302 STUDIES IN PHYSIOLOGY 



that soon this opening decreases in size, a result which 

 is brought about by the contraction of the circular in- 

 voluntary muscles found in the iris. 



The most important of the three coats of the eye is the 

 inner, which is known as the ret'i-na; for it is this layer 

 which is acted upon by the rays of light that enter the eye. 

 The retina is only about one eightieth (-g^) of an inch in 

 thickness, and yet in microscopical sections one can make 

 out ten distinct layers. We shall not, therefore, attempt 

 to give a minute description of this complex coat of the eye. 

 We need only say that the optic nerve passes into each eye- 

 ball through the sclerotic and choroid coats (Fig. 137), that 

 its five hundred thousand or more nerve fibers run over the 

 inner surface of the retina, and that each nerve fiber finally 

 comes into communication with certain sensory cells lying 

 just inside of the choroid coat ; the cells are known as the 

 rods and cones. 



The Lens of the Eye. Behind the iris is the crys'tal-Une 

 lens, a beautiful transparent object, both surfaces of which 

 are convex. It is highly elastic, and is attached to the 

 cil'i-a-ry muscles of the choroid coat by a suspensory liga- 

 ment. In a later section we shall see the wonderful ad- 

 justments of this crystalline lens in our every act of seeing. 



The Humors of the Eye. Between the back surface of 

 the cornea and the front surface of the iris and the crystalline 

 lens is a small cavity, which is filled with a watery liquid, 

 the a'que-ous hu'mor (Latin aquosus = watery -f- humor = 

 fluid). Behind the crystalline lens is another cavity of con- 

 siderable size. This is distended by the vit're-ous hu'mor 

 (Latin vitreus = glassy), which is perfectly transparent and 

 resembles jelly in its consistency. 



The Eye as a Camera. Any one who is at all familiar 

 with a camera knows that by means of a lens, or a combi- 

 nation of lenses, the scene to be photographed is made to 

 appear upside down on the ground glass plate at the back 

 of the camera. If the image is not clear, it is brought into 



