458 HANDBOOK OF PHYSIOLOGY. 



ratus closely comparable to that which regulates the secretion of saliva 

 or of sweat, by means of which the production of heat in the warm- 

 blooded animals is increased or diminished as occasion requires. This 

 apparatus probably consists of a centre or centres which may be reflexly 

 stimulated, as for example by impulses from the skin, and which act 

 through special nerves supplied to the various tissues. The evidence 

 upon which the existence of this regulating apparatus depends is the 

 marked effect in the increase of the oxygen taken in by a warm-blooded 

 animal when exposed to cold and the corresponding increase in the output 

 of carbon dioxide, indicating that there is an increase of the metabolism 

 and so an increased production of heat, under such circumstances and 

 not a mere diminution of the amount of heat lost by the skin, etc. A cold- 

 blooded animal reacts very differently to exposure to cold ; instead of as in 

 the case of the warm-blooded animal, increasing the metabolism, cold 

 diminishes the metabolism of its tissues. It appears clear, therefore, 

 that in warm-blooded animals there is some extra apparatus which 

 counteracts the effects of cold which in cold-blooded animals causes 

 diminished metabolism. In warm-blooded animals poisoned by urari, 

 or in which section of the bulb has been done, it has been found that 

 this regulating apparatus is no longer in action, and under such circum- 

 stances no difference appears to exist between such animals and those 

 which are naturally cold-blooded. Warmth increases their temperature 

 and cold lowers it, and with this there is of course evidence of dimin- 

 ished metabolism. The explanation of these experiments as given by 

 modern physiologists is that in such animals the connection which natu- 

 rally exists between the skin and the muscles through the nervous chain, 

 such as a thermotaxic nervous apparatus might be supposed to afford, is 

 broken either at the termination of the nerves in the muscles or at the 

 section point of the bulb. The position of this hypothetical centre is a 

 matter of some difference of opinion. It has been demonstrated that 

 stimulation of different parts of the brain may, among other symptoms, 

 produce increased metabolism of the tissues with increased output of 

 carbon dioxide and a raised temperature : the parts of which this may be 

 asserted are parts of the corpus striatum and of the optic thalamus. 

 The exact situation of the heat centres, however, is at present not known 

 with certainty. 



Experimental observations such as have been made upon animals 

 receive confirmation from the observations of patients who suffer from 

 fever or pyrexia; in them the temperature of the body may be raised 

 several degrees, as we have already pointed out (p. 450.) This increase 

 of temperature might of course be due to diminished loss of heat from 

 the skin, but this although in all probability entering into its causation, 

 is not the only cause. The amount of oxygen taken in and the amount 



