164 THE SENSOKIAL FUNCTIOX. 



act, the pupillaiy opening is closely contracted, ha^-ing tlie appearance of 

 a single line ; when, on the contrary, the radiating fibres are brought 

 iuto action, the pupil is dilated to its greatest extent. A strong light 

 induces the action of the former, to lessen its effect, and a dim light the 

 latter, to admit the greatest possible quantity of it. The light, hoTvever, 

 does not act on the iris itself, but on the optic nerve, and it is from a 

 reflected action from the brain that the muscular power of the iris is 

 influenced. The motions of the iris are not at all under the control of the 

 will, nor is the animal sensible of them. They are produced by sympathy 

 with the state of the retina. Wlien, however, a deficient portion of light 

 reaches the i-etina, and vision is indistinct, we are conscious of an apparent 

 effort to bring the object more clearly into view, and the fibres then con- 

 tract, and the aperture enlarges, and more light is admitted. 



This dilatation or contraction of the pupil gives a useful method of 

 ascertaining the existence of blindness in one eye or in both. The 

 cornea and crystalline lens remain perfectly transparent, but the retina is 

 palsied, and is not aff'ected by light ; and many persons have been deceived 

 when blindness of this description has been confined to one eye. A horse 

 blind in both eyes will usually have his ears in constant and rapid motion, 

 directing them in quick succession to every quarter. He will likewdse 

 hang back in his halter in a peculiar way, and wdll lift his feet high as if 

 he were stepping over some obstacle, when there is actually notliing to 

 obstruct his passage, and there will be an evident uncertainty in the put- 

 ting down of his feet. In blindness of one eye little or nothing of this 

 characteristic gait and manner can be perceived. Although a one-eyed 

 horse may not be absolutely condemned for the common business of the 

 carriage or the road, he is generally deterioriatod as a hunter, for he can- 

 not measure his distances, and will run into his leaps. Many a sportsman, 

 puzzled and angry at the sudden blundering of his horse, or injured by one 

 or more stunning falls, has found a very natural although unexpected 

 explanation of it in the blindness of one eye, and that pei'haps produced 

 through his own fault, by over riding his willing and excellent servant and 

 causing a determination of blood to the eye, which proved fatal to the 

 delicate texture of the retina. Even for the carriage or the road he is 

 considerably deteriorated, for his field of observation must be materially 

 lessened. 



Let the size of both pupils be carefully noticed before the horse is re- 

 moved from the stable, and, as he is led to the door, observe whether they 

 ])oth contract, and equally so, with the increase of light. If the horse 

 should be first seen in the open air, let it be observed whether the pupils 

 are precisely of the same size ; and let the hand be placed over each eye 

 alternately and held there for a little while, and let it be observed whether 

 the pupil dilates vnth the abstraction of light, and equally in each eye. 



Hanging from the upper edge of the pupil of the horse, are two or three 

 round black substances, as large as millet seeds, called the corpora nirjra. 

 When the horse is suddenly brought into an intense light, and the pupil is 

 closed, they present a singular appearance, as they are pressed out from 

 between the edges of the iris. An equal number, but much smaller, are 

 attached to the edge of the lower portion of the iris. Their general use is 

 probably to intercept rays of light which would be troublesome or injuri- 

 one, and their principal function is accomplished during the act of grazing. 

 They ai'e larger on the upper edge of the iris, and are placed on the outer 

 side of the pupil, evidently to discharge the same function which we have 

 attributed to the eyelashes, viz., to obstruct the light in those directions 

 in which it would come with greatest force, both from above and even from 



