THE EXTERNAL COAT THE FIBROUS TUNIC 631 



set in its rim; heim- the inner surface of the cornea possesses a slightly 

 greater diameter than the outer. 



The cornea may be said to consist of five layers: 1, the anterior 

 epithelium; 2, the anterior homogeneous membrane; 3, the corneal sub- 

 stance ; 4, the posterior homogeneous membrane ; 5, the posterior epithe- 

 lium. 



The anterior epithelium (corneal epithelium, corneal conjunctiva) 

 at the margin of the cornea is continuous with the scleral portion of the 

 conjunctiva. It consists of a relatively thin layer six to eight cells 

 deep of stratified squamous epithelium, the deepest cells of which are 

 elongated or columnar,, the middle cells polyhedral, and the superficial 

 cells somewhat flattened. The cells at all levels are nucleated and, like 

 the other corneal tissues, perfectly transparent. The columnar cells 

 are often slender and much elongated, their pointed apices extending 

 well toward the surface of the epithelial layer. 



The epithelium rests directly upon the anterior homogeneous lamella. 



The deeper cells of the epithelium present distinct intercellular lym- 

 phatic spaces and intercellular bridges. Between the cells are the termi- 

 nal ramifications of nerve fibrils from the plexus in the corneal substance. 



The anterior homogeneous membrane (anterior basal lamella, elas- 

 tic membrane of Bowman) was formerly thought to consist of elastic tis- 

 sue, but this supposition is disproved by its ready solubility on boiling 

 (His), as well as by the fact that it does not react typically to the spe- 

 cific stains for this tissue. Bowman's membrane is apparently a homo- 

 geneous or structureless coat except that it is slightly fibrillar at its 

 extreme margin where it becomes continuous with the fibrous tissue of 

 the sclera. It resembles elastic tissue in that it is highly refractive and 

 possesses a shining glassy appearance. It does not stain readily with the 

 ordinary dyes. 



The corneal substance (substantial, propria) forms the greater por- 

 tion of the cornea. It consists of a lamellated connective tissue, which 

 forms about sixty fibrous layers, parallel to the corneal surface. The 

 fibrous bundles of these lamella?, being arranged in meridional curves 

 parallel to the surface, appear to cross one another at right angles 

 in the central portion of the circular cornea. Other fibers, arcuate fibers, 

 pass from one layer to another ; so firmly uniting them that it is im- 

 possible to tease the cornea into its component lamellae. 



The intervals between the fibrous layers are occupied by interlamel- 

 lar cement, or ground substance, in which lymphatic channels and large 

 flattened cells, the corneal corpuscles, can be demonstrated. The cor- 



