PHYSIOLOGICAL OXIDATIONS. 833 



produced has failed to convince the majority of physiologists. The 

 existence of a special set of heat nerves, in fact, is still unproved. 

 Most physiologists, perhaps, believe that variations in heat pro- 

 duction occur, as stated above, by alterations in the intensity of the 

 oxidations in the muscles brought about by reflex excitation through 

 the motor nerve fibers, and that a special set of heat fibers does not 

 exist. We may at present adopt the conservative view that heat 

 production and heat dissipation in the body are controlled not 

 by a special heat-regulating apparatus composed of heat centers 

 and heat nerves, but by the co-ordinated activity of a number of 

 different centers in addition to the voluntary means already 

 specified. The unconscious regulation of the body temperature is 

 effected chiefly through the following centers : 



f 1. The sweat centers and sweat nerves. 



, ,. . ,. ) 2. The vasoconstrictor center and the vasoconstrictor 

 Heat dissipation < nerye fibers tQ the gkin 



v. 3. The respiratory center. 



f 1. The motor nerve centers and the motor nerve fibers 



, ' + ) to the skeletal muscles. 



Meat production i % The quantity and character of the food as deter- 



v mined by the appetite. 



Theories of Physiological Oxidations. Lavoisier compared 

 the oxidations in the body to the oxidation of organic substances 

 in combustions at high temperatures. He supposed that the mo- 

 lecular oxygen unites directly with the substances oxidized in one 

 case as in the other. It soon became evident, however, that this 

 direct analogy is not applicable. The material that is oxidized 

 in the body fats, carbohydrates, proteids is consumed with a 

 certain rapidity, in the case of muscular contractions with great 

 rapidity, and we know that these same materials out of the body 

 at a temperature of 39 C. are oxidized with extreme slowness. It 

 became customary, therefore, to speak of the oxidations in the body 

 as indirect, meaning thereby that the material is not acted upon 

 directly by the molecular oxygen. Within recent years it has been 

 shown that the oxidation in ordinary combustions the burning 

 of gaseous hydrogen, for instance is not explained by assuming 

 that the oxygen unites directly with the hydrogen. It is stated, 

 for instance, that this combustion does not take place if both gases 

 are entirely free from water vapor; the presence of water is necessary 

 for the oxidation. Chemists are not agreed as to the exact nature 

 of simple combustion, and it is therefore 'increasingly difficult to 

 compare these processes with the oxidations in the body. Leaving 

 aside the details of the process, it may still be believed that the 

 metabolism of material in the body by means of which its heat 

 energy is produced is at bottom comparable to ordinary combus- 

 tions. Oxvgen is absolutely necessary to the process in each case; 

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