928 THE SENSORY APPARATUSES. 



tissue interwoven in a very close manner, with some elastic fibres and little masses 

 of pigment "between, especially at its posterior part. Among these fasciculi, a 

 large number pass from before to behind, and these are intersected by others 

 which are placed in a circular manner around the globe. The superficial fibres 

 are continuous with the neurilemma of the optic nerve. (Delicate elastic fibres 

 are mixed with the others, and in the lacunae of the network are some connective 

 tissue corpuscles. Between the choroid and sclerotica is loose connective tissue 

 containing numerous elastic fibres, and branched pigment-cells and non-pigmented 

 flat endothelial cells ; this forms the hwiina supra-choroidea, or lamina fiisca 

 sclera. The optic nerve, at its entrance into the sclerotic, is very much con- 

 stricted, and passes through a funnel-shaped, porous mesh of fibrous tissue 

 named the lamina crihrosa, in the centre of which is a larger opening than the 

 others, for the passage of the arteria centralis retinas — theji;o/-MS opticus.) 



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. 

 Xerves have been found in the sclerotic of the Rabbit, but Leydig could not find 

 any in the Calf. Lecoq has remarked that in the Ass, particularly when it is 

 old, the back part of the sclerotic is encrusted with an unmistakable layer of 

 bony matter. (In Birds, bony plates are found in this region, and some Reptiles 

 also have them.) 



2. The Cornea (Fig. 502, e). 



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

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

 the margin of the membrane.) 



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

 to the interior of which it allows the light to pass. It closes 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 fares, 

 perfectly smooth — one external, convex ; the other internal — concave, forming 

 the external wall of the anterior chamber. 2. A rirrumference, bevelled on its 

 outer edge, and received into a 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, i\iQ p)roper 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 laminfe and laminellae. 

 These layers are formed by chondrigenous amorphous matter and connective 

 fibrillie, which may be disassociated by pyrogallic acid. They are perforated by 

 more or less irregular openings, and lacunae {spaces of Fontana) occupied by cells, 

 the prolongations of which anastomose through these slits. The cornea contains 

 in its substance a regular network of stellate cells, and of migratory cells wander- 

 ing through the slits and in the lacunae. Lecoq a long time ago remarked that 

 this layer became opaque when the eye was strongly squeezed, and attributed 

 this change to expression of the fluid it contained. Dubois has observed the 

 same opacity in the cornea of the Dog, as a consequence of anaesthesia by chloride 

 of ethylene. 



