ISO 



SENSOR I AL FUNCTIONS 



c54. The chief peculiarities in the eyes of these anirm is, 

 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 dis- 

 tance from the retina. On optical principles this arrange- 

 ment 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. 



555. The form of the eye in birds is preserved by a bony 

 circle, consisting of fifteen or twenty pieces overlappin 

 each other. By these bones the sclerotic coat is supporte 

 and its hemispherical prominence maintained. 



556. Nictitating Membrane. Most birds are furnished 

 with a winkifcg membrane (inembrana trictitans), 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. 



557. 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 

 each other by a peculiar and beautiful 

 piece of mechanism. 



558. 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 (lint 

 of the second muscle p, which is cabl- 

 ed the pyramidalis, and which has its 

 origin on the lower, and back part of 



What are the chief peculiarities of the eyes of birds ? How are the ob- 

 jects of these peculiarities answered? 



Fig. J12. 



Fig. 113. 



