184 



SENSORIAL FUNCTIONS. 



The chief peculiarities in the eyes of these animals, 

 are apparently designed to accommodate their vision to 

 a rare medium ; to strong degrees of light, and a ready 

 adjustment to objects situated at very different distances. 

 These ends appear to be answered chiefly by the great 

 prominence of the cornea, or front of the eye ball, which 

 contains an uncommon quantity of the aqueous humor, 

 so that the lens is situated far forward, or at the greatest 

 distance from the retina. On optical principles this 

 arrangement enables the eye to see near objects most 

 distinctly, while at the same time the refracting power 

 of the lens becomes susceptible of great variations. 



The form of the eye in Birds is preserved by a bony 

 circle, consisting of fifteen or twenty pieces overlaping 

 each other. By these bones the sclerotic coat is sup- 

 ported, and its hemispherical prominence maintained. 



Nictitating Membrane. Most Birds are furnished 

 with a winking membrane, (membrana nictitans^) which 

 they draw over the eye-ball, instead of closing the eye- 

 lids. This is a thin delicate structure, so translucent as 

 to admit a diffused light, while it intercepts the direct 

 rays from the eye. When not in use it is closely folded 

 up in the inner cornea of the eye. 



This membrane is represented at 

 Fig. 112, covering one-half the eye- 

 ball. Its motion is horizontal, and is 

 effected by two muscles, acting upon 

 | 7t/ each other, by a peculiar and beauti- 

 ful piece of mechanism. 



The first of these muscles is called 

 from its shape, the quadratus, q, Fig. 

 113, and arises from the upper and 

 back part of the sclerotica, its fibres 

 converging and terminating in a 

 round tendon as seen in the figure. 

 This tendon serves as a loop for that 

 of the second muscle p, which is call- 

 ed the pyramidalis, and which has its 

 origin on the lower, and back part of 



Fig. 112. 



Fig. 113. 



What are the chief peculiarities of the eyes of birds 7 How are the 

 objects of these peculiarities answered 1 



