864 NUTRITION AND HEAT REGULATION. 



practically constant, but the oxidations at the lower temperature 

 were over twice the amount of those at the higher temperature. 

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

 ner, is at its minimum. From 35 to 40 C. the heat regulating mech- 

 anism 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. Cold baths, cold 

 winds, and various climatic conditions, such as high altitudes and 

 seaside conditions, also 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 CO 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 intensity. 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 

 interpreted to mean that there exists in the brain anterior to the 

 medulla a general heat center of an inhibitory character. Under 

 normal conditions this center may hold the lower heat-producing 



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

 t See Wood, "Fever," 'Smithsonian Contributions to Knowledge." 

 Washington, 1880. 



