THE EYE. 819 



surface of the sclerotic is coated by a thin layer of areolar tissue stained 

 with black pigment the lamina fusca. ) 



The arteries of the sclerotic are derived from the anterior and posterior 

 ciliary arteries ; the veins pass into trunks lying parallel to the ciliary 

 arteries. Nerves have been found in the sclerotic of the Rabbit, but in none 

 other of the domesticated animals. " It is frequently found that in the Ass, 

 particularly when it is old, the back part of the sclerotic is encrusted with 

 an unmistakable layer of bony matter. This fact was unknown to Carus, 

 who states that in none of the mammalia does this membrane become 

 ossified." Lecoq. (In Birds, bony plates are found in this region, and 

 some reptiles also have them.) 



2. The Cornea. (Fig. 383, e.) 



(Preparation. The cornea should be removed with the sclerotic coat, by immersing the 

 eye under water, and making a circular incision with scissors about a quarter of an inch 

 from the margin of the membrane). 



The cornea is a transparent membrane forming the anterior part of the 

 eye, to whose interior it allows the light to pass. It closes up the anterior 

 opening of the sclerotic, and thus completes the external envelope or shell 

 of the globe, of which it forms about a fifth part. 



Elliptical, like the opening it closes, the cornea presents: 1, Two faces, 

 perfectly smooth one external, convex, the other internal, concave form the 

 external wall of the anterior chamber ; 2, A circumference, bevelled on its 

 outer edge, and received into the similar bevel around the sclerotic opening, 

 like the glass of a watch into its case. 



STRUCTURE. Three layers enter into the composition of the cornea : an 

 external, internal, and middle. 



Middle layer. This, the proper cornea, is remarkable for its thickness. 

 When pressed between the fingers, its two faces can be easily made to glide 

 over each other, a proof that its tissue is disposed in superposed and parallel 

 planes ; it is indeed possible to decompose the cornea into several laminae 

 and lamellae, but as their number varies with the amount of skill employed 

 in their separation, they should be considered as an artificial production of 

 dissection. Microscopically examined, it is found to be formed by bundles 

 of excessively fine conjuntival fibrillse, slightly undulating, and arranged 

 parallel to the surface of the cornea. These wavy fasciculi, when placed 

 alongside each other, leave numerous spaces which are oval in a transverse 

 section ; these communicate by means of fine canaliculi, and contain round 

 cellular elements which may move from one space to another. 



Between the fasciculi of the cornea is found a fluid amorphous substance, 

 " a kind of transparent serosity like the cornea itself, which maintains its 

 flexibility, and which, like it, also loses its transparency under the influence of 

 different causes. It is only necessary, in a fresh eye, to squeeze the globe in 

 order to produce a degree of opacity in the cornea which will be more or 

 less great in proportion to the amount of pressure exercised. Is a similar 

 effect produced by the swelling of the eye when the cornea becomes opaque 

 in ophthalmia?" Lecoq. 



The external layer is only the conjunctival epithelium spread over the 

 anterior face of the cornea. This epithelium is stratified, flattened on its 

 surface, but cylindrical below, where it rests on the middle layer, and from 

 which it is not separated, as in many species of animals, by a proper 

 limitary membrane. 



The inner layer is a portion of the membrane of the aqueous humour. 



