. A text-book of animal physiology, with introductory chapters on general biology and a full treatment of reproduction ... Physiology, Comparative. 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
. A text-book of animal physiology, with introductory chapters on general biology and a full treatment of reproduction ... Physiology, Comparative. 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 through the tubes, kept open by muscu- 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 Fig. -Lachrymal canals, irymal sac, and nasal canal, opened from the front (after Sappey). lachr Special Considerations. Comparative.—It seems to be established that certain animals devoid of eyes, as certain
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