104 PRACTICAL HISTOLOGY. 



THE EYE. 



THE CORNEA. 



PREPARATION. Cut out the fresh cornea from the eye of a cat or rabbit In excising 

 the eyeball, be careful not to squeeze or injure it. For the removal of the cornea, push a 

 narrow knife into it at its junction with the sclerotic, and then cut it out with scissors. 

 Harden it in a two per cent, solution of potassic bichromate for ten days, or in the chromic 

 acid and spirit mixture for a week. Make vertical sections, stain some with logwood and others 

 with picrocarmine, and mount in Farrant's solution, and also in dammar. 



EXAMINATION (L). Observe the epithelium occurring in many layers conjunctiva cornea; 

 or anterior epithelium covering the anterior surface of the cornea proper. In the human 

 cornea, and in the cornea of some animals, the epithelium rests on a narrow, elastic, trans- 

 parent membrane the anterior elastic lamina or Bowman's membrane but it is either absent 

 or but feebly developed in the cornea of the cat and dog. It seems to differ from the next 

 layer chiefly in the absence of corneal corpuscles. The substance forming the cornea proper 

 substantia propria is stained red, and on its posterior surface there is a section of a clear, 

 sharply-defined membrane stained yellow the membrane of Descemet, or posterior elastic 

 lamina and covering the posterior surface of this membrane there is a single layer of flat- 

 tened nucleated epithelial cells seen in profile. Examine each of these parts. (Indicate the 

 general arrangement in one half of I '1. XXV., Fig. i.) 



(H). The anterior epithelium. Begin at the surface, and note the flattened cells seen in 

 section. The deeper layers consist of two or three layers of polyhedral cells, each with a 

 spherical nucleus. They are so arranged as to fit into each other, and the deepest layer resting 

 on the cornea is composed of a row of columnar cells placed perpendicularly. Observe how 

 processes of the cells in the layer above this fit into depressions between the apices of the 

 columnar cells (Cleland and Stirling). The columnar cells are not all of the same height, and 

 their lower end possesses a flat expansion or foot-plate (Lott and Stirling), which rests on 

 the cornea. Prickle-cells are seen in the middle layers, especially where the epithelium is 

 in very many layers, as in the cornea of the ox. (See p. 9.) 



The anterior elastic lamina is a homogeneous, transparent membrane devoid of cornea- 

 corpuscles. It is perforated here and there by a few oblique channels, which transmit fine 

 nerve-fibrils the rami perforantes. 



The substantia propria is composed of a series of layers of fibrillar connective tissue, 

 arranged in the form of lamellae, placed one outside the other, and parallel to the surface of 

 the cornea. These lamellae, and also the fibres which compose them, are held together by an 

 albuminous cement-substance. Bundles of fibrous tissue pass 'from one lamella to another. 

 Near the anterior surface a few fibres perforate the lamellae obliquely, and constitute the 



