418 SPECIAL PHYSIOLOGY. 



up of a cluster of follicles, covered with mucous membrane, and stud- 

 ded with a few very fine hairs. Between the caruncle and the eyeball, 

 is a thin semilunar fold of the mucous membrane, the concavity of 

 which is turned towards the eye (Fig. 79) ; this is the rudiment of the 

 membrana nictitans (from nicto, to wink), or haw, of the horse and 

 mammalia generally, and of the third eyelid of birds. In front of the 

 edge of this membrane, on the margin of each eyelid, are two little 

 conical eminences, named the lachrymal papilla* ; in the apex of each 

 of these is a small aperture, the lachrymal punctum. These puncta 

 are the commencement of the lachrymal canals, which carry away the 

 secretion of the lachrymal glands. 



The lachrymal gland in each orbit, is a small almond-shaped body, 

 Fig. 79, 1, Fig. 80, y, situated in a slight depression at the upper and 

 outer part of the orbit, between it and the eyeball; it reaches forwards 

 to the upper eyelid, with which a portion of it is connected. This is 

 a compound racemose gland, and its ducts, from six to twelve in num- 

 ber, open on the inner surface of the upper eyelid, just above the 

 outer canthus. The tears are a clear, saline, alkaline fluid, and con- 

 tain a minute quantity of albuminoid matter; their total solid constit- 

 uents amount to only 1 per cent. 



The lachrymal canals, or canaliculi, Fig. 79, commence, as already 

 stated, at the lachrymal puncta ; they are two short tubes, placed be- 

 neath the skin, one above, and one below the lachrymal lake ; the 

 superior canal, which is smaller and longer than the lower one, passes 

 upwards and then inwards, the inferior downwards and then inwards, 

 and both terminate in a large membranous bag, called the lachrymal 

 sac. 



The lachrymal sac, 2, is lodged in the deep lachrymal groove, formed 

 in the inner wall of the orbit; it terminates below, in a narrower tube, 

 the nasal duct, which extends to the inferior meatus of the nose. The 

 lower end of the nasal duct is somewhat expanded, and is often par- 

 tially closed by a membranous fold or valve. 



The lachrymal sac and canals consist of fibrous and elastic walls, 

 lined internally by mucous membrane. The epithelium of the mucous 

 membrane, in the canals and upper part of the sac, is laminated, squa- 

 mous, and destitute of cilia; in the lower part of the sac. and in the 

 nasal duct, it is ciliated. This membrane is continuous above, through 

 the canalicuii, with the conjunctiva, and below, through the nasal duct, 

 with the pituitary membrane lining the nose. 



The eyebrows, by their elevation arid depression, influence the 

 amount of light reaching the eyes ; they also serve slightly to protect 

 these organs from foreign bodies, and from the perspiration running 

 down the forehead. 



The eyelids, eyelashes, and lachrymal apparatus, are parts admira- 

 bly adapted for the preservation and protection of the eyes. By means 

 of the two former, the entry of foreign bodies floating about in the 

 air, is prevented, and the eyes are protected from excessive light. 

 The eyelids, besides, play a most important part in cleansing and 

 moistening the surface of the eyeball. In the act of winking, which 

 may be voluntary, but is usually reflex, and consists merely in the 



