VISION. 559 
e 
We have not spoken of sensations derived from the internal 
organs and surfaces. These are ill-defined, and we know them 
mostly either as a vague sense of comfort or discomfort, or as 
‘actual pain. We are quite unable to refer them at present to 
special forms of end-organs. They are valuable as reports and 
warnings of the animal’s own condition. 
After-impressions (“ after-images ”’) of all the senses referred 
to exist, mostly positive in nature—i. e., the sensation remains 
when the stimulus is withdrawn. 
Synoptical—_The information derived from the skin in man 
and the other higher vertebrates relates to sensations of press- 
ure, temperature, touch, and pain. The muscles also supply 
information of their condition. In how far these are referable 
to certain end-organs in the skin is uncertain. There are der- 
mal areas that give rise to the sensations of heat, cold, pressure, 
and pain. Whether these are connected with nerve-fibers that 
convey no other forms of impulses than those thus arising is 
undetermined. 
In all these senses the laws of contrast, duration of the im- 
pression, limit of discrimination, etc., hold. 
The sensory judgments based on sensations derived from 
the skin are syntheses or the result of the blending of many 
component sensations simultaneous in origin. All our sensory 
judgments are very largely dependent on our past experience. 
VISION. 
Light and vision are to some degree correlatives of each 
other. Light is supposed to have as its physical basis the vibra- 
tions of an imponderable ether. Such is, however, to a non- 
seeing animal as good as non-existent, so that we may look at 
this subject either with the eyes of the physiologist or the phys- 
icist, according as we regard the cause of the effects or the 
latter and their relations to one another. It is, however, im- 
possible to understand the physiology of vision without a 
sound knowledge of the anatomy of the eye, and an apprehen- 
sion of at least some of the laws of the science of optics. The 
student is, therefore, recommended to learn practically the 
coarse and microscopic structure of the eye in detail. The eyes 
of mammals are sufficiently alike to make the dissection of any 
of them profitable. Bullocks’ eyes are readily obtainable, and 
from their large size may be used to advantage. We recom- 
