204 



ELEMENTARY PHYSIOLOGY 



with it any particles of dust which may have adhered to the eyeball. 

 The lachrymal fluid now enters the nasal or lachrymal duct by 

 means of two small lachrymal canals, and is then conveyed into 

 the nose. 



When there is a redundant secretion of the lachrymal fluid, such 

 as may arise from irritation of the conjunctiva by powerful vapours 

 &c., or from some strong mental emotion, the nasal duct cannot 



Fig. 1 88. Lachrymal Apparatus. 



i, lachrymal gland ; 2, nasal duct ; 3, the muscle which raises the upper lid. 



convey away the fluid as rapidly as it is sscreted. The tears then 

 accumulate between the eyelids and the eyeball, and at last roll in 

 drops over the cheeks. 



In addition to the muscles mentioned above, the ball of tt 

 eye is protected and supported by a large quantity of loose fatty 

 and connective tissue, which, acting like a cushion or pad, serves 

 to deaden the effect of pressure or blows. 



The general form of the eyeball is globular, but the front 

 portion projects somewhat, being a portion of a smaller sphere. I 

 measures about one inch in diameter. 



The eyeball has three distinct coats. The external consis 

 of the sclerotic coat and the cornea, the middle coat is formed b> 



