352 THE SENSORIAL FUNCTIONS. 



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 membrana nicitans or third eye-lid. 



Birds present a still farther development of all these 

 parts: their eyes are of great size compared 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 peculiari- 

 ties 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 ob- 

 jects 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 removal of the lens to a greater 

 distance from the retina, whereby the vision of near objects 

 is facilitated, while at the same time the refracting powers 

 are susceptible of great variation. 



For the purpose of preserving the hemispherical form of 

 the sclerotica, this membrane in birds is strengthened by a 

 circle of bony plates, which occupy the fore-part, and are 

 lodged between the two layers of which it consists. These 

 plates vary in number from fifteen to twenty, and they lie 

 close together, their edges successively overlapping each 

 other. There is manifest design in this arrangement: for it 

 is clear that a ring formed of a number of separate plates is 

 better fitted to resist fracture than an entire bony circle of 

 the same thickness. 



There is a dark-coloured membrane, called the Marsu- 

 pium, situated in the vitreous humour, the use of which is 

 unknown, though it appears to be of some importance, as it 



