Nervous System and Sense Organs 217 



diaphragm is the pupil. The iris is formed of two sets of muscles, one 

 dilates, the other contracts ; by means of their interaction, the size of 

 the pupil and hence the amount of light entering the eyeball proper 

 is accurately controlled. The pupillary reflex is thus important in vision. 



Just behind the iris, the ciliary body is located. This body is formed 

 chiefly of muscle fibers arranged in a series of tiny ridges. It is at- 

 tached to the lens by a series of ligaments. When these muscles con- 

 tract, the tension on the lens is reduced and the elasticity of the lens 

 allows it to bulge, bringing near objects into focus. The relaxation of 

 these muscles makes the lens flatter, bringing distant objects into focus. 

 This adjustment by the ciliary body is known as accommodation. 



The crystalline lens is composed of elongated epithelial cells which 

 are formed into ribbonlike fibers. It has no blood vessels, and is en- 

 tirely dependent for nourishment upon the fluids that bathe it. As 

 just noted, its shape is altered by the action of the ciliary body. 



The innermost coat of the eye is the retina which contains the light 

 receptors. This coat lines the entire inner surface of the eyeball except 

 for the anterior portion where its stops near the lens attachment. Two 

 layers form the retina : one is an outer pigmented layer, the other 

 is an inner sensitive region formed of sensory cells, nerves, and asso- 

 ciated structures. 



There are three chambers within the eyeball. The anteriormost 

 of these is that between the cornea and the iris, the anterior chamber. 

 Between the iris and the ciliary body is the posterior chamber. Both 

 of these are filled with a thin liquid, the aqueous humor. Behind the 

 lens is the large chamber of the vitreous humor filled with a viscous 

 fluid bearing the same name. 



Vision involves a number of factors : color, depth, distance, form, 

 and the perception of light. This requires the coordination of the 

 action of these many different structures which have just been described. 

 The actual perception, however, is done by the light-sensitive receptors 

 of the retina. These are of two types : the rods and cones. The rods 

 have been shown to contain visual purple, a reddish pigment which 

 bleaches when exposed to light. This pigment is constantly regenerated, 

 but its regeneration is dependent upon the presence of vitamin A in the 

 diet. The rods are concerned with black and white vision and number 

 about 1 30,000 jOOO in the human retina. The cones, which are concerned 

 with color vision, number about 6,500,000 and are not distributed so 

 generally over the retina. In fact, they tend to be scarce except at one 

 point, the jovca centralis, a depressed region toward the back of the 



