46 SPECIAL SENSES. 



tration of the optic nerve, it measures ^ of an inch. It is 

 thinnest at the middle portion of the eye, measuring about -^ 

 of an inch, and is a little thicker again near the cornea. This 

 membrane is composed chiefly of bundles of ordinary connec- 

 tive tissue. The fibres are slightly wavy, and arranged in 

 flattened bands, which are alternately longitudinal and trans- 

 verse, giving the membrane a lamellated appearance, though 

 it cannot be separated into distinct layers. Mixed with 

 these bands of connective-tissue fibres, are numerous small 

 fibres of elastic tissue. The vessels of the sclerotic are scanty. 

 They are derived from the ciliary vessels and those of the 

 muscles of the eyeball. The tissue of the sclerotic yields gela- 

 tine 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, and 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 -^ of an inch 

 in its central portion, and about -^ of an inch near its periph- 

 ery. Its substance is composed of transparent fibres, ar- 

 ranged 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 

 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 la- 

 mella." * This membrane, with its cells, is a continuation of 

 the conjunctiva. 



1 TODD AND BOWMAN, The Physiological Anatomy and Physiology of Man y 

 Philadelphia, 1857, p. 404. 



