CETACEANS. 159 



brane, or tapetum, of a bright-green colour. The scle- 

 rotic coat, at its posterior third, is excessively thick, 

 fibrous, and resisting, while its anterior third is thin 

 and flexible.* 



No lachrymal organs exist; but the surface of the 

 eye is lubricated by a secretion from mucous ducts, 

 which open abundantly on the lining membrane of the 

 lids. 



The external orifice of the ear is a longitudinal 

 fissure, situated a little below, and about one foot behind 

 the eye. Its structure is valvular, and its tube not larger 

 than will admit the first joint of the finger. 



A single spiracle, or spout-hole, opens upon the 

 upper and front part of the snout, near the median 

 line, but with a decided bias to the left side of the 

 head. It is but little raised above the surrounding 

 surface, and affords a longitudinal fissure, eight or ten 

 inches in length, and shaped like the orifice in the 

 sounding-board of a violin. The margins are thick, 

 rounded, and so closely approximated in the dead 

 whale as to require the exercise of some force to 

 introduce the hand between them. 



The tube of the spiracle (which is also single) is 

 capacious, of cylindrical form, and lined by a smooth 

 black membrane. Powerful muscles encircle both the 

 external nostril and the canal of the spiracle, to aid in 

 their functions ; and it is sufficiently obvious, that the 



* This conformation of the sclerotic may in some measure adapt the 

 eye of the whale for amphibious vision : the powerful muscles surrounding 

 the eye-ball being thus enabled to act upon the thin anterior margin of 

 the sclerotica and shorten the axis of vision, by pressing the lens towards 

 the posterior part of the globe ; in this manner reconciling the function 

 of vision to the changes from a denser to a thinner medium, or from 

 water to air. 



