694 MAMMALIA. 



an aquatic or aerial medium ? To smell in water would require 

 the nose of a fish, which could not be granted without infringing 

 upon the laws that regulate the progression of animal organiza- 

 tion. To smell in air would be useless to the Whale ; and, more- 

 over, its nasal passages are required for another function, with 

 which the exercise of smell would apparently be incompatible. 



Thus circumstanced, we find the whole nasal apparatus com- 

 pletely metamorphosed, and so disposed as to answer two impor- 

 tant purposes : viz. first, to allow the Cetacean to breathe air 

 whilst its mouth is immersed in water ; and, secondly, to provide 

 an outlet whereby the water that is necessarily taken into the 

 mouth may escape without being swallowed. 



The arrangement adopted to attain both these ends is very 

 beautiful. The nostrils, instead of occupying their usual position, 

 are situated quite upon the top of the head (Jig. 321, a), so that, 

 as soon as the vertex reaches the surface, air is freely obtained. 

 But another difficulty remains to be overcome : how is the 

 Cetacean to breathe air while its mouth is full of water ? 



To allow this, the upper extremity of the larynx is prolonged, 

 so as to form a thick cartilaginous plug (c). When the creature 

 breathes, this elongated larynx is introduced into the posterior nares, 

 as represented in the figure ; and, being firmly embraced by a 

 sphincter muscle whilst in that situation, the air is admitted into 



Fig. 321. 



the trachea through the passages a, 6, without ever entering the 

 oral cavity. 



