546 STUDIES IN ADVANCED PHYSIOLOGY. 



portion of it visible from the outside. It is not evenly 

 spherical, but consists of segments of two spheres, the 

 larger sphere composing most of the eyeball, the smaller 

 sphere forming the cornea. 



The globe is a completely closed ball pierced only at one 

 end, where the optic nerve and the blood-vessels for the 

 retina enter it. The walls are, of course, provided with 

 blood-vessels and nerves which enter it, but consist almost 

 wholly of very tough connective tissue so strong that in the 

 case of a beef's eye one may be able to stand on an eye 

 without breaking these coats. 



In a general way these coats are described as consisting 

 of three, called the outer or sclerotic coat, the middle or 

 choroid coat, and the inner coat or the retina. 



Enclosed within these coats are three refracting media; 

 the aqueous humor, the crystalline lens, and the vitreous 

 humor. 



6. Sclerotic Coat. The sclerotic coat is the outer coat 

 and is by far the strongest and toughest. It consists almost 

 entirely of closely packed white connective tissue. This 

 coat is visible from the outside as the white of the eye. 

 Immediately in front of the eyeball this coat is transparent 

 and is called the cornea. The sphericity of the cornea is 

 greater than that of the eyeball itself. 



The sclerotic coat covers the entire eye save at the back 

 where the optic nerve and the retinal blood-vessels pass 

 through it. The cornea is covered over in front with the 

 conjunctiva, and on its inner side is lined with a layer of epi- 

 thelial cells called the membrane of Descemet. 



7. The Choroid Coat. Immediately beneath the scle- 

 rotic coat is the choroid coat. This is not nearly so tough 

 as the sclerotic, but is much more vascular and readily 

 recognizable in the dissection of the eye by the black pig- 

 ment which it contains. It is, in fact, the choroid coat to 

 which the black inner walls of the eyeball are due, and the 

 presence of this pigment in the choroid explains largely its 



