902 PRACTICE OF AGRICULTURE. Part III. 



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

 atmosphere, no cognizance 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 have answered 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). In the passage of the rays through the cornea and aqueous humor, 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 gogglery 

 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 

 con.siderable ; in their progress through the vitreous humor a farther refraction is eiFected, till meeting in 

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

 ing in their passage numerous cones, the basis 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 imme- 

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

 apparently 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 different ; 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 favor 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 leapers, 

 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, meet together before the retina. 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 minute 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 

 iris becomes dilated, and the rays are then viewed through the edges of the lens, and their inclination is 

 thereby lessened. 



5704. The criteria of soundness in the eyes are gained by a careful examination of them ; and which 

 experience has shown to be best made by placing the horse within a stable, with his head nearly approach- 

 ing 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 termed glass eyes, but correctly gutta serena. 

 It is not however to be understood that all starters have defective eyes ; many are so from natural timi- 

 dity, 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 ap- 

 pears 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 j which 

 is the nucleus or central point of an incipient cataract. 



5705. A glassy greenish cast in the eye should occasion suspicion, and the hand should be placed 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. 



SuESECT. 9. The Nose and Sense of Smelling, 



5706. The organ af smell is, in most quadrupeds, the next in importance to that of vision, and in many 

 points of view it is even more so. With the herbivorous tribe, it forms their principal means of judging 

 between the noxious and the innoxious. It is not therefore to be wondered, 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 

 septum, (^fig. 616/). In this course they communicate with numerous openings and cavities, formed 

 within the bones of the skull, (5631.) the whole of which are lined by one continuous mem- 

 brane 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 mem- 

 brane 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 proceeds to disease the lungs ; nor why a common cold, which is at first a simple 

 inflammation of this membrane, so readily degenerates into inflammation of the lungs. The common in- 

 teguments 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. Wlien the nostrils are a little separated, a small canal may be seen, which is the 

 nasal duct for the transmission of the superfluous moisture from the eyes. The horse breathes or respires 

 wholly through his nostrils in all ordinary cases. 



5707. 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 cogni- 

 sance of their qualities appears derived as before pointed out,'by the expansion of nervous fibrillae from the 

 olfactory nerves which transmit impressions to the brain. 



SuBSECT, 10. The Cavity of the Mouth. 



5708. The external parts of the mouth are the lips, cheeks, and beard. The lips are made up of fleshy 

 masses so disposed as to give them motion every way ; they are covered over with a very fine expansion 

 of skin almost devoid of hair, their exquisite sensibility forms them into an organ of touch ; and in this 



