304 A Manual of VeUrinary Physiology. 



to a simple slit of pupil, such as is seen in sunlight ; in fact, 

 owing to the presence on the edge of the iris of some large 

 pigmentary bodies, which completely block up the centre 

 of a powerfully contracted pupil, it is wonderful how 

 sufficient light finds its way into the retina during sun- 

 light. 



The function of these black bodies (which are sometimes 

 so large as to cause serious apprehension about the vision) 

 is obscure; that they assist in absorbing rays of light 

 appears certain, but their position in the centre of the 

 pupillary opening would certainly not be the most suitable 

 position for a light-absorbing substance, and therefore they 

 must have some other function. We believe that they may 

 assist in rendering vision binocular when both eyes are 

 turned to the front ; but this point will be touched upon 

 later. It only remains here to note that the horse appears 

 to be the only animal possessing them. 



The colour of the iris is brown in the horse, occasionally 

 bluish white, as in : wall-eyed ' horses. It is of a brighter 

 brown in the ox, and is of a brownish yellow in the 

 sheep. 



The Retina— The expansion of the optic nerve within the 

 eye is termed the retina ; it is a thin, delicate membrane, 

 covered by the vitreous humour in front and the choroid 

 behind. Microscopically, it consists of several layers, of 

 which the so-called rods and cones is the most important, 

 for the reason that the cones are the parts most concerned 

 in vision. The entrance of the optic nerve is a good example 

 of this; there are neither rods nor cones over the optic 

 nerve at its entrance, and in consequence the part is quite 

 blind. There is no yellow spot or area of intense vision in 

 the horse or other herbivora; it is possible that the region 

 of the tapetum is the area of the retina which is most 

 excited by light. 



The Tapetum Lucidum is part of the choroid coat ; it is of a 

 remarkably brilliant, colour, being in the horse of a greenish 

 vellow, and occupying a somewhat semi-lunar spare above 

 the optic nerve. It is formed of 'numerous undulating 



