500 THE SENSORIAL FUNCTIONS. 



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 suscep- 

 tible of great variation. 



For the purpose of preserving the hemisphe- 

 rical form of the sclerotica, this membrane in 

 birds is strengthened by a circle of bony plates, 

 \yhich 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 thick- 

 ness. 



There is a dark-coloured membrane, called the 

 Marsypium, situated in the vitreous humour, the 

 use of which is unknown, though it appears to 

 be of some importance, as it is found in almost 

 every bird having extensive powers of vision.* 

 The comparative anatomy of the eye offers, 

 indeed, a great number of special structures of 



* It is shown at m, Fig. 433, which is a magnified section of 

 the eye of a Goose, c is the cornea; i, the iris; p, the cihary 

 processes, s, the sclerotic coat, and o, the optic nerve. 



f 



