340 DISEASES OF THE EYES. 



an ordinary photographic camera by, among other things, the " stop " (the 

 pupil) being in front of the lens, and by the focussing being performed by 

 alteration being made in the shape of the lens by the action of certain muscles. 

 The choroid is dark- coloured, so as to enable it to absorb any superfluous rays 

 of light. The pupil has the power of contracting and dilating in order to 

 regulate the admittance of light. 



Simple Ophthalmia {Co7ijimctivitis) 



is inflammation of the membrane (the conjunctiva) which covers 

 the surface of the eye, and lines the eyelids. It may be induced 

 by mechanical injury or cold. 



When it is caused by a blow directly inflicted on the cornea, the 

 opacity will radiate from the point struck. This appearance, which 

 serves to distinguish the affection from that due to cold, will not 

 be present when the eye has been injured at a moment when it was 

 protected by the eyelid. In catarrh, both eyes are usually affected ; 

 but in ophthalmia from injury, only one eye is as a rule inflamed. 

 The presence or absence of other signs of catarrh will also help 

 in obtaining a correct decision. In ophthalmia from a blow, there 

 will often be a mark of the injury on the skin of the eyelid. In 

 catarrh (" running ") of the eyes, the opacity commences at the 

 inner corner of the eye. 



SYMPTOMS. — The eye has the appearance of having received a 

 blow. The eyelids are closed and swollen. The eyeball is drawn 

 back. Tears flow copiously. The haw projects ; and there is in- 

 tolerance of light. The conjunctiva is inflamed. The cornea grad- 

 ually becomes clouded by an opacity which appears to be super- 

 ficial and of a bluish colour ; characteristics which distinguish this 

 disease from periodic ophthalmia (p. 341). 



TREATMENT. — Examine the part to see if there be any external 

 injury. If a foreign body be found in the eye, remove it carefully. 

 To do this, and also to facilitate examination, it is well to pro- 

 duce insensibility of the part by applying to the surface of the 

 eyeball a few drops of a 5 per cent, solution of hydrochlorate 

 of cocaine (p. 608) in water, or we may have to pull back the 

 haw by transfixing it with a needle and thread. Give a dose 

 of physic, and keep the animal on laxative food. Foment the 

 part with warm water, and keep the horse in darkness. From 

 time to time smear the skin round the eyelids with extract of bella- 

 donna, which can be made to adhere by mixing it with a little 

 glycerine; or place in the eye a few drops of liquor atropinse sul- 

 phatis. The belladonna, by virtue of its active principle, atropine, 

 relieves the congestion of the blood-vessels by causing their mus- 



