THE REASON WHY. 235 



'And tl.e eye cannot say unto the hand, I have no need of thee; nor again the 

 head tc the feet, I have no need of you." COEINTH. xii. 



winking', or brightening the eye, is so instantaneous that it does not 

 impede the sight. 



976. Whence are the humours and secretions of the ey 

 derived ? 



From the blood, which flows abundantly to the eyes, and is 

 circulated in capillary vessels that are spread out upon the 

 membranous coats of the eye-balls. 



Fig. 63. SECTION OF THE EYE. 



A and B. Tne sclerotic, choroid, and retina, the three layers or coats which 

 form the walls of the glohe of the eye, and enclose its humours. 

 C C. The iris. 



D. The front chamber of the eye, filled with watery humour . 



E. The pupil, through which the rays of light pass to 



F. The crystalline lens. 



G G. The vitreous humour enclosed in cells formed by the hyaloid membrane. 

 H. An artery which supplies blood to the crystalline lens, and which passea 

 through the centre of the optic nerve. 



G. The optic nerve, showing the sheath in which the nerve is enclosed. 



977. Why do tears form in the eyes ? 



Because, under the emotions of the mind, the circulation of blood 

 in the brain, and in its nearest branches, becomes considerably 

 quickened. The eyes receive a larger amount of blood, and the 

 eecretion of the lachrymal glands being increased, the fluid 

 overflows, and tears are formed. The use of tears is probably to 

 keep the eyes cool during the excitement of the brain. They ar 

 formed also during lyughing, but less frequently. 



