REGULATION OF HEAT PRODUCTION. 831 



tain its body temperature when the outside temperature is changed. 

 The rabbit behaves, in fact, like a cold-blooded animal. In the 

 calorimeter it shows a marked loss of heat production, and its 

 temperature may be made to go up and down with the outside 

 temperature. The same result may be obtained by section of all 

 the motor nerves, that is, section of the spinal cord in the upper 

 cervical region. Rubner has shown by calorimetric experiments 

 upon normal animals (dogs) that the body temperature, as we 

 know, remains constant when the outside temperature is changed, 

 but that the heat production is increased as the outside tempera- 

 ture is lowered. Finally, Johannson* has shown that the increased 

 oxidations that occur under the influence of outside cold, as meas- 

 ured by the C0 2 output, occur only when muscular tension is 

 increased or shivering is noticed. We may believe, therefore, that 

 the increased oxidations 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 contractions (shivering) according to their inten- 

 sity. This fact accords with one's personal sensations regarding 

 the condition of his muscles in cold weather. 



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 supposed 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- 

 lied upon is the effect of lesions, experimental or pathological, of 

 definite portions of the brain or cord. The following facts are 

 significant: A number of observers f have found that section or 

 puncture of the brain at the junction of medulla and pons causes 

 an increase in heat production and a rise of temperature. Section 

 of the cord in the cervical region is, on the other hand, attended 

 usually by a fall in body temperature. These experiments might be 



* Johannson, " Skandinavisches Archiv f. Physiologic, " 7, 123, 1897. 

 t See Wood, "Fever," "Smithsonian Contributions to Knowledge." 

 Washington, 1880. 



