II.] 



THE SPIRAL SKELETON. 



Snakes, which glide along by the successive application to the 



ground of the edges of ventral horny plates, each plate being 



attached to the ends of a pair of 



ribs. There being no sternum in 



Serpents, all the ribs are, as before 



said, " false ; " but all the ribs may 



also be false notwithstanding the 



presence of a sternum, as in 



Tailed-Batrachians. 



The ribs may form a solid case 

 for the protection of all the other 

 parts. Thus in Tortoises the head 

 and limbs can be drawn into such 

 a case (called the carapace), which 

 is formed of greatly expanded ribs 

 joining each other, and also the 

 expanded neural spines before no- 

 ticed, by suture (Fig. 57). 



In man we find the floating ribs 

 float by their anterior ends only, 



their hinder ends articulating with FIG. 7 8. RIBS OF THE FLYING 

 the vertebras. The very reverse 

 condition to this may appear to 

 obtain, as in the Crocodile, where 

 we have ventral rib-like structures 

 (towards the hinoler end of the ab- 

 domen), which are attached ventrally, but are free at their 

 vertebral ends, and thus float in the reverse direction. These, 

 however, are hardly true ribs, but are ossifications of a more 

 superficial region. 



The presence in the ribs of a distinct "head" and "tubercle," 

 as in man, is a very general but not a constant character. 

 Very often, however, if not always, when there is but one 

 articular surface for attachment to the vertebral column, 

 that surface represents and is equivalent to a " head " and 

 " tubercle," as it were, united and fused together ; though in 

 Monotremes the ribs are attached only to the sides of the 

 bodies of the vertebras. 



This gradual fusion is well shown in the different vertebrae 

 of the Crocodile, where, as we proceed post-axially, these two 

 parts become more and more approximated together. 



As in man, so generally, it is at the more pre-axial part of 

 the series of ribs that this distinction into head and tubercle 

 is most marked. 



LIZARD (Draco volans). 



sternum ; x, one of the di- 

 verging branches of the xiphoid 

 process ; t, true ribs ; f t floating 

 ribs. 



