402 CHAPTER 57. 



In n state of health there. is always a slight secretion from the lachry- 

 mal glands, which is carried along and conveyed away by two small canals 

 into the lachrymal sac at the inner canthtis of the eye, and thence by 

 means of a duct into the nose. This duct has been sometimes mistaken 

 for the ulcer of glanders. 



837. Muscles of tlie Eye. The Retractor oculi. 



The muscles of the eye, as we might expect in a part so sensitive and 

 so rapid in its motions, are both numerous and powerful. None however 

 call for any special remark, except the Retractor oculi. Owing to the 

 great strength of this muscle the horse possesses in a peculiar degree the 

 power of withdrawing at will the eye into its orbit. 



This power is greatly assisted by the fact of the eye resting posteriorly 

 on the cushion or bed of semi-fluid fat mentioned above, which on the 

 application of pressure yields very rapidly and thereby admits of easy 

 retraction of the eye. The displacement of this body also causes the haw 

 or membrana nictitans to come forward over the eye. 



838. The Membrana nictitans or Haw. 



The Membrana nictitans or Haw, which is situated in the inner corner 

 of the eye, constitutes a very marked peculiarity of that organ in the 

 horse. " The haw has two principal uses, first, by moving backwards and 

 forwards it wipes off the dust or other extraneous particles which may 

 lodge on the surface of the eye ; and, secondly, by protruding forward 

 when the eye is drawn back on the approach of danger, it guards the 

 cornea to a certain degree from any accidental blow. Its motion may be 

 said to be involuntary, because it is forced over the eye mainly by the 

 withdrawal of that organ into its socket. 



This peculiarity of structure renders it very difficult to perform on the 

 eye of the horse many of those delicate operations which are so useful in 

 human surgery for the relief of various ocular diseases. With some 

 trouble however the eye may be got at. The operator must frequently 

 touch or threaten to touch the eye ; and though for some time it will be 

 drawn back at each threat, yet at last the retractor muscle will be wearied 

 out and certain operations may then be performed. This practice is 

 adopted with success in India in cases of worm in the eye. Of late years 

 chloroform and cocaine have been used in operations on the eye. 



839. Nature and seat of the Diseases of the Eye. 



By the nature of its organisation, by its office and situation, and by 

 its connections with other parts of the system, the eye is exposed to 

 numerous external "and internal causes of disease. 



The diseases of the eye may be divided into two great classes ; namely, 

 those which result from external influences, and those arising from con- 

 stitutional causes. Most, but not all of the diseases of the eye, whether 

 resulting from the one or the other classes of causes, are of an inflamma- 

 tory character. 



