Book VII. PHYSIOLOGY OF THE HORSE. 971 



or choanoid, peculiar to quadrupeds, to draw the eye within the socket and thus preserve it from danger, 

 which draws the globe inwards. 



6376. The phenomena of vision. If the diagram oe examined, it will be evident that the eye of the 

 horse presents an optical instrument of exquisite workmanship and mechanism, admirably fitted to collect 

 the luminous rays from the various objects around, and to transmit them with truth to the brain. If the 

 luminous rays reflected from objects passed through the eye in a rectilinear course, as they do through the 

 atmosphere, no cognisance at all useful to the animal could be taken of them by the eye ; all would be 

 glare and indistinctness : but being refracted or bent by the media through which they pass, the rays 

 finally meet at a point called their focus or focal point. Neither would one simple line of refraction 

 have been sufficient to answer all the purposes of perfect vision, under its various modifications. It is 

 necessary that the refraction should be increased in its passage by increased degrees of density in the 

 media of its transit {n n). In the passage of the rays through the cornea and aqueous humour, they must 

 encounter their first refraction ; and it is evident, that the more convex the anterior portion of the eye 

 may be, the more will this refraction be increased. We need not, therefore, be surprised that a goggler, 

 or horse with this form of eye, should start. The next and largest degree of bending which the rays receive 

 occurs in their passage through the crystalline lens, which from its lenticular form must necessarily be 

 considerable ; in their progress through the vitreous humour a farther refraction is effected, till meeting in 

 a point on the retina, a perfect representation of the object or objects viewed is obtained ; the rays forming 

 in their passage numerous cones, the bases of which will be the object viewed, and the apex of each a 

 radiant point. Amidst the number of objects around, it appears that the eye has a capability of collecting 

 rays from such only as are immediately necessary for the purposes of the animal it belongs to ; hence, 

 although the general field of view may fall under an angle of vision, yet such rays only as are im- 

 mediately capable of this convergency produce effect, all others are lost in the black pigment of the eye, 

 api)arently placed there purposely to absorb the superfluous rays. As the eye must necessarily have a 

 vast variety of objects painted on it whose distances are widely difl^rent, there must be some optical 

 adjustment of the powers of the part to enable it to effect a distinct vision of all objects near or remote; 

 but whether this takes place by means of the angle formed on the two opposite axes, or as has been more 

 lately taught, by a muscular power in the lens itself, is not yet satisfactorily ascertained : certain it is that 

 after the loss of one eye, time is required both in the human and brute subject for the remaining eye to 

 learn to adjust itself to judge of relative distances ; which fact is certainly in favour of the opinion that 

 an angle formed between the eyes regulates the judgment of distances. In this way we can account for 

 the well known fact, that hunters, which have before the loss of an eye been excellent and sure Icapers, 

 have afterwards lost the power of measuring their leaps. Were it not for some adjustment of the optical 

 organ itself, the rays reflected from objects very near the eye would fall behind it, and those from distant 

 ones would, from being almost parallel, me?t together before the rdtina. The mechanical adjustment of 

 the focus is also assisted in some measure by the iris, which contracts almost to a point when we look at a 

 very rninute object ; and by this means only permits such rays to pass through as penetrate the centre of 

 the lens, by which such rays will be very much refracted ; but when the eye regards distant objects, the 

 kis becomes dilated, and the rays are then viewed through the edges of the lens, and their inchnation is 

 thereby lessened. 



6377. The criteria of soundness in the eyes are gained by a careful examination of them ; and which ex- 

 perience has shown to be best made by placing the horse within a stable, with his head nearly approaching 

 the stable door, which should be fully open. Small eyes are found more prone to inflammation than large, 

 and large goggling eyes are more liable to accompany a starting horse than lesser ones : and when the 

 convexity is extreme, not only is the starting in proportion, but such eyes are more liable than others to 

 become affected with the disease popularly called glass eyes, but medically gutta serena. It is not, however, 

 to be understood that all starters have defective eyes ; many are so from natural timidity, and still more 

 from harsh usage. The eyes should be examined together, not only to observe whether each presents an 

 equal degree of clearness in the transparent part and within the pupil, but also that an equal degree of 

 contraction exists between each of the pupils. This is of much consequence: if any inequality in size or 

 form be observable between the pupils, the least of them has been in some way affected, and will probably 

 become so again. It is even more suspicious when a turbid milkiness appears on any part of the transparent 

 portion ; and equally so, when the inferior part looks other than clear ; or, in a very strong light, with 

 a lively bluish tinge. When it is at all turbid, viewed under various aspects, regard it attentively, and 

 there may probably be found an inward speck of perfect white; which is the nucleus or central point of 

 an incipient cataract. 



6378. A glassy greenish cast in the eye shottld occasion suspicion, and the hand should be i>laced over 

 such eye so as to exclude the light ; remove the hand suddenly and watch the motions of the Iris or cur- 

 tain of the pupil. If it do not contract, carry the examination' still further, and it will probably be found 

 such eyes are totally blind. A blind horse usually carries his ears about, as though in alarm, on his 

 leaving the stable; he also lifts his feet on such occasions, particularly in strange quarters, higher than a 

 sound horse. 



SuBSECT. 9. The Nose and Sense of Smelling. 



6379. The organ of smell is, in most quadrupeds, the next in importance to that of vision, and in many 

 points of view it is even of more consequence. With the herbivorous tribe, it forms their princij)al means of 

 judging between the noxious and the innoxious. It is not therefore to be wondered at, that it should in. 

 these tribes form so large a portion of the head ; nor that it should be so exquisitely gifted with sensibility, 

 or so admirably formed to answer its important purposes. The external parts of the nasal organ are the 

 two nostrils, and as much of their convolutions and linings as come into immediate view. Internally 

 these two cavities are carried upwards into the pharynx, but completely divided by a cartilaginous sep- 

 tum {Jig. 83]./). In this course they communicate with numerous openings and cavities, formed within 

 the bones of the skull (6300.), the whole of which are lined by one continuous membrane of exquisite 

 vascularity and sensibility ; being largely furnished with blood-vessels, which gives them such a ready 

 tendency to inflame and become red, as we witness under only a slight degree of exertion, and as we see 

 more evidently when violent colds or inflammations on the chest are present. Its sensibility is derived 

 from the olfactory nerves, which are spread over all its surface. It is this membrane which is the peculiar 

 seat of glanders, becoming first inflamed, and next ulcerated throughout its extent ; and as the membrane 

 itself appears to be continued to the pharynx and larynx, so we need not wonder why the glanders pro- 

 ceeds to disease the lungs; nor why a common cold, which is at first a simple inflammation of this mem- 

 brane, so readily degenerates into inflammation of the lungs. The common integuments or coverings of 

 other parts are extended over the nose, but it is little furnished with fat. Of hairs it has a fine thin 

 covering to the edges of the nostrils, and a longer set, which are carefully removed in trimming. By a fold 

 of the skin, within which is a cartilage, the false nostril, as it is termed, is formed, whose use appears to 

 be to keep open the canal for the transmission of air, and yet to offer an interruption to extraneous matter. 

 When the nostrils are a little separated, a small canal may be seen, which is the nasal duct for the trans- 

 mission of the superfluous moisture from the eyes. The horse breathes or respires wholly through his 

 nostrils in all ordinary cases. 



6380. The sense of smelling. The volatile particles from all odorous bodies are continually passing off 

 from them, and consequently some must reach the olfactory organs, whose capability of taking cognizance 

 of their qualities appears derived as before pointed out, by the expansion of nervous fibrilte from the olfac- 

 tory nerves which transmit impressions to the brain. 



