492 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



or less characteristic ; and that the different regions seem to be so intimately 

 related to one another as to constitute a co-ordinate mechanism. Certain of these 

 regions when irritated give rise, as a direct result, to increased thermogenesis, 

 hence they are of the nature of thermo-accelerator centres; and others to 

 diminished thermogenesis, hence are thermo-inhibitory centres. Both kinds of 

 centres seem to be associated with and to govern a third kind which is dis- 

 tinguished as the general or automatic thermogenic centres. The mechanism 

 may be theoretically expressed in this form: The general thermogenic centres 

 may be regarded as maintaining by virtue of independent activity a fairly con- 

 stant standard of thermogenesis, and as being influenced to increased activity by 

 the thermo-accelerator centres and to diminished activity by the thermo-inhib- 

 itory centres. The finer or smaller variations iu thermogenesis are presumably 

 effected by the general centres, whereas the grosser variations are probably ef- 

 fected by the influences of the thermo-accelerator and thermo-inhibitory centres. 



Specific heat-centres (thermogenic and thermolytic) have by various ob- 

 servers been held to exist in certain regions of the brain cortex, in the base of 

 the brain just in front of and beneath the corpus striatum, in the corpus stri- 

 atum, in the septum lucidum and the tuber cinereum, in the optic thalamus, 

 in the corpora quadrigemina, in the pons and medulla oblongata, and in the 

 spinal cord. Some of these centres have been regarded as being thermogenic 

 and others as being thermolytic. Many errors in deduction have, however, 

 been made because of the many inherent difficulties attending experimenta- 

 tion upon the cerebro-spinal axis, and because almost all the methods used 

 necessarily involve injury or excitation of contiguous parts. The methods 

 adopted of studying these various regions have been chiefly destruction or 

 injury by means of a probe, actual cautery, excision, and the injection of 

 cauterants; by transverse incisions across the cerebro-spinal axis so as to sepa- 

 rate higher from lower portions of the cerebro-spinal axis; and by excitation 

 by small punctures, electricity, etc. 



In classifying these centres Ave are governed by the results which follow 

 excitation and destruction. When irritation or destruction directly affects 

 thermogenesis, the centre is regarded as being thermogenic, but if heat-dissi- 

 pation is the process directly affected, the centre is regarded as being thermo- 

 lytic. In classifying thermogenic centres we would regard the centre as being 

 a general thermogenic centre if it is capable, after the destruction of other 

 thermogenic centres, of causing the normal output of heat; a thermo-acceler- 

 ator centre is distinguished by the lad that excitation increases thermogenesis, 

 while destruction does not diminish thermogenesis, unless the centre happens 

 to be active at the time, and further by the fact that after its destruction the 

 normal output of heal may continue; a thermo-inhibitory centre is distinguished 

 by a decrease of heat-production following stimulation and by the absence of 

 any permanent effect on thermogenesis when the centre is destroyed. The 

 general or reflex thermogenic centres are undoubtedly continuously active, the 

 degree of activity varying according to the immediate demands of the organism 

 for heat ; while the thermo-accelerator and thermo-inhibitory centres are prob- 



