs. As | 
the latter developed into seizing organs, their muscles became 
more powerful and more specialised, and reacted, in turn, on the 
form of the ribs and the arch of the thorax. Further conse- 
quences of this are seen in the greater compactness of the internal 
organs, in the gradual fusion of certain lobes of the liver and 
lungs, and in the approximation and final union of the peri- 
cardium and diaphragm, which may also imply the gradual 
