SENSE ORGANS AND SENSATIONS 243 



covering of the eye. Over a small area in front this coat is 

 transparent, and this part of it is known as the cornea. In- 

 side the sclerotic is the middle coat, or choroid, richly sup- 

 plied with blood vessels and containing in its connective 

 tissue large quantities of black pigment, which prevents the 

 passage of light into the eyeball except through the cornea. 

 The choroid lines the sclerotic everywhere except in front, 

 where in the region of the cornea it leaves the sclerotic and 

 projects toward the long axis of the eye as a kind of cur- 

 tain, the iris that part of the eye which is black or gray 

 or blue. The pupil is the dark round opening, or hole, in 

 the iris. Immediately inside the choroid is the third and 

 innermost coat, the retina. This is a thin membrane, not 

 more than one eightieth of an inch in thickness, and lining 

 the chamber of the vitreous humor as far forward as the 

 ciliary region (Fig. 93). The retina is the part of the eye 

 sensitive to the stimulation of light. Here also begin the 

 fibers of the optic nerve, which passes through and perforates 

 the choroid and sclerotic coats behind on its way from the 

 retina to the brain. These and other parts of the eye may 

 be easily seen by dissecting the eye of an ox or sheep. 



7. The lens and the muscle of accommodation. Immedi- 

 ately behind the pupil is the lens, a biconvex, transparent, 

 compressible, and elastic body fastened by a circular liga- 

 mentous sheet to the choroid coat immediately above and 

 behind the iris. The lens and its suspensory ligamentous 

 sheet thus divide the eye into two distinct chambers: the 

 one, in front of the lens and behind the cornea, filled with 

 a watery fluid, the aqueous humor; the other, behind the 

 lens and surrounded by the retina, filled with a jellylike, 

 transparent substance, the vitreous humor (Figs. 93, 96). 



The elastic choroid coat is not long enough to reach 

 around and inclose the vitreous humor without stretching, 

 and hence it constantly exerts a steady, elastic pull or ten- 

 sion on the ligament of the lens. This tension flattens the 



