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 eacli 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- 

 il angle of the eye-lids. It is a roundish reddish mass, 

 ;reting 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 in 

 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 eve, and term 



2 M 



