. The physiology of the domestic animals; a text-book for veterinary and medical students and practitioners. Physiology, Comparative; Domestic animals. 864 PHYSIOLOGY OF THE DOMESTIC ANIMALS. supply of the iris will influence the size of the pupil. Thus, in forced expiration, by which the return of the blood from the head is retarded, the pupil is contracted. So, also, when the intra-ocular pressure is dimin- ished, as by puncture of the anterior chamber, there is less resistance to the flow of blood to the blood-vessels of the iris and the pupil is imme- diately contracted. On the other hand,
. The physiology of the domestic animals; a text-book for veterinary and medical students and practitioners. Physiology, Comparative; Domestic animals. 864 PHYSIOLOGY OF THE DOMESTIC ANIMALS. supply of the iris will influence the size of the pupil. Thus, in forced expiration, by which the return of the blood from the head is retarded, the pupil is contracted. So, also, when the intra-ocular pressure is dimin- ished, as by puncture of the anterior chamber, there is less resistance to the flow of blood to the blood-vessels of the iris and the pupil is imme- diately contracted. On the other hand, strong muscular exertion, which leads to the blood flowing freely into the contracting muscles, will produce dilatation of the pupil. The size of the pupil may be modified by various drugs. Substances which dilate the pupil are called mydriatics ; those which lead to its con- traction, myotics. Of the former may be mentioned atropine, hom- atropine, duboisine, daturine, and hyoscyamine. They act chiefly by paralysis of the oculo-motor nerve, while also acting slightly upon the dilator fibres, for after complete paralysis of the oculo- motor nerve the moderate dila- tation thereby produced may be intensified by the administration of atropine. Atropine appears to act mainly by a local mechanism, since it produces dilatation of the pupil even after destruction of the ophthalmic ganglion and division of all the nerves of the eye except the optic, and even, according to some authorities, will produce dilatation of the pupil in an excised eye. Myotics, of which physostig- mine or eserine is the best known, may produce contraction of the pupil either by stimulation of the oculo-motor nerve or paralysis of the sympathetic. 2. Visual Sensations.—Our considerations of the action of the organ of vision have thus far dealt simply with physical processes. The rays of light entering the eye have been traced backward through the trans- parent media of the eye until they resulted in the for
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