332 THE SENSORIAL FUNCTIONS. 



sides effectually guarded from injury by Ijeing contained in 

 a hollow bony socket, termed the orbit, and composed of 

 seven portions of bone. Tliese seven elements may be re- 

 cognised in the skulls of all the mammalia, and perhaps also 

 in those of all other vertebrated animals, affording a remark- 

 able illustration of the unity of the plans of nature in the 

 construction of the animal fabric. 



§ 4. Physiologi/ of perfect Vision. 



The rays of light, proceeding from a distant object, strike 

 upon the convex surface of the cornea, which being of great- 

 er density than the air, refracts them, and makes them con- 

 verge towards a distant focus. This effect, however, is in 

 part counteracted on their emergence from the concave pos- 

 terior surface of the cornea, when the rays enter into the 

 aqueous humour. On the whole, however, they are refract- 

 ed, and made to converge to a degree equal to that which 

 they would have undergone if they had at once impinged 

 against the convex surface of the aqueous humour, supposing 

 the cornea not to have been interposed. 



A considerable portion of the light which has thus en- 

 tered the aqueous humour is arrested in its course by the iris; 

 so that it is only those rays which are admitted through the 

 pupil that are subservient to vision. These next arrive at 

 the crystalline lens, where they undergo two refractions, 

 the one at the anterior, the other at the posterior surface of 

 that body. Both these surfaces being convex outwardly, 

 and the lens being a denser substance than either the aque- 

 ous or the vitreous humours, the effect of both these refrac- 

 tions is to increase the convergence of the rays, and to bring 

 them to unite in a focus on the retina at the bottom of the 

 eye. The most considerable of these refractions is the first; 

 because the difference of density between the air and the 

 cornea, or rather the aqueous humour, is greater than that 

 of any of the humours of the eye compared with one ano- 

 ther. 



