ANATOMY OF VERTEBRATES. 



55 



47 



Two vertebras of the Rattlesnake 

 (Crotalus) 



base of the spine ; the lower apex of the wedge being, as it were, 

 cut off, and its sloping sides presenting two smooth, flat, articular 

 surfaces. This wedge is received into a cavity, the ' zygantrum,' 

 excavated in the posterior expansion of the neural arch, and 

 having two smooth articular surfaces to which the zygosphenal 

 surfaces are adapted. Thus the vertebra? of serpents articulate 

 with each other by eight joints in addition to those of the cup and 

 ball on the centrum ; and interlock by parts reciprocally receiving 

 and entering one another, like the joints called tenon-and-mortice 

 in carpentry, fig. 47. In the caudal vertebra?, the hypapophysis 

 is double, the transition being effected by 

 its progressive bifurcation in the posterior 

 abdominal vertebra?. The diapophyses be- 

 come much longer in the caudal vertebra?, 

 and support in the anterior ones short ribs 

 which usually become anchylosed to their 

 extremities. 



The pleurapophyses or vertebral ribs 

 have an oblong articular surface, concave 

 above and almost flat below in the Python, 

 with a tubercle developed from the upper 

 part, and a rough surface excavated on the fore part of the ex- 

 panded head for the insertion of the precostal ligament. They 

 have a large medullary cavity, with dense but thin walls, and a 

 fine cancellous structure at their articular ends. Their lower end 

 supports a short cartilaginous ha?mapophysis, which is attached 

 to the broad and stiff abdominal scute. These scutes, alternately 

 raised and depressed by muscles attached 

 to the ribs and integument, aid in the glid- 

 ing movements of serpents ; and the ribs, 

 like the legs in the centipede, subserve 

 locomotion ; but they have also accessory 

 functions in relation to breathing and con- 

 striction. The anterior ribs in the cobra, 

 fig. 46, pi, are unusually long, and are 

 slightly bent ; they can be folded back one 

 upon another, and can be drawn forward, 

 or erected, when they sustain a fold of 

 integument, peculiarly coloured in some species — e.g., the 

 spectacled cobra — and which has the effect of making this 

 venomous snake more conspicuous at the moment when it is 

 about to inflict its deadly bite. The ribs commence in the 

 cobra, as in other serpents, at the third vertebra from the head. 



47a 



Front view of a vertebra, 

 Rattlesnake 



