THE EYE AND ITS APPENDAGES 121 



The posterior layer of the cornea is an elastic lamina (Membrane 

 of Descemet), on which is a layer of polygonal cells. This should 

 be noted because this is the layer which replenishes the anterior 

 chamber of the eyeball with aqueous humour when the latter 

 escapes through the chamber being perforated by injury. Behind 

 the cornea is the anterior chamber into which the iris is seen to 

 project. Posteriorly to the iris again is the Crystalline Lens, which 

 is more convex on its posterior than on its anterior aspect, whilst 

 the lens is accommodated posteriorly in a depression of the vitreous 

 humour — the material which fills the large posterior chamber of the ball. 



The entrance of the optic nerve with the central artery of the 

 retina is placed in the inferior half of the ball. 



Excision of the 'Eyeball. — This is an operation which is rather 

 frequently necessary in the horse, particularly in colliery ponies, 

 owing to the injuries to which the latter are liable. 



If the ball has not burst, it is advisable to preserve it intact, as 

 this facilitates the operation of removal. 



A thread should be passed with a fine needle through the 

 conjunctiva and superficial portion of the cornea, a sufficient thickness 

 of the cornea only being included as to enable the operator to obtain a 

 firm hold with the thread ; but, for the reason stated, namely, that of 

 preserving the ball intact, care should be taken not to pierce the 

 cornea completely. 



The thread should be utilised to draw the ball outwards towards 

 the operator. 



A puncture should then be made in the conjunctiva, and the latter 

 should be slit all round with a curved pair of scissors, thus severing the 

 connection between the ball and the eyelids. This will enable the 

 ball to be drawn further out of the bony socket. 



The next step is to cut through each of the straight and oblique 

 muscles of the eyeball separately, and lastly, the retractor oculi with its 

 contained optic nerve should be severed. 



Q 



