THK KYE.] 



MODERN VETERINAPvY PT^NrTTri' 



["tiik kvk, 



lii^lit pas3 through a tninsparont eonirx body, 

 nud become bent, whilst convcyiiii; objoots to 

 v focus, which again convoys tliom to the 

 retina, thus constituting vision. In somo men 

 tl\o cornea is frequently too convex, in conse- 

 ijueuce of which tho rays of light are brought 

 to a focus before they reach tho retina. 

 This constitutes near-sightedness ; the reverse 

 taking place with old people, tho cornea 

 being too flat. Of defective sight iu the 

 horse it is more difficult to judge ; but there 

 is no doubt that many horses with prominent 

 eves, and very convex, have imperfect vision, 

 as they, invariably, are very apt to shy, or to 

 start. 



In eiamining the eye, and looking into the 

 anterior chamber, which contains the aqueous 

 humour, the iris appears, the centre of which 

 is perceived in the opening called the pupil. 

 Looking steadily at it, there is a blueness 

 apparent, showing that there is free access to 

 the back part of the eye through the pupil, 

 and also showing that the iris divides the 

 humours of the eye into two spaces, which are 

 called the anterior and the posterior chambers. 

 The iris appears to hang as a curtain between 

 the cornea and the crystalline lens, and is com- 

 posed of two orders of muscular fibres. It 

 demonstrates the colour of the eye ; as, for 

 instance, when it is black, or blue, it forms 

 an eye of the same colours. In bay horses it 

 is of a cinnamon colour. Sometimes it is 

 white, constituting a wall eye, with which 

 tliere are generally associated white hairs on 

 the eye-lashes and orbit. The colour of tho 

 outer part of the iris is no criterion for the 

 posterior, which is generally black, and is the 

 part that is of service to the sight of the 

 animal. The shape of tho iris, at the circum- 

 ference, is oblong, like tho transparent cornea. 

 It is very muscular, and its fibres are radiated, 

 and wound round the circumference with 

 another order of muscular fibres. Another 

 order winds round the inner margin, and the 

 union of the two is by the radiated expansion 

 of one order of fibres and of the blood-vessels. 

 The opening of the iris, or the pupil, is not 

 a solid body, but a passage left for the rays of 

 light to penetrate to the posterior chambers ol' 

 the eye. The iris undergoes many changes, as 

 to size, in all animals. In the horse it changes 

 its form, from round to oblong, and vice versd. 

 20 



In cats, the changes are well Ht-en ; for, in a 

 strong liglit it becomes cxtrenK'ly oblung, but 

 perpendicular; whil.st in tlio horso it ia hori- 

 zontal. The object of this adaptation ia at 

 once obvious. In order to secure ita prey tho 

 cat haa to look upwards and downwards, but 

 not laterally ; whilst the horso has to look ia 

 all directions ; so that, although the pupil be- 

 comes horizontal, it is never very narrow. 

 Tliere is also, in tho horse's eye, a peculiarity- 

 of structure which seems to have the power of 

 excluding light. This is effected by four glan- 

 dular bodies, two placed at the lower edge of 

 the upper margin of tho iris, and two at tho 

 lower margin. These a'-e not seen in a weak 

 light, when the pupil is large ; and seldom in 

 the dead subject ; but make their appearance 

 in prominent eyes, under a strong light. They 

 are black, and covered with the nigrum pig- 

 mentum, or black paint, which, in the dead eye, 

 becomes a mucous. These bodies may almost 

 be called an internal eyelid, and, united to the 

 iris, they complete the curtain. 



By observing the effect of different degrees 

 of light on the iris, it will be seen that it be- 

 comes stimulated by the action of the rays, 

 and expanding, through the nervous energy, 

 makes the pupil much smaller. On the other 

 hand, if the iris contract, the pupil appears so 

 much larger. This portion of the optical organ 

 is possessed of an abundance of nerves and mus- 

 cular fibres. The muscular fibres are of two 

 orders, the one appearing a kind of sphincter — 

 signifying to shut up— at the inner margin, 

 and which when excited by a strong light, at 

 tlie same moment, contracts. The transverse 

 order relax and elongate, and thus the pupil is 

 diminished. The stimulus of light being re- 

 moved, the transverse preponderate and dimi- 

 nish the iris, whilst they enlarge the pupil. 

 It would appear that the circular or sphincter 

 order, require a very strong stimulus to be 

 able to overcome the constant superabounding 

 power of the transverse order, the varied action 

 of the pupil taking on different forms, accord- 

 ing to the strength of light. In a weak light 

 these become circular, from the transverse 

 fibres being in full play ; but, as in a strong 

 light the pupil becomes oblong, it would appear 

 that the transverse fibres have not an equal 

 power of relaxation, except at the top and 

 bottom ; for, at the corners, they scarcelv relax 



201 



