THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA 



401 



A high development of sense organs for the senses of sight, hear- 

 ing, smell, taste, and touch is characteristic of vertebrates. The organs 

 are receptors and they are stimulated by changes in external environ- 

 mental conditions, such as light, sound waves, chemical changes, and 

 contact. The eye, which is the organ of sight, is a highly developed 

 organ. It is constructed on the plan of a camera with the eyeball 

 forming the light-tight box. The wall of this is composed of an 

 outer fibrous sclera (white of eye) which continues anteriorly as a 

 transparent front, the cornea. Beneath the sclera is a black, pig- 

 mented and vascular layer, the chorioid, which continues anteriorly 

 as the iris, the colored part of the eye. The iris is like a curtain 



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crjo.-y\/. 



Fig. 221. — Diagram of the eyeball; c, cornea; a, aqueous humor; I, lens; v, 

 vitreous humor; sc, sclerotic coat; ch, chorioid coat; r, retina: /, fovea centralis; 

 i, iris; s.L, suspensory ligaments; c.p., ciliary process; cm., ciliary muscles; op.n., 

 optic nerve. (From Zoethout, Textbook of Physiology, published by The C. V. 

 Mosby Company.) 



surrounding a space at the anterior surface of the eye and this space 

 between its medial margins is the pupil. The pupil appears black 

 because there is no light behind it. Behind the pupil is a. transparent 

 lens whose surfaces are curved to bend the rays of light in such a 

 way as to focus them on the sheetlike retina behind. The retina is 

 a lateral extension of the brain and is the sensory part of the eye. 

 It lies as a lining of the inside of the posterior half of the cavity of 

 the eye and is connected directly with the brain by the optic nerve. 

 The general cavity of the eyeball is divided into some chambers. The 



