CETACEA 1563 



of the water; and this air, with which they fill their 

 lungs in respiration, 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 further their 

 specific gravity, Nature has provided that a large 

 quantity 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 blow- 

 ing 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 toward the surface, 

 when the animal is constrained to rise, or downward, 

 when, by diving, it hastens to escape from danger. 



All the essential and permanent parts of the skele- 

 ton of vertebrated animals, that is, the spinal column, 

 and its immediate dependencies, the skull, the caudal 



* The substance called Spermaceti is lodged in cells, formed 

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

 head of the Cachalot. 



