536 A MANUAL OF VETERINAkY PHYSIOLOGY 



smell. They are animals which are hunted, instead of hunt- 

 ing. Hearing and smell give earlier indications of approaching 

 danger than sight. 



As the pupil of the horse dilates under artificial light, the 

 fact is taken advantage of in ophthalmoscopic work, and the use 

 of atropine rendered unnecessary. There are certain drugs 

 which dilate the pupil, such as atropine and cocaine, and others 

 which contract it — for example, morphine and eserine. Dilata- 

 tion of. the pupil is spoken of as mydriasis, and contraction as 

 miosis. Atropine and cocaine are therefore termed mydriatics, 

 and their antagonists miotics.^ It is believed that atropine acts 

 by paralysing the constrictor fibres furnished by the third pair. 

 In greater strength the fibres supplying the ciliary muscle may 

 also be paralysed, and the animal unable to use the eyes for 

 close vision. All animals, however, are not so affected. The 

 ciliary muscle of the horse is not paralysed by atropine. It is 

 curious to observe in the horse that, although the pupil when 

 normally contracted is elliptical, yet when dilated it becomes 

 circular. The chief radiating fibres are above and below, and 

 but very few at the sides. Eversbusch* has studied the struc- 

 ture of the iris of the horse, and states that the elongated form 

 of the pupil is due to the presence of an accessory apparatus 

 on the posterior surface of the iris, which he calls the ligamentum 

 inhibit orium.. Through this ligament the sides of the iris are 

 not pulled in by the contraction of the sphincter muscle. 



The long axis of the pupil in the horse is always horizontal, 

 or practically so, no matter what the position of the head may 

 be. This is a point which will be touched on again in dealing 

 with the muscles of the eyeball. The pupil of the horse dilates 

 moderately after the animal has been galloped. Immediately 

 after a violent death it dilates widely, but in the course of twenty- 

 four hours or so it gradually contracts until the pupil becomes a 

 mere slit. In the horse there exists on the edge of the iris, at 

 the centre and upper part of the pupil, one or more large soot- 

 like bodies, known as corpora nigra (Fig. 159). A small one 

 may be found on the lower margin of the iris, but the upper are 

 the most prominent. When the pupil is strongly contracted in 

 direct sunlight, the centre of it is entirely blocked out by these 

 pigmentary masses, and divided into an inner and outer portion. 

 It would appear as if this might cause an imperfect image to be im- 

 printed on the retina ; but on subjecting the question to actual 

 experiment, no broken image was found to result from the use 

 of a diaphragm the centre of which was blocked out. The use 

 of these bodies is doubtless to assist in absorbing rays of light, 



* Zeitschrift fur Vergleichende Augenheilkunde, Heft 1, 1882. 



