VISION*. 25 1 



or luminous lines; also another pattern, which appears to 

 be the network of the choroidal capillaries; and sometimes 

 patches of small points closely set together, which have exactly 

 the appearance of the extremities of the rods and cones of the 

 retina itself. These experiments may be carried to the extent 

 of giving pain, arid are certainly bad for the eyes. 



The retinal vessels, however, can be seen in a less unplea- 

 sant way, by holding a light a few inches from the side of the 

 eye in a dark room, and gazing forwards into the darkness 

 while the light is gently moved. In. a little while the field 

 of vision becomes yellowish, and dark lines are seen ramifying 

 in it in the position of the branches of the retinal artery. 

 These are what are termed figures of Purkinje, and are 

 occasioned by the vessels intervening between the light and 

 the rods and cones behind them. The blood corpuscles, as 

 they traverse the anterior layers of the retina, can also be 

 seen as luminous spots, when, one gazes intently into a clear 

 sky. 



187. Lachrymal apparatus. The secretion of tears is 

 primarily useful for keeping the surface of the cornea clear, 

 and, in connection with this object, there are channels pro- 

 vided by which they may be removed without unduly accu- 

 mulating or rolling over the cheeks. But the flow of tears is 

 likewise influenced by the emotions; and any one who is 

 familiar with the important effects Avhich may be wrought 

 by the action of one or two leeches, will be slow to doubt 

 that a copious flood of tears may be of great utility in reliev- 

 ing a congested condition of the brain. 



The lachrymal gland is similar in structure to the salivary 

 glands, is about the size of an almond, is situated in the 

 upper and outer angle of the orbit, under cover of the upper 

 eyelid, and pours out its secretion by several ducts. 



The eyelids are closely associated with the lachrymal gland 

 in function. The upper lid is the more important of the 

 two; it is larger than the lower, has twice as many eyelashes, 

 has a special muscle for raising it, and is stiffened by a cres- 

 centic plate of thin cartilage (superior tarsal cartilage) 

 beneath its mucous membrane, while the lower lid has only 

 a linear strip of that substance inside its margin. The 

 upper and lower tarsal cartilages are united at the inner 



