674 



SPECIAL SENSES. 



same phenomenon is observed when the eyeball is pressed upon or contused, 

 a fact which is sufficiently familiar. 



PHYSIOLOGICAL ANATOMY OF THE EYEBALL. 



The eyeball is a spheroidal body, partially embedded in a cushion of fat 

 in the orbit, protected by the surrounding bony structures and the eyelids, 

 its surface bathed by the secretion of the lachrymal gland, and movable in 

 various directions by the action of certain muscles. It is surrounded by a 

 thin, serous sac, the capsule of Tenon, which exists in two layers. The outer 

 layer lies next the fatty layer in which the globe is embedded, and the inner 

 layer invests the sclerotic coat. When the axis of the eye is directed for- 

 ward, the globe has the form of a sphere, in its posterior five-sixths, with the 

 segment of a smaller sphere occupying its anterior sixth. The segment of 

 the smaller sphere, bounded externally by the cornea, is more prominent 

 than the rest of the surface. 



The eyeball is made up of several coats enclosing certain refracting 

 media. The external coat is the sclerotic, covering the posterior five-sixths 

 of the globe, which is continuous with the cornea, covering the anterior 

 sixth. This is a dense, opaque, fibrous membrane, for the protection of the 

 inner coats and the contents of the globe. The cornea is dense, resisting 

 and perfectly transparent. The muscles that move the globe of the eye are 

 attached to the sclerotic coat. 



Were it not for the prominence of the cornea, the eyeball would present 

 very nearly the form of a perfect sphere, as will be seen by the following 

 measurements of its various diameters ; but the prominence of its anterior 

 sixth gives the greatest diameter in the antero-posterior direction. 



The form and dimensions of the globe are subject to considerable varia- 

 tions after death, by evaporation of the humors, emptying of vessels, etc., 

 and there is no way in which the normal conditions can be restored. The 

 most exact measurements are those made by Sappey. As an illustration of 

 the post-mortem changes in the eye, Sappey has given comparative measure- 

 ments made three hours and twenty-four hours after death, the results of 

 which presented very considerable differences. 



In measurements made by Sappey, one to four hours after death, of the 

 eyes of twelve adult females and fourteen adult males, of different ages, the 

 following mean results were obtained : 



From these results it is seen that all the diameters are less in the female 

 than in the male. The antero-posterior diameter is the greatest of all, and 

 the vertical diameter is the shortest. The measurements at different ages, 



