ANIMAL HEAT. 577 



cold moisture. This influence, the existence of which is a matter of ordinary 

 experience, is probably exerted directly upon the nervous system. 



723. Having thus considered the general facts which indicate the faculty 

 possessed by the living system, in the higher Animals, of keeping up its tem- 

 perature to an elevated standard, and of preventing it from being raised much 

 beyond it by any degree of external heat, we have next to inquire to what this 

 faculty is due. We shall be more likely to arrive at accurate results in such 

 an inquiry, the more comprehensive our survey is of the phenomena to which 

 it relates.* The most recent experiments on the temperature of Plants (those 

 made by MM. Becquerel and Breschet with the thermo-multiplier) have de- 

 monstrated, that in those parts in which the vital processes are taking place 

 with activity, a sensible amount of caloric is being constantly evolved. The 

 amount of this evolution of heat is generally very low, not more, in fact, than 

 a single degree (Fahr.) ; and as it does not more than counterbalance the effect 

 of the evaporation, which is continually taking place from the surface, there 

 is no sensible difference between the temperature of the plant and that of the 

 surrounding air. At the time of Flowering, however, a much greater degree 

 of heat is generated in many plants ; especially in those in which a large 

 number of flowers are crowded together, as in the case of the Arum tribe : 

 thus a thermometer placed in the midst of twelve spadixes has been seen to 

 rise to 121, whilst the temperature of the air was only 66. During the 

 Germination of seeds, again, a considerable development of heat takes place ; 

 this, which is soon carried off from a single seed, becomes very sensible when 

 a large number are heaped together, as in malting ; the thermometer plunged 

 into a heap of germinating barley having been seen to rise to 110. 



724. These facts are of more importance than might appear at first sight ; 

 for they indicate unequivocally, that the source of the heat is to be looked for 

 in the Organic functions not in those of Animal life. The evolution of Calo- 

 ric has been attributed by many physiologists to the Nervous system ; the 

 influence which this system evidently possesses over the function, being mis- 

 taken for the efficient cause of it. As has been remarked on several former 

 occasions, however, the fact that any change takes place in Vegetables to 

 the same degree (under certain conditions) with that in which it ever presents 

 itself in Animals, is a sufficient proof that it cannot be dependent upon nervous 

 agency, although it may be influenced by it. Moreover, it may be remarked, 

 that the production of Heat is an operation of an entirely physical character, 

 and that it may be referred to physical causes ; whilst the operations in which 

 the Nervous system is concerned, are such as we cannot liken in any degree 

 to physical phenomena, and are of a purely vital character. In our inquiry 

 into the sources of the Heat evolved by living beings, we are limited, there- 

 fore, to those which can operate in the Vegetable kingdom ; and on examining 

 into the phenomena which present any relation to this, we are at once struck 

 with the fact, that an absorption of Oxygen from the air, with an extrication of 

 Carbonic acid, is continually taking place (constituting the true Respiratory 

 process of Plants, 522) ; and that these changes occur with excessive activity, 

 at the very periods at which the evolution of Heat is most remarkable, those, 

 namely, of germination and flowering. The quantity of Oxygen consumed 

 by flowers is enormous, those of the Arum Italicum having been found to 

 convert 40 times their own bulk of that gas into Carbonic acid, between the 

 periods of their first appearance and their final decay ; and of this, the far 

 larger proportion is consumed by the sexual apparatus, which has been found 

 to consume 132 times its own bulk of Oxygen in 24 hours. That this change 



* This subject is more fully treated in the Author's Principles of General and Com- 

 parative Physiology, 548 567. 

 49 



