770 SPECIAL SENSES. 



of the Brain, printed by Charles Bell, in 1811. A few years later, Magendie, in operating 

 for cataract, passed the needle to the bottom of the eye and irritated the retina, in two 

 persons. The patients experienced no pain but merely an impression of flashes of light. 

 The insensibility of the optic nerves has also been repeatedly noted in surgical operations 

 in which the nerves have been exposed. If a current of galvanism be passed through the 

 optic nerves, a sensation of light is experienced. The 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 cer- 

 tain muscles. 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 surface. 



The eyeball is made up of several coats enclosing certain refracting media. The exter- 

 nal 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 variations 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 men- 

 tions comparative measurements made three hours and twenty -four hours after death, the 

 results of which presented very considerable differences. 



In measurements made by Sappey, apparently with great care and accuracy, from 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, 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 -fa 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 connective tissue. 



