THE SKIN AS AN ORGAN OP SENSE. 525 



On the other hand, the groups with hoofs are notably inferior 

 in the mental scale. When we pass to the lower forms of in- 

 vertebrates the appreciation of vibrations of the air or water 

 in which they live, of its temperature, of its pressure, etc., must 

 be considerable to enable them to adapt themselves to a suitable 

 environment. 



We have not spoken of sensations derived from the internal 

 organs and surfaces. These are ill-deflned, 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 conditions. 



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 when the stimulus is withdrawn.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 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. 



