484 THE MECHANICAL FUNCTIONS. 



gives their bodies the buoyant force that is 

 required to facilitate their ascent, and supersedes 

 the necessity of a swimming bladder, an organ 

 which is so useful to the fish. 



With the intent of diminishing still farther 

 their specific gravity, nature has provided that a 

 large q^iantity of oily fluid shall be collected 

 under the skin, a provision which answers also 

 the purpose of preserving the vital warmth of 

 the body. A great accumulation of this lighter 

 substance is formed on the upper part of the 

 head, apparently with a view to facilitate the 

 elevation to the surface of the blowing hole, or 

 orifice of the nostrils, which is placed there.* 



Another peculiarity of conformation, in which 

 the cetacea differ from fishes, and which has also 

 an obvious relation to their peculiar mode of 

 breathing, is in the form of the tail, which, instead 

 of being compressed laterally, and inflected from 

 side to side, as in fishes, is flattened horizontally, 

 and strikes the water in a vertical direction, 

 thereby giving the body a powerful impulsion, 

 either towards the surface, when the animal is 

 constrained to rise, or downwards, when, by 

 diving, it hastens to escape from danger. 



All the essential and permanent parts of the 

 skeleton of vertebrated animals, that is, the spi- 

 nal column, and its immediate dependencies, the 



* The substance called Spermaceti is lodged in cells, formed 

 of a cartilaginous substance, situated on the upper part of the 

 head of the Cachalot. 



