EXTERNAL STRUCTURE. 9t 



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 con- 

 cerned. During this examination the horse should not be in the open air, 

 but in the stable, standing in the door-way 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 

 round the edge of it, where it unites with the sclerotica, there should be a 

 narrow ring or circle of haziness, the conclusion is equally true, but the in- 

 flammation occurred at a more distant period. Whether, however, the 

 inflammation has lately existed, or several weeks or months have elapsed 

 since it was subdued, there is every probability that it will recur. 



There is one Httle caution to be added. The cornea in its natural state 

 is not only a beautifully transparent body, but it reflects, even in proportion 

 to its transparency, many of the rays which fall upon it, and, if there be a 

 white object immediately before the eye, as a very hght waistcoat, or much 

 display of a white neckcloth, the reflection may puzzle an experienced 

 observer, and has misled the 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 co- 

 vered ; the intention of which, like the inside of our telescopes and micro- 

 scopes, has been supposed to be, to absorb any stray 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 per- 

 fection of many of his best pleasures, and, particularly of his intellectual 

 powers, man wants the vivid impression which will be caused by the ad- 

 mission 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. Our servant 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 nourish- 

 ment and repose. Then it is necessary that, by some peculiar and excel- 

 lent contrivance, these hours of comparative or total darkness to us should 

 be partially yet sufficiently illuminated 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 rather, it is not found on any 

 part on which the rays proceeding from the objects could fall. It is not 

 found in any part of what may be called the field of vision ; but, in its 

 place, a bright variegated green is spread, and more over the upper part 

 than the lower, because the animal's food, and the objects which it is of 

 consequence for him to notice, are usually below the -/evel of his head — 



