PHYSIOLOGICAL ANATOMY OF THE EYEBALL 649 



Their mechanical or electric stimulation produces luminous impres- 

 sions. 



PHYSIOLOGICAL ANATOMY OF THE EYEBALL 



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

 in the orbit, protected by the surrounding bony structures and the eye- 

 lids, its surface bathed with the secretion of the lachrymal gland and 

 movable in various directions by the action of certain muscles. It is 

 surrounded with a thin serous sac, the capsule of Tenon, which exists 

 in two layers. The outer layer lies next the fat in which the globe is em- 

 bedded and the inner layer invests the sclerotic coat. When the axis 

 of the eye is directed forward, 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 globe. 



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 pro- 

 tection of the inner coats and the contents of the globe. The cornea is 

 dense, resisting and transparent. The muscles that move the globe are 

 attached to the sclerotic coat. 



Were it not for the prominence of the cornea, the eyeball would pre- 

 sent 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. 



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 great- 

 est of all, and the vertical diameter is the shortest. The measurements 

 at different ages, not cited in the table just given, show that the excess 

 of the antero-posterior diameter over the others diminishes with age. 



