I20 THE SURGICAL ANATOMY OF THE HORSE 



distant from the orbit, and rice i-ersa should the liquid escape by the 

 nasal opening. 



Injections will usually prove sufficient to relieve the obstruction, 

 but, tailing these, the duct should be probed with a fine bouge, the 

 thickness of which should not exceed that of a goose-quill. 



An inflammatory swelling of the wall of the duct may cause an 

 obstruction of the lumen, in which case an astringent injection should 

 be utilised. Failing relief by injection, an artificial opening may be 

 made into the duct by trephining the superior maxillary bone along the 

 line represented in Plate XXV., which indicates the course of the duct. 



The Eyeball 



Upon looking at the exposed portion of the eyeball in situ, we see 

 that the cornea is somewhat elliptical. Close observation, however, 

 will reveal the fact that it is not regularly so, since the inner end of 

 the ellipse would form an arc of a greater circle than would the 

 outer end. Surrounding the cornea is the visible white zone of 

 the sclerrjtic. 



Looking through the cornea in a dim light, where the direct rays 

 are cut off by shading the eye, the pupil is seen to be also elliptical, 

 with its long axis directed transversely as in the case of the cornea. 

 Surrounding the pupil is the iris, and hanging into the pupil are 

 two or three dark-coloured bodies. These are the peculiar corpora 

 nigra, which are nothing more nor less than ball-like masses of uvea, the 

 material forming the pigmented layer on the posterior aspect of the 

 iris. Apart from the presence of the easily visible corpora nigra, the 

 whole of the elliptical area forming the pupil should be quite clear. 

 This is a point of great importance in the examination of horses as to 

 soundness. 



Sagittal Section of the 'Eyeball. — From the study of such a section we 

 find that covering the front of the cornea is the layer of conjunctiva 

 which is continuous with that lining the inner aspect of the eyelid. 



