SUMMARY OF THE ANATOMY OF THE GLOBE OF THE EYE. 783 



explanation of Fig. 250, which represents a section of the human eye and shows the 

 relations of its various coats, humors, etc. 



The eyeball is nearly spherical in its posterior five-sixths, its anterior sixth being 

 formed of the segment of a smaller sphere, which is slightly projecting. In its posterior 

 five-sixths, it presents the following coats, indicated in the figure : 



S. The sclerotic ; a dense, fibrous membrane, chiefly for the protection of the more 

 delicate structures of the globe, and giving attachment to the muscles which move the 

 eyeball. Attached to the sclerotic, are the tendons of R, R, the recti muscles. 



...CKo. 



Cor. 



FIG. 250. Section of the human eye, copied from Helmholtz and slightly modified, 



Cor. The cornea ; a transparent structure, forming the anterior, projecting sixth of 

 the globe ; dense and resisting, allowing, however, the passage of light ; covered, on its 

 convex surface, with several layers of transparent epithelial cells, and, on its posterior 

 surface, with the membrane of the aqueous humor. 



Oho. The choroid coat, lining the sclerotic and extending only as far forward as the 

 cornea ; connected with the sclerotic by loose connective tissue, in which ramify blood- 

 vessels and nerves, and presenting an external, vascular layer and an internal, pigmentary 

 layer, which latter gives its characteristic dark-brown color. 



0. P., 0. P. The ciliary processes ; peculiar folds of the choroid, which form its ante- 

 rior border, and which embrace the folds of the suspensory ligament of the lens. 



0. M., C. M. The ciliary muscle, formerly called the ciliary ligament ; a muscular ring, 

 situated just outside of the ciliary processes, arising from the circular line of junction of 

 the sclerotic with the cornea, and passing over the ciliary processes, to become lost in 

 the fibrous tissue of the choroid. This is sometimes called the tensor of the choroid. Its 

 action is to tighten the choroid over the vitreous humor and to relax the ciliary processes 

 and the suspensory ligament of the lens, when the lens, by virtue of its elasticity, becomes 

 more convex. The ciliary muscle is the active agent in accommodation. In the figure, 

 the mechanism of accommodation is shown by the dotted lines, which represent the sus- 



