54 COMPARATIVE ANATOMY OP 



rertieal, not a lateral motion of their spines, some- 

 what after the fashion of the caterpiller. Re- 

 ferring the curious reader to the valuable memoir 

 quoted below,* we shall now give in a few words 

 a popular account of this strange phenomenon. 



The remarkable fact is, that during the progres- 

 sion of Seals on land, the hind-feet are never employ- 

 ed, and the fore-feet not necessarily. In explanation, 

 we observe, first, that there is a remarkable change 

 effected on the structure of the spine, or back-bone. 

 This is well known to be not a single bone, but a 

 strong column of many bones, amounting to twenty- 

 four in man, and to many more in the majority of 

 quadrupeds. In most, there is between each verte- 

 bra, as it is called, an inter vertebral cartilage, which 

 we may compare to a piece of Indian-rubber, placed 

 between the bones, to take off, as in man, the 

 effects of every severe shock, keeping the brain 

 on its summit free from all jar and agitation. 

 Now, the inter vertebral cartilages in the Seal are 

 quite peculiar, and very large, especially in the 

 regions of the neck and loins. Each of them is 

 composed of a number of fibrous coverings, form- 

 ing concentric rings, the more external of which are 

 broad and strong, whilst the internal are smaller and 

 more slender, the last, which lines the central cavity, 

 being so fine, that it seems to assume the character 

 of a serous membrane. The centre of this consider 



For an able paper on the Osteology and Myology of the Seal, 

 by M. Duvernov, see Mem. du Mm. d'Hist. Nat. ix. pp. 49, 

 165. 



