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TEXTBOOK OF ZOOLOGY 



external or aqueous chamber between the cornea and the lens, with 

 the iris extending into it, is filled with aqueous humor. This cavity 

 is subdivided by the iris. Behind the lens is the large internal or 

 vitreous chamber which is filled with a jellylike vitreous humor. The 

 curvature of the lens can be controlled by the action of the ciliary 

 muscle which encircles its margins. This makes possible an adjust- 

 ment of the eye to near and far objects and particularly so in higher 

 vertebrates. This power is known as accommodation. As people 

 get older they tend to lose this accommodation because of loss of 

 elasticity in the lens. The tension on it due to the attachment of 

 the inside of the eyeball by the ciliary process tends to hold it out 



Fig. 222. — Diagram of a section through the right ear. B, semicircular canal ; 

 a, external auditory meatus; o, oval window (fenestra ovale) ; P, tympanic cavity 

 containing the three auditory ossicles ; Pt., scala tympani ; r, round window 

 (fenestra rotunda) ; below r is seen the Eustachian tube; 8, cochlea; T, membrana 

 tympani; Yt, scala vestibuli. (From Zoethout, Textbook of Physiology, published 

 by The C. V. Mosby Company, after Czermak.) 



in a flattened condition. This focuses the eyes very well on distant 

 objects but does not provide the necessary curvature of the lens 

 to bring near objects in focus. Eyeglasses are used by older people 

 to supply this lost phase of accommodation. A ray of light enters 

 the eye by passing through the cornea, then aqueous humor, pupil, 

 lens, vitreous humor and then to the retina where the sensory cells 

 are stimulated and the impulse carried to the brain by the nerve 

 fibers of the optic nerve. 



