138 COMPARATIVE ANATOMY. 



IX. The five senses are common to most large ani- 

 mals : nor have ne much difference to remark in their con- 

 stitution ; or much, however, which is referrible to mech- 

 anism. 



The superior sagacity of animals which hunt their prey, 

 and which, consequently, depend for their livelihood upon 

 their nose, is well know^n in its use ; but not at all known 

 in the organization which produces it. 



The external ears of beasts of prey, of lions, tigers, 

 wolves, have their trumpet part or concavity standing for- 

 wards, to seize the sounds which are before them, viz. the 

 sounds of the animals which they pursue or watch. The 

 ears of animals of flight are turned backward, to give no- 

 tice of the approach of their enemy from behind, w hence he 

 may steal upon them unseen. This is a critical distinc- 

 tion ; and is mechanical : but it may be suggested, and, I 

 think, not without probability, that it is the effect of con- 

 tinued habit. 



The eyes of animals which follow their prey by night, 

 as cats, ow4s, ^c. possess a faculty, not given to those of 

 other species ; namely, of closing the pupil entirely. The 

 final cause of which seems to be this. It was necessary 

 for such animals to be able to descry objects with rery 

 small degrees of light. This capacity depended upon the 

 superior sensibility of the retina ; that is, upon its being 

 affected by the most feeble impulses. But that tenderness 

 of structure, which rendered the membrane thus exquisite- 

 ly sensible, rendered it also liable to be offended by the 

 access of stronger degrees of light. The contractile range 

 therefore of the pupil is increased in these animals, so as 

 to enable them to close the aperture entirely ; which in- 

 cludes the power of diminishing it in every degree ; where- 

 by at all times such portions, and only such portions of 

 light are admitted, as may be received without injury to 

 the sense. 



There appears to be also in the figure, and in some prop- 

 erties of the pupil of the eye, an appropriate relation to 

 the wants of different animals. In horses, oxen, goats, 

 sheep, the pupil of the eye is elliptical ; the transverse 

 axis being horizontal ; by which structure, although the 

 eye be placed on the side of the head, the anterior elon- 

 gation of the pupil catches the forward rays, or those 

 which come from objects immediately in front of the ani- 

 mal's face. 



