238 HUMAN PHYSIOLOGY. 



The Sclerotic and Cornea. The sclerotic is the opaque fibrous 

 membrane covering the posterior five sixths of the ball. It is com- 

 posed of connective tissue arranged in layers, which run both trans- 

 versely and longitudinally ; it is pierced posteriorly by the optic nerve 

 about T Vof an inch internal to the optic axis. The sclerotic, by its 

 density, gives form to the eye and protects the delicate structures 

 within it, and serves for the attachment of the muscles by which the 

 ball is moved. 



The cornea is a transparent non-vascular membrane covering the 

 anterior one sixth of the eyeball. It is nearly circular in shape and is 

 continuous at the circumference with the sclerotic, from which it 

 can not be separated. The substance of the cornea is made up of 

 thin layers of delicate, transparent fibrils of connective tissue, more 

 or less united ; between these layers are found a number of inter- 

 communicating lymph-spaces, lined by endothelium, which are in 

 connection with lymphatics. Leukocytes or lymph-corpuscles are 

 often found in these spaces. The anterior surface of the cornea 

 is covered by several layers of nucleated epithelium, which rest upon 

 a structureless membrane known as the anterior elastic lamina. The 

 posterior surface is covered by a similar membrane, the membrane 

 of Descemet, which at its periphery becomes continuous with the iris ; 

 it is also covered by a layer of epithelial cells. At the junction of 

 the cornea and sclerotic is found a circular groove, the canal of 

 Schlemm. 



The choroid, the iris, the ciliary muscle, and the ciliary processes 

 together constitute the second or middle coat of the eyeball. 



The choroid is a dark brown membrane which extends forward 

 nearly to the cornea, where it terminates in a series of folds, the 

 ciliary processes. In its structure the choroid is highly vascular, 

 consisting of both arteries and veins. Externally it is connected with 

 the sclerotic by connective tissue ; internally it is lined by a layer of 

 hexagonal pigment cells, which, though usually classed as belonging 

 to the choroid, is now known to belong, embryologically and physio- 

 logically, to the retina. From without inward may be distinguished 

 the following layers : 



1. The lamina suprachoroidea. 



2. The elastic layer of Sattler, consisting of two endothelial layers. 



3. The chorio-capillaris, choroid proper, or membrane of Ruysch a 

 thick, elastic network of arterioles and capillaries lying within 

 the outer layer of veins and arteries called the venae vorticosse. 



