VISION. 597 
mal fluid, and prevents the latter running over their’ edges as 
oil would on the margins of a vessel. The lachrymal gland is 
not in structure unlike the parotid, the secretion of which its 
own somewhat resembles. 
The saltness of the tears, owing to abundance of sodium 
chloride, is well known to all. The nervous mechanism of se- 
cretion of tears is usually reflex, the stimulus coming from the 
action of the air against the eyeball or from partial desiccation 
owing to evaporation. When the eyeball itself, or the nose, is 
irritated, the afferent nerves are the branches of the fifth, to 
which also belong the efferent nerves. The latter include also 
the cervical sympathetic. But it will, of course, be understood 
that the afferent impulses may be derived through a large num- 
ber of nerves, and that the secreting cen- 
ter may be acted upon directly by the 
cerebrum (emotions). The excess of lach- 
rymal secretion is.carried away by the 
nasal duct into which the lachrymal 
canals empty. While it is well known 
that closure of the lids by the orbicularis 
muscle favors the removal of the fluid, 
the method by which the latter is ac- 
complished is not agreed upon. Some 
believe that the closure of the lids forces 
the fluid on through the tubes, when 
they suck in a fresh quantity ; others that : 
the orbicularis drives the fluid directly 9,8 harhigmal canals, 
through the tubes, kept open by muscu- ganaly ee 
lar arrangements; and there are several 
other divergent opinions. The prevention of winking leads to 
irritation of the eye, which may assume a serious character, so 
that the obvious use of the secretion of tears is to keep the eye 
both moist and clean. 
SPECIAL CONSIDERATIONS. 
Comparative.—It seems to be established that certain animals 
devoid of eyes, as certain myriopods, are able to perceive the 
presence of light, even when the heat-rays are cut off. The most 
rudimentary beginning of a visual apparatus appears to be a 
mass of pigment with a nerve attached, as in certain worms; 
though it is questionable whether mere collections of pigment 
without nerves may not in some instances represent still earlier 
