VISION. 499 



usually a single perforated eye-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 

 lacrymal glands, and with a very moveable 

 memhrana nictitans, or third eye-lid. 



Birds present a still further developement of 

 all these parts : their eyes are of great size com- 

 pared with the head ; as may be seen from the 

 large portion of the skull which is occupied on 

 each side by the orbits. The chief peculiarities 

 of the internal structure of these organs are ap- 

 parently designed to accommodate them to vision 

 through a very rare medium, and to procure their 

 ready adjustment to objects situated at very dif- 

 ferent 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 



