84 SPINE. 



T. Is this wonderful mechanism the same in all 

 animals ? 



A. In quadrupeds the number of the vertebrae is 

 from thirty to fifty. In the common serpent it is 

 about three hundred. In the shark the number is 

 somewhat upwards of two hundred. In the eel it ex- 

 ceeds an hundred ; while in common fish it is nearly 

 the same as in quadrupeds. Nor is this all with re- 

 spect to the serpent. Not only is the flexibility of 

 the back increased by the multiplicity of the joints, 

 but the manner in which the vertebrae are united is 

 truly remarkable. They are not united by surfaces 

 which are nearly smooth, as in the spine of a quadru- 

 ped, nor is their flexion impeded in any direction by 

 any projections of the bone. They play freely into 

 one-another like a cup and ball. One extremity is 

 rounded and received into a corresponding cavity in 

 the contiguous vertebra. At the other extremity the 

 arrangement is reversed. Here is a cavity which re- 

 ceives the point of the next bone. So that the whole 

 spine is a continued chain of ball and socket joints, 

 affording a free motion on every side. As a piece of 

 mechanism for pliancy and flexibility, without too 

 much sacrificing a secure union of the joints, it is as 

 palpable a contrivance as a watch-chain, which it 

 does not a little resemble. 



B. This explains why these nimble creatures are 

 able to twist themselves into so many forms. 



T. Yes ; but the more interesting explanation is, 

 it affords them a compensation for their want of feet. 

 Having no feet they need this structure of the spine. 



