182 ANATOMY OF THE HEAD. 



in its capsule (Fig 19 . 5), which encircles it. A certain 

 water, or liquid, is said to surround the crystalline lens ; 

 hut this is found only some time after life has departed, 

 and is not present immediately after death. The lens itself, 

 when considered independent of its capsule, is organized. 

 We know this, because it undergoes change ; because ves- 

 sels are seen entering into its substance in disease ; also 

 because the crystalline lens itself can be absorbed, and bone 

 deposited in the space a crystalUne lens once occupied. It 

 is convex both before and behind, or on each of its sides ; 

 though more convex on one side than the other. Tlie 

 crystalline lens is lamellated in structure, and is more dense 

 as the centre is approached. Its use is to refract the rays 

 of light still more than the cornea and the aqueous humour 

 have done ; and to transmit them thus refracted to the 

 vitreous humour, which likewise brings them closer to- 

 gether (see Fig 19). 



The vitreous humour {Fig 19 .3) is all that w^e have 

 said ; but besides this it has an artery which runs along its 

 centre to the crystalline lens, which it nurtures (Fig 19 . /). 

 This artery likewise supplies the membrane of the vitreous 

 humour with blood for secretion. Besides the parts we 

 have spoken of, there are others named the canal of Petit, 

 &c., which do not exist in a perfectly fresh eye, but require 

 maceration, &c. for their development. Of such fanciful 

 structures, therefore, we shall take no notice. 



The coats of the eye. — The conjunctiva is a fine, delicate, 

 transparent and mucous membrane which lines the internal 

 surface of each eyelid, and is reflected from thence over 

 the anterior part of the globe of the eye : being thus 

 reflected, freedom of motion is allowed, yet the eye is amply 

 secured within the orbit ; and the entrance of any foreign 

 substance beyond the point where the membrane folds is 

 completely prevented. The conjunctive coat, where it is 

 opposed to the pupil, is transparent ; too fine to allow of 

 demonstration, so that it may unchanged admit of the 

 visual rays ; and here, therefore, although it be not entirely 

 deprived of vascularity, it does not allow, when the organ 

 is in health, of vessels circulating, sufficiently large to be seen. 

 That portion which lines the eyelids, on the contrary, is 



