96 



body is obedient to these general laws ; for all the 

 solid matter of animals has been once in a fluid state, 

 and, having passed into the solid form, becomes a 

 recipient for other fluids, out of which the solids 

 therrtselves may, in turn, be increased and reno- 

 vated *." . 



80. This conversion of fluid into solid matter, 

 cannot, however, go on in the animal body without 

 the constant presence of heat, the agency of which 

 is essentially necessary to carry forward these trans- 

 formations. When, to a certain degree, heat is ab- 

 stracted from the body, its vital functions gradually 

 decline, and at length finally cease. Many animals 

 experience these effects periodically, without injury 

 to the vital organs, and the actions of life re-ap- 

 pear as the temperature of the season returns. In 

 our own climate, the hedge-hog, the bat, the dor- 

 mouse, and several birds, pass into a state of torpidi- 

 ty during the winter season ; and, in the more nor- 

 thern parts of Europe and America, the bear and al- 

 ligator do the same : from which we may conclude, 

 that a certain degree of heat is necessary to sustain 

 the actions of life in the superior, as well as in the 

 inferior animals. 



81. The operation of light on the colour of ani- 

 mal bodies (50.), and probably on some of their other 

 proper ties, has been already noticed. In the human 

 subject, the colour of the skin depends on that of 

 the reticular membrane placed beneath the cuticle, 

 which assumes various colours in different parts of 



Treatise on the Bipod, p. 12. 



