650 SPECIAL SENSES 



Sclerotic Coat. The sclerotic is the dense, opaque, fibrous covering 

 of the posterior five-sixths of the eyeball. Its thickness is different in 

 different portions. At the point of penetration of the optic nerve, it 

 measures ^ of an inch (i millimeter). It is thinnest at the middle 

 portion of the eye, measuring about -$ of an inch (0.5 millimeter), and 

 is a little thicker again near the cornea. This membrane is composed 

 chiefly of bundles of ordinary connective tissue. The fibres are slightly 

 wavy, and are arranged in flattened bands, which are alternately longi- 

 tudinal and transverse, giving the membrane a lamellated appearance, 

 although it can not be separated into distinct layers. Mixed with these 

 bands of connective-tissue fibres are small fibres of elastic tissue. The 

 vessels of the sclerotic are scanty. They are derived from the ciliary 

 vessels and the vessels of the muscles of the eyeball. The tissue of the 

 sclerotic yields gelatin on boiling. 



Cornea. The cornea is the transparent membrane that covers about 

 the anterior sixth of the globe of the eye. As before remarked, this is 

 the most prominent portion of the eyeball. It is in the form of a seg- 

 ment of a sphere, attached by its borders to the segment of the larger 

 sphere formed by the sclerotic. The thickness of the cornea is about g 1 ^ 

 of an inch (0.8 millimeter), in its central portion, and about -% of an inch 

 (i millimeter) near its periphery. Its substance is composed of trans- 

 parent fibres, arranged in complete layers something like the layers of 

 the sclerotic. It yields chondrin instead of gelatin on boiling. 



On the external, or convex surface of the cornea, are several layers 

 of delicate, transparent, nucleated epithelium. The most superficial 

 cells are flattened, the middle cells are rounded, and the deepest cells 

 are elongated and arranged perpendicularly. These cells become 

 slightly opaque and whitish after death. Just beneath the epithelial 

 covering of the cornea, is a very thin, transparent membrane, described 

 by Bowman under the name of the "anterior elastic lamella." This 

 membrane, with its cells, is a continuation of the conjunctiva. It is 

 sometimes called Bowman's membrane. 



The proper corneal structure is composed of flattened bundles of 

 very pale fibres interlacing with each other in every direction. Their 

 arrangement is lamellated, although they can not be separated into 

 complete and distinct layers. Between the bundles of fibres, lie a great 

 number of stellate, anastomosing, connective-tissue corpuscles. In these 

 cells and in the intervals between the fibres, there is a considerable 

 quantity of transparent liquid. The fibres constituting the substance of 

 the cornea are continuous with the fibrous structure of the sclerotic, 

 from which they can not be separated by maceration. At the margin 

 of the cornea the opaque fibres of the sclerotic abruptly become trans- 



