126 HISTORY OF THE HUMAN BODY 



special adaptations, admirably fitted to the habits and environ- 

 ment of the various species, but covering up the original race- 

 history which we are seeking to interpret. While, then, one 

 cannot be dogmatic about this portion of the history, the fol- 

 lowing seems to be the most likely course of development, and 

 its various stages are to be found, with some little modifica- 

 tion, in living species. 



It would seem, then, that the primary pairs of neural 

 processes, for the purpose of better fulfilling their mission of 

 protecting the nerve cord, became more elongated until they 

 finally met in the middle line above the nerve cord, thus form- 

 ing a series of intersegmental neural arches, shaped like inverted 

 Vs; and since the aorta, lying immediately beneath the noto- 

 chord, needed a similar protection, other arches became 

 developed for this purpose, situated immediately below the 

 former, and with their points directed downwards. 



When these two systems of arches were well established, 

 they would naturally seek to secure a firmer attachment to the 

 notochordal sheath by spreading out their bases, and as the 

 two sets of arches, neural and hcemal, were opposite each 

 other, each neural arch being associated with its corresponding 

 haemal arch, the enlarged bases would grow together. To 

 support the increasing weight of these parts, the notochordal 

 sheath would then chondrify or ossify beneath these bases in 

 the form of rings, and the fusion of three elements, a neural 

 and a haemal arch and a notochordal ring, would form a 

 vertebra of the type found in the ordinary bony fish. The 

 condition just described, with neural and haemal arches alike, 

 is that found in the tail region, posterior to the visceral cavity, 

 while in the trunk the haemal arches are open and their halves 

 widely divergent, forming the ribs, which embrace the visceral 

 cavity. The rings, which are formed from the notochordal 

 sheath, begin in tne center of the future vertebrae, and as they 

 grow, expand along both edges until they come in contact 

 with the preceding and succeeding ones. At the same time, 

 however, the ossification has proceeded inwards as well, re- 

 stricting the notochord, and as the central portion of the ring 



