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 fossse 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 

 (cf. p. 45). 



Eeturning now to the more important factors which deter- 

 mine the transformation of the thorax, we must, as Huge rightly 

 points out, take into account the influence of the fore-limbs. 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 



