TEARS. VISION. 159 



the lachrymal ducts, which are destined to receive the tears 

 after they have bathed the eye, and of which the orifices or 

 lachrymal points are seen near the internal commissure of the 

 lids; fourthly, the lachrymal sac, in which the lachrymal 

 ducts terminate, and which empties the tears into the nasal 

 canal. 



The tears by running over the surface of the conjunctiva 

 render it supple and facilitate the movements of the globe 

 and eyelids by lessening the friction. They serve the same 

 purpose in the eye as the synovia does in the articulations. 

 The influence of moral or physical causes increases their 

 secretion, and the lachrymal ducts do not suffice to carry 

 them off when they run over the lids. 



Vision. Among the phenomena, all of which constitute 

 the sight, some belong to the domain of physics, and may 

 be submitted to investigation, many may even be demon- 

 strated by experiment; while others, on the contrary, are 

 patent to the observation, but little known as to their cause 

 or their mechanism, and await from the progress of science 

 an- explanation which physiology has as yet been unable to 

 give. Even in those phenomena which at first seem purely 

 physical, we must not forget that the refracting media of the 

 eye are organized, and cannot be compared except by 

 approximation to inorganic bodies, on the form and density 

 of which physicists base their calculations. This is necessarily 

 the cause of the differences in the theories promulgated in 

 regard to vision; for although the eye may in some respects 

 be considered as an optical instrument, we can never arrive 

 at exact deductions by comparing organs analogous or even 

 similar in their construction, but different in their nature. 



Physicists claim as belonging to their proper province the 

 visual phenomena produced between the cornea and retina; 

 everything on the other side of that membrane belongs to 

 physiology. 



The retina renders the eye sensible of light, and we may 

 therefore consider it as the essential organ of vision. The 

 function of the other portions is to convey the luminous rays 

 to its surface under conditions necessary to a nervous im- 

 pression, which all combine to insure, but which is accom- 



