172 PHILOSOPHY OF ZOOLOGY. 
compared, have the same temperature. When a sensa- 
tion is felt, it arises from the difference of temperature, and 
the changes which the caloricis experiencing to bring about 
an equilibrium. When the body feels cold, the caloric is 
passing out of it into the neighbouring object; when the 
body feels warm, it is receiving heat. As the body is 
thus constantly either giving or receiving heat, in con- 
sequence of being surrounded with a variety of objects, 
changeable in their temperature, the information which ~ 
it can communicate to the mind, in reference to its own 
temperature, or that of other bodies, is merely relative. 
Thus, an object which would feel warm to the body in one 
state, would feel cold in another, But though the infor- 
mation communicated to the mind by this sense, be not 
equally accurate with the indications of the thermometer, 
it exercises a powerful influence over our volition, and is 
intimately connected with our comfort. It gives warning 
of the approach or retreat of certain objects, when no other 
sense could have indicated their presence or change. It is 
a faculty common to all animals. 
III. Sense or Sicurt. 
The organ of Sight, or the Eye, exhibits, in the diffe- 
rent classes of animals, so many modifications of form and 
structure, that it is difficult to assign to each part its pro- 
per function, and to form a just estimate of the sensations 
which, under different circumstances, may be excited, or 
the ideas which may be communicated to the mind. 
1. Structure oF THE Eye. ~Among all the Vertebral 
Animals, including Man, there is a considerable resemblance 
between their organs of sight. Hence a very general 
description of their structure will suffice, to enable us to 
comprehend their peculiar functions, and to estimate the 
