480 CLASS MAMMALIA. 



The want of external eyes is a character which the Ba- 

 laenae, and all the Cetacea share in common with some 

 of the Phocae, and most aquatic animals. 



The internal arrangement of this organ presents some 

 interesting peculiarities, which we shall notice in speaking 

 of the senses. The external orifice is nothing but a very 

 narrow cartilaginous canal, proceeding from the tympanum, 

 winding through the bed of fat as far as the surface of the 

 skin, and opening externally by a little hole almost imper- 

 ceptible, and terminated by no vestige of a conch. This 

 canal pierces the upper maxillary bone, and terminates 

 above the spiracle in an orifice which a small valve ren- 

 ders impenetrable to water. 



It was believed that the Balaenae preserved after birth the 

 oval foramen which in the mammiferae is open previously 

 to birth, and by means of which the blood can pass from 

 one part of the heart into the other, without circulating 

 through the lungs. This opinion however is erroneous ; 

 and it is now ascertained that the Cetacea can remain under 

 water but a very short time, and are obliged to come fre- 

 quently to the surface to respire the atmospheric air 

 through their spiracles. 



These same spiracles are two canals situated towards the 

 middle of the great vault of the head, a little behind that 

 part where a hump is raised on which this orifice is 

 situated. They proceed from the bottom of the mouth, 

 traverse obliquely and in a curved direction the interior of 

 the head, terminating towards the middle of the upper 

 part. They have not however the same form and situation 

 in all the Balsenae. In some they have the form of two 

 crescents with the convexities opposed and a little sepa- 

 rated one from the other ; in others, there are two aper- 

 tures completely circular, sometimes considerably remote 

 from each other, and sometimes so near that they seem to 



