THE EYE AND THE PHYSIOLOGY OF VISION. 547 



function. All good optical instruments are painted black 

 inside, so as to do away with the possibility of reflection 

 from the walls, which would materially interfere with the 

 clearness of the image. The cornea is not in contact with 

 the choroid, but at this point the sclerotic coat and the 

 choroid are separated and so a space produced which is 

 filled with a watery liquid called the aqueous humor. The 

 pigment of the choroid is also different at this place, and in- 

 stead of being black is brown or blue, or whatever other 

 color we find the eyes to have. This colored portion of the 

 choroid coat seen through the cornea is called the iris. 



Just in the middle line of the eyeball the iris has an 

 opening known as the pupil, and visible on the eye as a 

 dark disk within the iris. This dark disk is really a hole, 

 but seems dark because no light conies out of it, just as an 

 opening into a mine would seem dark if there were no light 

 beneath. 



Within the choroid coat lies the retina. The retina is 

 but an expansion of the optic nerve and does not run en- 

 tirely around the eye. The impossibility of throwing an im- 

 age on the forward portions of the inner wall of the eye 

 would render useless the extension of the retina so far. 



Filling the large posterior chamber is the vitreous 

 humor, a transparent glassy-looking liquid having about 

 the consistency of the white of an egg. This vitreous 

 humor is enclosed in a delicate membrane extending all 

 around it, known as the hyaloid membrane. This mem- 

 brane seems a single layer in most portions, but towards 

 the front it divides into two layers, and in the space so 

 formed there is imbedded the rather large crystalline lens. 



As the eyeball is distended even to a slight pressure 

 with the vitreous humor, all the coats are held in place and 

 a collapse of the eyeball is prevented. In fact, the pressure 

 of the vitreous humor in the hyaloid membrane is normally 

 such as to stretch this somewhat, and as the lens is im- 

 bedded between the layers of this membrane it is by the 

 stretching of this hyaloid membrane always in a state of 



