. The chordates. Chordata. Mammalia: Nervous System 687. Tarsal glands of lower lid Lacrimal sac and nasolacrimal duct Fig. 513. Dissection of the human left eyelids and lacrimal apparatus, viewed from behind. (Natural size.) (After Fox. Courtesy, Jackson: "Morris' Human Anatomy," Philadelphia, The Blakiston Company.) The watery "tear" fluid, usually produced mostly by the larger lacrimal gland, bathes the external surface of the eyeball, filling the conjunctival sac and prevented from flowing unduly beyond the confines of that space by the rim of oily substance deposited b
. The chordates. Chordata. Mammalia: Nervous System 687. Tarsal glands of lower lid Lacrimal sac and nasolacrimal duct Fig. 513. Dissection of the human left eyelids and lacrimal apparatus, viewed from behind. (Natural size.) (After Fox. Courtesy, Jackson: "Morris' Human Anatomy," Philadelphia, The Blakiston Company.) The watery "tear" fluid, usually produced mostly by the larger lacrimal gland, bathes the external surface of the eyeball, filling the conjunctival sac and prevented from flowing unduly beyond the confines of that space by the rim of oily substance deposited by the Meibomian glands along the edges of the lids. Any excess of the fluid drains away via the lacrimal sac and nasolacrimal duct into the nasal cavity. Movements of the lids ("blinking" in man) promotes flow of the fluid across the eyeball. Strong emotions, a vagrant puff of tobacco smoke, or proximity of raw onion may so augment the activity of the human lacrimal gland as to overtax the nasolacrimal drainage system, with the result that the fluid bursts through the Meibomian dam and floods the cheeks, and meanwhile the nasal cavity also is flooded. The glandular apparatus of the lids prevents drying of the delicate and more or less exposed conjunctival layer and provides lubrication which facilitates movements of the lids. The flow of fluid across the eyeball keeps its surface clean. The lacrimal secretion is probably the chief source of nutriment for the corneal conjunctiva, which is re- mote from the blood-vessels. The antiseptic tendency of the fluid is protective. There seems to be no good reason why a submarine mammal should produce salty tears. In fact, in all aquatic mammals, the eyelids and their glands exhibit a great variety of modifications—in the case of the glands, mostly by reduction. In general, the extent of reduction depends on the degree to which the animal is aquatic. In seals Meibo- mian glands are lacking, the Harderian is larger than the lac
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