ORGANS OF PERCEPTION. 179 
sists of water, mucus, muriat of soda, soda, phosphat of 
lime, and phosphat of soda. Its taste is perceptibly saltish, 
although the saline ingredients do not exceed a hundredth 
part of the whole. The use of the tears is to lubricate the 
surface of the eye-ball, and protect it from dust and the 
drying influence of the air. They are not secreted in 
those animals which live in water. When the eye-lids 
close, the tears are pressed towards the internal angle of 
the eye, from whence they are conveyed to the nose through 
the lachrymal sac. The opening into this sac is either by a 
single fissure, or by two small pores, the mouths of two 
canals which are situated at the nasal angle of the eye, and 
termed puncta lachrymalia. 
The Meibomian Glands are situated in the substance of 
both eye-lids. They consist of small follicles, arranged in 
vertical lines, terminating in small round holes on the edge 
of each eye-lid. The albuminous substance which they 
secrete, covers the margins of the eye-lids, and while it pre- 
vents the tears from flowing out, preserves the tarsi from 
adhering to each other. 
The Caruncula Lachrymalis is situated at the inter- 
nal angle of the eye-lids. It is a roundish reddish mass, 
secreting a thick whitish humour, which is supposed to pro- 
tect the lachrymal pores.. The Harderian gland, which 
occurs nearly in a similar situation, and secretes a fluid 
somewhat similar in its properties, although not found m 
man, is observable in many quadrupeds, and in birds. 
6. Coverings of the Eye—The external coverings of 
the eye are all derived from the common integuments, more 
or less altered in their texture. They may be regarded as 
three in number, and described under the names, Eye-lids, 
Nictitating Membrane, and Conjunctiva. 
The Eye-lids, when present, are generally two in num- 
ber, the one protecting the upper half of the eye, and term- 
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