PHYSIOLOGICAL ACTION OF LIGHT. 159 
position under the action of light, it might possibly have been expected that 
there would be a change in the electro-motive force. It has been found, how- 
ever, in this research, that this current is not affected by light. 
2. Pigment Cells of the Choroid.—A. current is readily obtained from the pig- 
ment cells of the choroid, when a portion of this membrane is laid on the clay 
points. This current does not exhibit any sensitiveness to light. 
3. Sclerotic and Nerve without the Retina.—These structures give a consider- 
able current not affected by light. 
4, Anterior Segment of the Eyeball_—tThe eye of a frog was bisected so as to 
include in the anterior segment the cornea, iris, and lens. This preparation was 
then placed on the pads, so that the corneal surface was in contact with the one 
pad while the posterior surface of the lens was in contact with the other. <A 
strong deflection was obtained, which was, however, not affected by light. 
5. Posterior Segment of the Eyeball.—On bisecting an eye of a frog, so as to 
remove completely the anterior portion, including cornea, aqueous humour, iris, 
ciliary muscle, and lens, and on bringing the retina into actual contact with one 
of the clay pads, we obtained a large deflection, which was as sensitive to light 
as when the whole eye was employed, thus eliminating any possibility of con- 
tractions of the iris under the stimulus of light, having to do with the results 
previously obtained. 
6. Distribution of the Electro-motive Force of the various Textures of the Eye. 
—The distribution of the electro-motive force between the different portions of 
the eye and cross section of the nerve may be stated as follows :—The most 
positive structure is the cornea, then the sclerotic, then the longitudinal surface 
_ of the nerve ; the cornea is also positive to the posterior surface of the crystal- 
line lens, and the retina itself is positive to the transverse section of the nerve. 
X.—EXPERIMENTS ON THE Eves or Livine Brrps AND MAMMALS. 
At this stage of the inquiry, we examined the action of light on the eyes of 
living animals. In our earlier experiments on the eyes of rabbits, cats, and 
dogs, we found that sensibility to light disappeared with great rapidity. This 
phenomenon is in conformity with the well-known fact, that the sensibility of 
the nervous system in warm-blooded animals quickly disappears after death 
from loss of arterial blood. It was important to ascertain, if possible, whether 
or not the variations in the electro-motive force, by the action of light, which we 
had noticed in the eyes of various animals removed from the head, also occurred 
in the eye while it remained in the head, and was nourished by blood. We, 
accordingly, instituted a series of experiments which were practically very 
troublesome. We examined the eye—(1), of the living cat; (2), of the living 
pigeon ; and (3), of the living owl (Strix fammea, Lin.) In all cases the animals 
were deeply under the influence of chloroform during the experiments. 
