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 lacrimal 

 gland and secretes an oily substance, and there is no nasolacrimal duct. 



