162 THE SENSORIAL FUXCTIOX. 



and extends over nearly 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 lio-ht which might dazzle and confuse. The black paint, pigmentum 

 nigrum, seems perfectly to dischai-ge this function in the human eye. It 

 is placed immediately outside the retina or expansion of the optic nerve. 

 The rays of hght fall on the retina, and penetrating its delicate substance, 

 are immediately absorbed or destroyed by the black covering of the cho- 

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

 particularly of his intellectual powers, man wants the vivid impression 

 which ^\\\\ 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 are awake and 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 oui- conve- 

 nience and profit, has often little more than the period of night allotted to 

 hun for nourishment and repose. It is necessary then that, by some 

 pecuHar and adequate 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 Avhole 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 does not occupy the smallest portion of what may be called 

 the field of vision ; but, in its place, a bright variegated green is spread, 

 called the tapetum lucidum, 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 level of his head— thus, by suffering 

 the impression to remain longer on the retina, or by some portion of hght 

 reflected from this variegated bed on which the retina reposes, or in some 

 other inexpHcable but efl&cient way, enabHng the animal, even in compa- 

 rative darkness, to possess a power of vision equal to his wants. 



The reader may see in the dusk, or even when duskiness is fast yielding 

 to utter darkness, the beautiful sea-green reflection from the eye of the 

 horse. It is that lucid variegated carpet of which we are now speaking. 



Who is unaware that in the fading gUmmering of the evening, and even 

 in the darker shades of night, his horse can see surrounding objects much 

 better than his rider : and who, resigning himself to the guidance of that 

 sagacious and faithful animal, has not been carried in safety to his jour- 

 ney's end, when he would otherwise have been utterly bewildered ? 



If the reader has not examined this beautiful pigment in the eye of the 

 horse, he should take the earhest opportunity of doing so. He will have 

 a beautiful illustration of the care which that Being who gave all things 

 life has taken that each shall be fitted for his situation. The horse has not 

 the intelhgence of man, and may not want for any purpose of pleasure or 

 improvement the vivid picture of su.rrounding objects which the retina of 

 the human being presents. A thousand minute but exquisite beauties 

 would be lost upon him. If, therefore, his sense of vision may not be 

 so strong during the day, it is made up to him by the increased power of 

 vision in the night. 



Perfectly white and cream-coloured horses have a pecuhar appearance 

 of the eyes. The pupil is red instead of black. There is no black paint or 



