THE ORDER CETACEA. 33 



enormous size, until at least one-third of the carcass 

 appears above water, and sometimes the body bursts 

 with the force of the air generated within. 



Formerly it was believed that the whales were pre- 

 served, after birth, under water, in consequence of the 

 existence of the oval foramen, which in the mammiferce 

 is open previously to the birth of any viviparous animal, 

 by which means the blood is enabled to pass from one 

 part of the heart to another, without first circulating 

 through the lungs. This opinion, however, is erroneous ; 

 for it is now ascertained that all the cetdcea can remain 

 under water but a very short time, and are compelled 

 to come forward frequently to the surface to respire 

 atmospheric air through their spiracles. 



These spiracles consist of two canals, situated towards 

 the middle of the great vault of the head ; a little behind 

 is a lump or protuberance raised, upon which the orifices 

 are situated. They proceed from the bottom of the mouth, 

 traversing obliquely, and in a curved direction, the in- 

 terior of the head, terminating towards the middle of the 

 superior portion. They have not the same form and 

 situation in all the oalcena: in some, they have the form 

 of two crescents, with the convexities opposed and a 

 little separated one from the other; in others, there are 

 two apertures, completely circular, sometimes consider- 

 ably remote from each other, and sometimes so near that 

 they appear to form but one and the same orifice, the 

 external diameter of which constitutes about a hundredth 

 of the total length of the animal. 



The spiracles answer the purpose of expelling the 

 water which penetrates the interior of the whale's throat, 

 and prevents its entering the larynx, allowing the air 

 that is necessary for the respiration of the animal to 

 enter the lungs. They send forth so considerable a 



D 



