228 THE NATURE AND TREATMENT OF 



rounded by a capsule of its own, on the inner surface of which 

 is secreted the aqueous humor ; the uses of this limpid fluid 

 appears to be, to transmit the rays of light and permit free mo- 

 tions of the iris ; the fluid is frequently evacuated and repro- 

 duced. 



Chrystalline Lens. — These are situated in a concavity be- 

 hind the aqueous humor, and anterior to the vitreous, opposite 

 to the pupil ; in appearance it resembles a chrystal, or lens of 

 magnifying glass, and is composed of concentric lamellae, and 

 these of radii. Its form is spherical, yet the posterior surface 

 is more convex than the anterior. Like the other humors of 

 the eye it is enveloped in a capsule, and the disease known as 

 cataract, consists of altered structure, in the lens or its capsule ; 

 sometimes both are involved. Acids, alcohol, and boiling wa- 

 ter convert it into an opaque, solid body, resembling the white 

 of an egg when boiled. The function of the chrystalline lens 

 is to concentrate the rays of light so as to form a distinct image 

 on the interior of the eye. 



Vitreous Humor. — This jellylike substance derives its 

 name from its glassy appearance ; it occupies the posterior 

 concavity of the globe of the eye, known as the " dark-cham- 

 ber." It fills the membranes, aids to maintain the form of the 

 eye, and keeps the chrystalline lens at a proper, and physio- 

 logical distance from the retina. Anatomists have hitherto 

 failed in detecting any signs of vascularity in the vitreous 

 humor. 



COATS OF THE EYE. 



\st. Tunica Conjunctiva. — This membrane is very highly 

 organized, lines the eyelids, and is reflected over the anterior 

 part of the eye-ball ; it covers the region known as the pupil, 

 yet is pervious to the rays of light. Previous, however, to 

 being reflected over the globe, it gives a covering to the mem- 

 brana nictitans, carunculae lacrymale, and puncta lachrymalia. 



2nd. Tunica Sclerotica. — This tunic is made up of fibres, 

 which are very dense, yet elastic ; it constitutes what is com- 

 monly known as the "white of the eye," extends from the 



