746 OPERATIONS UPON THE EYE AND EAE. 



Neither of these operations is often followed by successful re- 

 sults, although the perforation of the bone enables the tears still 

 to escape in the nasal cavities. The formation of a fistula fur- 

 nishes a channel for the flow of the tears over the lachrymal sur- 

 face of the face. 



3d. Obliteration of the Natural Tract. — The intention of this 

 operation is to effect the entire obliteration of the duct from the 

 lachrymal puncta and the lachrymal duct down. It is obtained by 

 the cauterization of these parts, either alone or inclusive of the 

 obhteration of the lachrymal sac. Tincture of iodine often pro- 

 duces the same effect. As the result of this treatment, the flow 

 of the tears takes place over the face. 



4th. Extirpation of the Lachrymal Gland. — This is not re- 

 ferred to as a practicable measure, but only because it is some- 

 times mentioned in the way of theorizing. The situation of the 

 gland in our domestic animals renders the operation an impossi- 

 bility. 



III. — Operations on the Essential Organs of Si&ht. 



On the Globe. — The essential organ of vision, or ocular globe, 

 is a membranous ball, completely closed, and filled with transpar- 

 ent fluids of different densities, and popularly known as the humors 

 (or media) of the eye. 



This baU, nearly spherical, flattened from backwards in front, 

 has its greater convexity in front, where it is closed by the cornea, 

 a transparent expansion, thick and resisting ; the glass of the eye. 

 Posteriorly, it is composed of three capsular, concentrical sheaths, 

 proceeding from without inward. These are the fibrous sclerotic, 

 the choroid and the retina (Fig. 538). 



In the cavity of the globe one of these membranes — the cho- 

 roid — throws out, perpendicularly to the great axis of the organ, 

 a septum, the iris, a kind of contractile diaphragm, perforated in 

 its center by the pupil. 



The retina is a membrane of special nature, being an expan- 

 sion of the optic nerve, and performs the function of receiving 

 the impressions of light, and transmitting its impressions and 

 images to the brain. 



The humors of the eye are three, considered from before back- 

 ward, the most anterior being the aqueous, the most posterior the 

 vitreous, with the crystalline lens in the intermediate position. 



