EXISTENCE OF HEAT CENTERS AND HEAT NERVES. 965 



At about 33° C. the metabolism of the mammal, according to 

 Rubner, is at its minimum. From 35° to 40° C. the heat regula- 

 ting mechanism in the experiments quoted broke down, in that 

 heat loss was prevented to such an extent by ' the outside high 

 temperature that the body temperature rose in spite of the 

 diminution in heat production. The increased production of 

 heat in the body in consequence of a fall in external temperature 

 is a characteristic property of warm-blooded animals. Rubner 

 designates this side of the regulating mechanism as the chemical 

 regulation, and he calls attention, moreover, to the fact that in 

 mankind, owing to our custom of protecting the surface of the 

 body by clothing and by artificial heat, chemical regulation 

 plays less of a role than in the lower animals. Man, in fact, 

 keeps most of his skin surrounded by a warm layer of air at 

 about the temperature (33° C.) at which the metabolism, as 

 affected by temperature, is minimal. Cold baths, cold winds, 

 and various climatic conditions, such as high altitudes and sea- 

 side conditions, may cause a marked increase in body metabolism, 

 Johannson* has shown that the increased oxidations that occur 

 under the influence of outside cold, as measured by the CO2 out- 

 put, occur only when muscular tension is increased or shivering 

 is noticed. We may believe, therefore, that the increased oxida- 

 tions caused by cold are due to motor reflexes upon the skeletal 

 muscles. These reflexes take place doubtless through the motor 

 fibers, and lead to an augmented muscular tone or to small con- 

 tractions (shivering), according to their intensity. It should be 

 added, perhaps, that some authors believe that this reflex effect 

 is not exerted immediately upon the muscles, but indirectly by an 

 action on the internal seci'etion of the thyroid gland (p. 937). 



The Existence of Heat Centers and Heat Nerves. — Physi- 

 ologists have long supposed that there may be in the body a special 

 set of heat nerves and heat centers, separate in their action from the 

 motor, secretory, and other efferent nerves that influence the me- 

 tabolism of the peripheral organs. It is suj^posed that these fibers, 

 if they exist, when in activity augment or inhibit the physiological 

 oxidations in the tissues, and that this effect has for its specific 

 object an increase or decrease in heat production, outside of any 

 functional activity of the tissues. Bernard thought at first that 

 he had demonstrated the existence of calorific fibers in the cervical 

 sympathetic, but it was afterward recognized that the fibers in 

 question are vasoconstrictors. Since that time very numerous 

 experiments have been made with this object in view, but it must 

 be admitted that no conclusive proof has yet been obtained of the 

 existence of such a system. The evidence that has been most re- 



* Johannson, "Skandinavisches Archiv. f. Physiologie," 7, 12.3, 1897. 

 Also Sjoestroem, ibid., 30, 1, 1913. 



