ANATOMY OF THE EYEBALL. 675 



not cited in the table just given, show that the excess of the antero-posterior 

 diameter over the others is diminished by age. 



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 (1 mm.) It is thinnest at the middle portion of the eye, measuring 

 about -fa of an inch (0-5 mm.), 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 longitudinal 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 scle- 

 rotic yields gelatine on boiling. 



Cornea. The cornea is the transparent membrane which 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 segment 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 -fa of an inch (0-8 mm.), 

 in its central portion, and about ^ of an inch (1 mm.) near its periphery. Its 

 substance is composed of transparent fibres, arranged in incomplete layers, 

 something like the layers of the sclerotic. It yields chondrine instead of 

 gelatine on boiling. 



Upon 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 whit- 

 ish 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. 



The proper corneal membrane is composed of very pale, flattened bun- 

 dles of fibres, interlacing with each other in every direction. Their arrange- 

 ment 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 inter- 

 vals 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 transparent. The corneal substance is very tough, and it 

 will resist a pressure sufficient to rupture the sclerotic. 



Upon the posterior, or concave surface of the cornea, is the membrane of 

 Descemet or of Demours. This is elastic, transparent, structureless, rather 

 loosely attached, and covered with a single layer of regularly polygonal, nu- 



