THE FIBROUS TUNIC. 



1451 



radius of curvature of the posterior surface is only 6 mm. ; the cornea is consequently 

 thicker in the periphery than at the center, in the proportion of i.i mm. to .8 mm. 

 The degree of curvature varies in different individuals and at different periods of life, 

 being greater in youth than in 



 Fig. 



old age. 



1205. 



Corneal corpuscles (connective tissue cells), surface view. 



350- 



As the radius of curva- 

 ture of the sclera, with which its 

 bundles are continuous, is 12 

 mm. , the cornea rests upon the 

 sclera as a watch-glass upon a 

 watch. At the junction of the 

 two membranes, on the outer 

 surface, is the shallow groove, 

 the S2ilcus sclera:. 



Structure of the Cor- 

 nea. — The cornea is composed 

 of five distinct layers, which 

 from without in are: (i) the 

 anterior epithelium, (2) the an- 

 terior limiting membrane, (3) 

 the substantia propria, (4) 

 the posterior limiting mem- 

 brane, and (5) the posterior 

 €7idothelinm. 



The anterior epithelium 

 of the cornea is continuous 

 with that covering the surface 



of the adjacent conjunctiva sclerae. It is of the stratified squamous variety, usually 

 five cells deep in man, and measures .045 mm. in thickness at the center, and 

 .080 mm. at the periphery. The deepest cells are columnar in form, with broad 

 basal plates resting upon the anterior limiting membrane, to which they are firmly 

 attached by means of minute projections which roughen the anterior surface of 



the latter. The outer parts of the basal 

 cells contain the nucleus and fit into corre- 

 sponding depressions in the cells of the 

 superimposed layers. The middle layers are 

 composed of irregular polyhedral cells, 

 which usually present fine protoplasmic 

 denticulations, and resemble prickle cells. 

 The superficial layers consist of flattened 

 cells which lie parallel to the free surface 

 and contain well-staining nuclei. 



The anterior limiting membrane, or 

 Bozuman s membrane, is situated immedi- 

 ately below the epithelium, and appears as a 

 homogeneous band, about .02 mm. in thick- 

 ness at the center and thinner at the periph- 

 ery, where it terminates without extending 

 into the conjunctiva of the sclera. The mem- 

 brane may be split into fine fibrillae by 

 the use of suitable reagents, is connected 

 firmly with the cornea proper by delicate 

 filaments, and is to be considered a special condensation of the latter. I.t contains 

 no elastic tissue. 



The substantia propria constitutes the main portion of the cornea, and is 

 made up of interlacing bundles of connective tissue, which are directly continuous 

 with those of the adjacent sclera. The bundles are composed of fine fibrillae, have 

 a flattened form, and are so disposed as to produce regular lamellae, about sixty 

 in number, running parallel with the surface. The alternating lamellae have a direction 

 approximately at right angles to each other and are frequently joined together by 



Fig. 1206. 



Corneal spaces, after action of argentic nitrate ; 

 surface view. X 350- 



