THE EXTERNAL COAT THE FIBROUS TUNIC 



633 



the latter, though formerly considered an elastic membrane it does not 

 give the specific reactions of elastic tissue. It is somewhat thicker than 

 Bowman's membrane. At its margin the membrane is continuous with 

 fibrous bundles which are directed outward into the ligamentum pectina- 

 tum, and, at least in some animals, through this ligament into the ciliary 

 margin of the iris. The membrane of Descemet can be readily detached 

 from the corneal substance by teasing. It prevents filtration of fluid 

 from the anterior chamber into 

 the corneal stroma. 



The posterior epithelial layer 

 (corneal cndothelium) is a mes- 

 enchymal epithelium consisting 

 of clear, cuboidal or flattened 

 cells, placed edge to edge, and 

 bound together by intercellular 

 bridges. At the margin of the 

 cornea it is reflected over the 

 lateral wall of the anterior 

 chamber to the anterior surface 

 of the iris. Its cells rest upon 

 the posterior homogeneous mem- 

 brane. 



All the tissues of the cornea, 

 during life, are absolutely trans- 

 parent. The elements of which they consist are of almost identical 

 refractive indices, and about that of water, so that in fresh, or in 

 living tissue, it is almost impossible for the microscope to discover any 

 of the structure of the cornea. After death the cornea becomes opaque 

 and its elements are then easily distinguished. 



Vascular and Nerve Supply. The cornea itself is an absolutely 

 non-vascular tissue, having neither blood nor true lymphatic vessels. 

 It is, however, well supplied with nerve fibers, derived from the ciliary 

 nerves, which form an annular plexus in the sclera about the margin 

 of the cornea, from which point bundles of naked axis-cylinders pass 

 into the corneal substance to form a basal plexus, near the anterior homo- 

 geneous membrane. From this latter plexus, fibers are distributed to 

 the corneal substance and to a subepithelial plexus, anterior to Bowman's 

 membrane, whence the terminal sensory fibrils penetrate the anterior 

 epithelium. For a distance of several millimeters within the margin of 

 the cornea special nerve endings (bulbous corpuscles) may also occur. 

 40 



FIG. 541. CORNEAL CELLS, ISOLATED. 

 Highly magnified. (After Waldeyer.) 



