110 LECTURE m. 



half a century ago, was accustomed to illus- 

 trate its use by comparing it to a bladder 

 partly filled with water, and placed between 

 two trenchers ; in which case, the water 

 would readily diminish in bulk as the cir- 

 cumference of the trenchers became ap- 

 proximated on the one side, and would 

 occupy the increasing space on the other. 

 As the exterior ligaments are unyielding, 

 this variation of form in the central pivot 

 could never have taken place, had not an 

 elastic substance intervened between them. 



It is interesting and creditable to human 

 intellect, which thus penetrates into the de- 

 signs of nature, to know, that in the great 

 fish, the squalus maximus, or basking shark 

 of Pennant, this, then merely suppositious, 

 structure is actually found. There is in the 

 centre of the intervertebral connection, a 

 bag of water, and so great is the elasticity of 

 the substance by which it is surrounded, 

 that when Mr. Clift cut into the bag, the 

 expansion of the elastic matter, projected 

 the fluid to the height of four feet in a 



