

INFLUENCE OF HEAT ON ANIMALS. 79 



sions of the cells, too, of which the tissue is composed, appear to have 

 an influence ; the liability to freeze being diminished by a very minute 

 subdivision of the fluids. And when the roots are implanted deep in 

 the soil, where the temperature does not fall by many degrees so low as 

 that of the surface, the fluidity of the sap may be maintained, in spite 

 of an extremely cold state of the atmosphere. 



111. It is in Cryptogamic plants, that the greatest power of sustain- 

 ing Cold exists; as might be inferred from what has been already stated 

 in regard to their geographical distribution. The little Fungus (Torula 

 Gerevisice) which is one of the principal constituents of Yeast, does not 

 lose its vitality by exposure to a temperature of 76 below zero ; though 

 it requires a somewhat elevated temperature for its active growth. It 

 would appear that Seeds are enabled to sustain a degree of cold, without 

 the loss of their vitality, which would be fatal to growing plants of the 

 same species ; thus grains of corn, of various kinds, will germinate after 

 being exposed for a quarter of an hour to a temperature equal to that 

 of frozen mercury. It is not difficult to account for this, when the close- 

 ness of their texture, and the small quantity of fluid which it includes, 

 are kept in view. The act of Germination, however, will only take place 

 under a rather elevated temperature; and we find in the Chemical 

 changes which it involves, a provision for maintaining this, when the 

 process has once commenced. 



112. The influence of Heat upon the vital activity of Animals, is quite 

 as strongly marked as we have seen it to be in the case of Plants ; but 

 the mode in which it is exerted is in many instances very different. In 

 those animals which are endowed with great energy of muscular move- 

 mejit, and in which, for the maintenance of that energy, the nutritive 

 functions are kept in constant activity, we find that a provision exists 

 for the development of heat from within, so as to keep the temperature 

 of the body at a certain uniform standard, whatever may be the climate 

 in which they live. Their energy and activity are, in fact, so dependent 

 upon the steady maintenance of a high temperature in their bodies, that, 

 if this be not kept up nearly to its regular standard, a diminution or 

 even a complete cessation of vital action takes place, and even a total 

 loss of vitality may result. In these warm-blooded animals, as they are 

 termed, we do not so evidently trace the effects of Heat, because they 

 are constantly being exerted, and because external changes have but 

 little influence upon them, unless these changes are of an extreme kind. 

 But if those internal operations, on which the maintenance of the tem- 

 perature is dependent, are from any cause retarded or suspended, the 

 effect is immediately visible, in the depressed activity of the whole 



1 system. In the class of Birds, whose muscular energy, and whose ge- 

 | neral functional activity, are greater and more constant than those of 



any Bother animals, the temperature is pretty steadily maintained at from 



! to 112 ; and we shall presently see, that a depression of the heat 



; of the body to about 80 is fatal. Among Mammalia, the temperature 



'is usually maintained at from 98 to 102 ; and it seems that in them 



too a. depression of about thirty degrees is ordinarily fatal. 



113. In the different tribes of Birds and Mammals, we find a very 

 diversified power of generating heat ; and on this depends their adapta- 



