172 OF SIGHT. 



The lens itself is very pellucid and cellular, but so 

 much more dense than the vitreous humour that in the 

 hand it seems like a very tenacious, although an amaz- 

 ingly clear, gluten. Its nucleus is more dense than 

 the exterior lamella?. These may, by management, be 

 reduced into extremely delicate fibres, converging from 

 the circumference to the centre.* 



In an adult man the lens is proportionally smaller 

 than in quadruped mammalia ; also less convex, espe- 

 cially on its posterior surface. 



2G6. The remaining space of the eye is filled by the 

 aqueous humour, which is very limpid, and divided by 

 the iris into two chambers: the anterior and larger sepa- 

 rating the cornea and iris ; and the posterior, in which 

 the uvea lies towards the corpus ciliare, so small, as 

 scarcely believed by some to exist. 



267. These most valuable parts are defended from 

 injuries both by the depth of their situation in the orbits 

 and by the valvular coverings of the eye-lids. 



In the duplicature of the palpebra, lie the sebaceous 

 follicles of MeibomiuSjf thickly distributed; and their 

 edges are fringed by a triple or quadruple series of 

 cilia :'j the cartilaginous tarsi serve for their support 

 and expansion, and also facilitate their motion upon 

 the eye-ball. 



Above the eye-lids, to use the language of Cicero, 

 the skin is covered by the supercilia, which preserve 

 the eyes from the sweat that flows from the head and 



* Tli. Young, Phifos. Trans. 1793. tab. xx. fig. 2, 3. 



Dav. Hosack, ib. 1794. tab. xvii. fig. 4. 



J. C. Reil, De lentis crystallines structure, fibrosa. Hal. 17!/4. Kvo. 



t H. Meibomius, I)r rasis Palpcbrarum iiovis c]>. Hclmst. U'^O. 4to 



J B. S. Albinus, Aiuvitat. Avadvm, L, iii. tab. iii. lisr. 1. 



