344 



ADVANCED PHYSIOLOGY 



Beneath the sclerotic and cornea is the choroid coat; Fig. 

 167. It contains a large number of blood vessels and is the 

 principal nourishing layer of the eye. The choroid covers 

 the whole ball save a small spot exactly in front, the pupil. 

 Around the pupil is a colored area, blue, brown or black, as 

 the case may be, called the ins. The pigment area of the iris 

 is for the purpose of shutting out all light except an amount 

 sufficient to stimulate properly the nerves of sight. When the 

 light is dim, more must be admitted to produce the requisite 

 degree of stimulation; consequently the iris is provided with 

 muscles by whose contraction and relaxation the aperture of the 



pupil is made smaller 

 or larger. Certain other 

 muscles belonging to 

 the choroid coat will 

 be mentioned in a 

 later paragraph in con- 

 nection with the focus- 

 ing mechanism of the 

 eyes. 



The Retina. Inside 

 the choroid covering 

 is the retina, the only 

 layer of tissue in the 

 eye which is sensitive 

 to light. This does not go entirely about the eye, being 

 absent in front in the region of the iris and pupil. 



The Vitreous and Aqueous Humors. Filling up the center 

 of the eye, and making the whole organ spherical, is a mass of 

 clear, jelly-like material, the vitreous humor. In front in the 

 space between the iris and the cornea is also a mass of clear, 

 transparent material, the aqueous humor, which causes the 

 cornea to protrude slightly; Fig. 167. 



The Lens. The lens also is made of perfectly clear, trans- 

 parent material of the consistency of thick jelly. In shape it 



FIG. 167. DIAGRAM 

 * a section throi^h the eyeball 



