446 THE SENSORIAL FUNCTIONS. 



lid, which, when closed by its orbicular muscle, 

 exhibits merely a horizontal slit. There is also a 

 small internal fold, forming the rudiment of a third 

 eye-lid. The Chameleon has remarkably projecting 

 eyes, to which the light is admitted through a very 

 minute perforation in the skin constituting the outer 

 eye-lid. This animal has the power of turning each 

 eye, independently of the other, in a great variety 

 of directions. 



The eyes of Tortoises exhibit an approach to 

 those of birds : they are furnished with large la- 

 crymal glands, and with a very moveable membrana 

 nic titans, or third eye-lid. 



Birds present a still further developement of all 

 these parts : their eyes are of great size compared 

 with the head ; as may be seen from the large por- 

 tion of the skull which is occupied on each side by 

 the orbits. The chief peculiarities of the internal 

 structure of these organs are apparently designed 

 to accommodate them to vision through a very rare 

 medium, and to procure their ready adjustment to 

 objects situated at very different distances. The 

 form of the eye appears calculated to serve both 

 these purposes ; for the great prominence of its 

 anterior portion, which has often the shape of a 

 short cone, or cylinder, prefixed to the front of a 

 hemispherical globe, and which is terminated by a 

 very convex cornea, affords space for a larger 

 quantity of aqueous humour, and also for the re- 

 moval of the lens to a greater distance from the 

 retina; whereby the vision of near objects is faci- 

 litated, while at the same time the refracting powers 

 are susceptible of great variation. 



For the purpose of preservins; the hemispherical 



