SENSITIVE SYSTEM. 367 



triangular-shaped conduit is formed by their approximation, by 

 which the tears are conducted into the puiicta. The edge of the 

 lid, the boundary hne between the two margins, presents a series 

 of pinholes, which are the orifices of the ciliary glands : they 

 give issue to a sebaceous secretion that serves to prevent ad- 

 hesion of the lids during sleep, as well as keep the tears from 

 escaping as they run along the triangular canal. 



Eyelashes. 

 The cilia or eyelashes are the long, fine-pointed horse-hairs 

 growing from the anterior margins of the lids. They are gene- 

 rally black, unequal in their lengths, and are implanted in ranks, 

 two, three, and four deep. They are much longer, and their 

 ranks are more regular, in the upper than in the lower lid : in 

 the former they grow thin towards the upper canthus, and 

 ultimately become discontinued altogether; in the latter the 

 same thing happens as they approach the lower canthus. It 

 requires but little discrimination to perceive that this is the 

 most advantageous arrangement they could possibly have in 

 both of the lids : it being obvious, from the position of the head, 

 that light coming from above must dart upon the temple, while 

 that which is reflected from the ground will be directed to the 

 nasal canthus ; consequently, the cilia are suitably disposed to 

 intercept rays in either direction. 



Structure of the Eyelids. 



The eyelids in composition are cuticular, muscular, cartila- 

 ginous, and membranous ; also, it may be added, glandular, vas- 

 cular, and nervous. These several parts and textures are most 

 developed in the upper lid. 



The skin, covering and in part composing the lids, is thin, 

 fine, soft, and very extensible in its texture ; it becomes more 

 attenuated as it approaches the ciliary borders, around which, 

 to a broader extent in the lower than in the upper lid, it is desti- 

 tute of hair. This marginal portion (which is commonly black, 

 though in some horses pied) is pierced by the eyelashes, perforat- 

 ed by the ciliary holes, and turns in to join the conjunctive mem- 

 brane, from which it can only be distinguished along the line of 

 union by their respective colours. 



The orbicularis palpebrarum, consisting of a broad stratum of 

 fleshy fibres encircling the lids, and lying immediately under- 

 neath the skin, has been already described (at page 86). 



Underneath the muscle is found a dense, tou^h, Jibrous ex- 

 pansion ; and interposed between the two is a long, loose, cellular 

 tissue, which admits of very free motion between them. This 



