186 



ESSENTIALS OP ZOOLOGY 



name, blind spot, has been applied to this area. The eyeball is divided 

 into two compartments, the aqueous chamber in front of the lens and 

 the vitreous chamber behind it. The aqueous humor filling the outer 

 chamber is of a watery consistency while the clear vitreous humor 

 filling the inner chamber is jellylike. The light, in reaching the retina 

 of the rat (or human being), would enter the eye through the cornea 

 and pass through the aqueous humor, pupil, lens, and vitreous humor 

 in that order. 



The eye is capable of perceiving several features of light such as 

 intensity, distance, color, etc. There are several theories of visual 

 perception, but those most commonly accepted are based upon chemi- 

 cal changes in the rods and cones of the retina. Such an explanation 



Con/uncf/Vcr 



Poster/or. 

 chamber 



■ScJerofic layer 



Choroid layer 

 Retinal layer 



Pigmerrh 

 layer 



Fovea 

 centralis 



Optic 

 nen^e 



Anterior 

 chamber 



Eyelid 



Rectus 

 muscle 



Fig. 55. — ^Diagram of a section through the eyeball. (From Wolcott, Ln\r)xal 



Biology, McGraw-Hill Book Company, inc.) 



considers the presence of a phototropic substance which is called 

 visual purple, as an essential factor. The rods and cones seem to take 

 different roles in dim light and white light, but these differences are 

 interpreted as varying amounts of visual purple in them. The 

 theories of color vision utilize the presence of this chemical also. 

 According to this idea, color blindness would be due to deficiency 

 of visual purple or a peculiar distribution of it. 



Sense of Hearing. — The ear, which is the organ of hearing, is com- 

 posed of external ear, middle ear, and internal ear. The first is com- 

 posed of the pinna, which is funnellike, and the audit oj-y meatus, or 



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