586 A MANUAL OF PHYSIOLOGY 



blood was the hotter by 0-4. But such observations, like 

 the corresponding ones on the salivary glands, are open to 

 many errors, and 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 produced 

 in the animal. 



In the case of the brain there is some evidence, obtained by com- 

 parison 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 demonstrate 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, especially the occipital portion, of the scalp, which some 

 observers have stated to take place during mental activity, cannot 

 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. The increase observed by Mosso in the temperature of 

 the brain during intense psychical activity, sometimes to o'2 C., 

 or 0*3 C. above the rectal temperature, may also have been due, 

 in part at least, to vascular changes. And, indeed, if we remember 

 how large a proportion 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 to 

 the precise relation of the energy which appears as heat and of 

 that which is converted into work. The original source of both 

 is, of course, the oxidation (and cleavage) of the food substances, 

 but it has been the subject of discussion whether in a muscle, 

 as in a heat-engine, the chemical energy is first converted into 

 heat, and part of the heat then transformed into work, or 

 whether the chemical energy is immediately changed into work, 

 or whether there is an intermediate form of energy other than 

 heat. Some have supposed that the chemical energy is first 

 converted into electrical energy (p. 640) . 



It has been very generally admitted that the chief seat of 

 excessive metabolism in fever is the muscles ; but U. Mosso has 

 stated that cocaine fever the marked rise of temperature 

 produced by injection of cocaine can be obtained in animals 



