172 PHILOSOPHY OF ZOOLOGY. 



compared, have thd same temperature. When a sensa- 

 tion is felt, it arises from the difference of temperature, and 

 the changes which the caloric is experiencing to bring about 

 nn 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 OF SIGHT. 



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 

 desmption of their structure will suffice, to enable us to 

 comprehend their peculiar functions, and to estimate flu- 



