THE SENSORIAL FUNCTION. 129 



scarcely he pierced by the sharpest instrument. The cornea is composed of 

 many different plates, laid over one another ; and between each, at least in a 

 state of health, is a fluid that is the cause of its transparency, and the evapo- 

 ration of which, after death, produces the leaden or glazed appearance of the 

 eye. When it appears to be opaque, it is not often, and never at first, that the 

 cornea has undergone any change. 



There is nothing that deserves attention from the purchaser of a horse more 

 than the perfect transparency of the cornea over the whole of its surface. The 

 eye should be examined for this purpose, both in front, and with the face 

 of the examiner close to the cheek of the horse, under and behind the eye. 

 The latter method of looking through the cornea is the most satisfactory, so far 

 as the transparency of that part of the eye is concerned. During this examina- 

 tion the horse should not be in the open air, but in the stable standing in the 

 doorway and a little within the door. If any small, faint, whitish lines appear 

 to cross the cornea, or spread over any part of it, they are assuredly the remains 

 of previous inflammation ; or, although the centre and bulk of the cornea should 

 be perfectly clear, yet if around the edge of it, where it unites with the sclero- 

 tica, there should be a narrow ring or circle of haziness, the conclusion is equally 

 true, but the inflammation occurred at a more distant period. Whether how- 

 ever the inflammation has lately existed, or several weeks or months have 

 elapsed since it was subdued, it is too likely to recur. 



There is one caution to be added. The cornea in its natural state is not 

 only a beautifully transparent structure, but it reflects, even in proportion to its 

 transparency, many of the rays which fall upon it; and if there is a white object 

 immediately before the eye, as a light waistcoat, or much display of a white 

 neckcloth, the reflection may puzzle an experienced observer, and has misled 

 many a careless one. The coat should be buttoned up, and the white cravat 

 carefully concealed. 



Within the sclerotica, and connected with it by innumerable minute fibres 

 and vessels, is the choroid coat, I. It is a very delicate membrane, and extends 

 over the whole of the internal part of the eye, from the optic nerve to the 

 cornea. It secretes a dark-coloured substance or paint, by which it is covered ; 

 the intention of which, like the inside of our telescopes and microscopes, is 

 probably to absorb any wandering rays of light which might dazzle and confuse. 

 The black paint, pigmentum nigrum, seems perfectly to discharge this function 

 in the human eye. It is placed immediately under the retina or expansion of 

 the optic nerve. The rays of light fall on the retina, and penetrating its delicate 

 substance, are immediately absorbed or destroyed by the black covering of the 

 choroides underneath. For the perfection of many of his best pleasures, and 

 particularly of his intellectual powers, man wants the vivid impression which 

 will be caused by the admission of the rays of light into a perfectly dark 

 chamber; and when the light of the sun begins to fail, his superior intelligence 

 has enabled him to discover various methods of substituting an artificial day, 

 after the natural one has closed. Other animals, without this power of kindling 

 another, although inferior light, have far more to do with the night than we 

 have. Many of them sleep through the glare of day, and awake and are busy 

 during the period of darkness. The ox occupies some hours of the night 

 in grazing ; the sheep does so when not folded in his pen ; and the horse, 

 worked during the day for our convenience and profit, has often little more 

 than the period of night allotted to him for nourishment and repose. Then it 

 is necessary that, by some peculiar and adequate contrivance, these hours of 

 comparative or total darkness to us should be partially yet sufficiently illumi- 

 nated for them ; and therefore in the horse the dark brown or black coat of the 

 choroides does not extend over the whole of the internal part of the eye, or 



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