5 o4 A MANUAL OF PHYSIOLOGY 



changes in the animal body are undoubtedly associated with 

 the setting free of heat, other, and not less weighty and 

 characteristic, reactions may cause the absorption of heat ; 

 and it is possible that some of the syntheses which the 

 hepatic and other glandular tissues seem to be capable of 

 performing may be included in this latter category. For 

 example, when urea is decomposed so as to yield ammonium 

 carbonate (p. 388), heat is set free. We must assume, 

 therefore, that if ammonium carbonate were transformed into 

 urea in the liver, an equal amount of heat would be, on the 

 whole, absorbed. So that the heat-production of an organ 

 may depend, not only on the quantity, but also on the 

 quality, of its chemical activity. When we consider the 

 enormous tide of blood which during digestion sets through 

 the portal system, we shall look with suspicion upon results 

 that announce a difference of more than a small fraction of 

 a degree in the temperature of the incoming and outgoing 

 blood of the liver. Probably not less than 200 litres of 

 blood pass in twenty-four hours through the liver of a 2 kilo 

 rabbit. If the temperature of this blood is raised even 

 one-tenth of a degree in its passage through the hepatic 

 capillaries, this would correspond to a heat-production of 

 20,000 small calories, or one-tenth of the whole heat pro- 

 duced in the animal. 



In the case of the brain ifc has been shown by comparison of the 

 gases of blood taken from the carotid and from the venous sinuses 

 (torcula Herophili) that the metabolism is feeble as compared even 

 with that of resting muscles (Hill). Nor is it possible to demon- 

 strate any marked or constant increase when the cerebral cortex is 

 roused to such an active discharge of impulses as leads to general 

 epileptiform convulsions. The rise of temperature of certain regions 

 of the scalp observed by Lombard during mental activity cannot, 

 therefore, be supposed to be due to conduction of heat from the 

 brain through the skull. It is perhaps caused by vaso-motor changes in 

 the scalp, associated, it may be, with corresponding changes in related 

 areas of the cortex. And, indeed, if we remember how large a pro- 

 portion of the central nervous system is made up of nerve fibres, in 

 which, or at any rate in the fibres of peripheral nerves, no sensible 

 production of heat has ever been demonstrated, it will not appear 

 surprising if even a considerable increase in the metabolism of the 

 really active elements should fail to make itself felt. 



With regard to the muscles, we are as yet in the dark as 



