ANIMAL HEAT 451 



metabolism at all times and thus provide for a continuous production of heat, 

 the amount of which from day to day, varying with the extent of the losses. 

 In this case the heat liberated by muscles at the time of their contraction 

 would be incidental and not to be regarded as a necessary part of the fun- 

 damental heat-producing mechanism. 



This view is apparently supported by the increase in heat-production 

 that follows injuries or lesions of certain portions of the nerve system. Thus 

 injury to the caudate nucleus or to the tuber cinereum, in an animal is promptly 

 followed by a rise in the body-temperature of from four to about seven de- 

 grees; while lesions of the cortex of the animal in the neighborhood of the 

 cruciate sulcus and at the junction of the supra-sylvian and post-sylvian 

 fissures is promptly followed by a fall in the body-temperature. These and 

 many other facts have led to the formation of a theory as to the manner in 

 which heat-production is excited and regulated by the nerve system in ac- 

 cordance with variations in external temperature. In this theory it is 

 assumed that, throughout the anterior horns of the gray matter of the spinal 

 cord there are nerve-cells that give origin to nerve-fibers that pass out in the 

 ventral roots of the spinal nerves, to be distributed in company with them 

 to muscles at least, and excite heat-production even though the muscles are 

 not in active contraction. These centers and nerves are termed, respectively, 

 thermogenic centers and ihermogenic nerves. The thermogenic centers it is 

 further assumed, though continuously active, are not influenced directly by 

 nerve impulses transmitted to them from the skin in consequence of changes 

 in the external temperature nor by changes in the temperature of the blood, 

 but are influenced by nerve impulses descending the nerve system from 

 higher levels. 



Experimental investigations have apparently demonstrated that an injury 

 or stimulation of the caudate nucleus or of the tuber cinereum is very 

 promptly followed by an increase in heat-production, which leads to the 

 inference that there are in these regions groups of nerve-cells which when 

 stimulated increase heat-production. To this group of cells the term thermo- 

 accelerator has been given. 



Experimental investigations have also apparently demonstrated the fact 

 that injuries or stimulation of the cerebral cortex in the neighborhood of the 

 cruciate sulcus and at the junction of the supra-sylvian and post-sylvian 

 fissures (dog) is promptly followed by a decrease in heat-production which 

 leads to the inference that there are in these regins groups of nerve-cells stimu- 

 lation of which decreases heat-production. To this group of cells the term 

 thermo-inhibitor has been given. Both these centers are in direct connec- 

 tion by nerve-fibers with the thermogenic centers in the spinal cord and 

 through which they alternately accelerate or inhibit their activity. 



The thermo-accelerator and thermo-inhibitor centers it is assumed are 

 both connected with the cutaneous surface through the intermediation of 

 afferent nerves and, therefore, in a position to be influenced by changes ^in 

 the external temperature. Evidence is also at hand that they may be in- 

 fluenced by changes in the temperature of the blood passing to and around 

 them. A fall of the external temperature develops nerve impulses which 

 transmitted to the thermo-accelerator center excite it to action and thus 

 increase heat-production; a rise in external temperature, on the other hand, 

 has an opposite effect by stimulation of the inhibitor center. 



