INCOME AND EXPENDITURE OF ENERGY 689 



respiration is responsible for the production of about 210 calories 

 (without including the heat produced by the smooth muscle of the 

 bronchi and bloodvessels), or nearly one-twelfth of the total produc- 

 tion of a man doing ordinary labour. 



The glands, and then the central nervous system, rank after the 

 muscles, though at a great distance, as seats of heat-production. 

 The liver and brain (?) are the hottest organs in the body; and that 

 this is not altogether due to their being well protected against loss 

 of heat is shown, in the case of the liver, by the excess of tempera- 

 ture of the blood of the hepatic over that of the portal vein. In 

 view, however, of the exaggerated importance which some have 

 given to these organs as foci of heat-production, it may be well to 

 point out that although many of the chemical changes in the animal 

 body are undoubtedly associated with the setting free of heat 

 (exothermic reactions), other, and not less weighty and character- 

 istic, reactions may cause the absorption of heat (endothermic 

 reactions) ; and it is possible that some of the syntheses which many 

 of the tissues are capable of performing may be included in this 

 latter category. For example, when urea is decomposed so as to 

 yield ammonium carbonate (p. 478), heat is set free. We must 

 assume 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 upon 

 the quantity, but also upon the quality, of its chemical activity. 

 In all the tissues, including the muscles, it is necessary to assume 

 that some of the energy transformed is expended in so-called ' resti- 

 tution ' processes that is, in replenishing the store of nutritive 

 material within the cells and in building up the protoplasm. Claude 

 Bernard observed an excess of 0-6 C. in the temperature of the 

 blood of the hepatic vein over that of the portal during hunger, and 

 as much as i'6 at the height of digestion, although at the beginning 

 of digestion the portal 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 



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