DISEASES OF THE EYE. 401 



to a lesser degree, even in affections arising from external causes, one 

 eye generally sympathises to a certain extent with the other. 



832. Arteries of the Eye. 



The internal structures of the eye are supplied with blood by the oph- 

 thalmic artery, which enters by a foramen at the back of the globe. 



From the fact that the internal structures of the eye are wholly sup- 

 plied by a deep-seated artery entering at its posterior part, it is plain that 

 local bleeding can have little or no effect in specific ophthalmia and other 

 diseases of the internal structures. 



833. Appendages of the Eye. 



The principal appendages of the eye are the eyelids ; the lachrymal 

 glands, which secrete fluid for its lubrication ; the lachrymal ducts, which 

 carry off these secretions ; the muscles which move the eye and its various 

 appendages ; the nerves, which affect those muscles ; a mass or cushion of 

 adipose membrane or fat, on which the globe of the eye rests ; and lastly, 

 the membrana nictitans or haw. 



834. The Eyelids. 



The Eyelids form a sort of curtain, by means of which the horse closes 

 at will the anterior aperture of the orbit. They are made up of skin, 

 muscular fibre, cartilage, mucous membrane, and glands. The corners of 

 the eyelids, where they meet, are termed canthi. 



The skin of the eyelid is peculiarly thin and delicate. The interior is 

 lined with conjunctival membrane. Its glands, termed the meibomian, 

 are very numerous, and secrete an oily material which prevents the lids 

 from adhering to each other or to the cornea. The size of the aperture 

 between the eyelids varies in different horses. In some a greater, in 

 others a less portion of the globe is exposed to view. 



835. The Eye-lashes. 



The Eye-lashes grow from the anterior margins of the lids. A distinct 

 branch of the nerve is supplied to each bulb, from which a hair grows. 

 Hence the eye-lashes are exceedingly sensitive. 



836. The Tears. 



The Lachrymal glands are situated in a depression in the outer and 

 upper angle of the orbit. Under certain circumstances, such as the irri- 

 tation of foreign bodies, various deep-seated inflammatory affections, or 

 recent inflammation of the conjunctival membrane or of any of the ex- 

 ternal tunics, the glands are excited to increased action, and the surface 

 of the eye is washed by a more than usually copious amount of saline 

 watery secretion. When the ordinary channel is unable to carry off this 

 secretion fast enough, it is poured over the lower lid and is then recog- 

 nised as tears. 



9ft 



