598 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



total heat-production. The quantity formed as by-products of the activity of 

 various structures during a state of muscular quiet is doubtless small compared 

 with the quantity produced by the muscles. 



The Thermogenic Nerves and Centres. Heat-production may occur independ- 

 ently of, but under normal circumstances it is regulated by, the nervous system. 

 A muscle separated from all nervous influences continues to produce heat, but con- 

 siderably less than before, and it ceases to respond to the demands of the system 

 for more or less heat as do muscles with their nerves intact. Injuries to certain 

 parts of the cerebro-spinal axis affect heat-production in muscles, in some in- 

 stances causing an increase and in others a decrease ; but these changes do not occur 

 if the nervous communication between the centres and muscles is destroyed. 



Thermogenic Nerves. Specific thermogenic nerve-fibres have not as yet 

 been isolated, although the researches by Kemp 1 and Reichert 2 indicate that 

 such fibres exist. In muscles probably two kinds of katabolic processes 

 go on, one subservient to muscular contraction and the other to heat-produc- 

 tion. From the fact that there may be two kinds of katabolic processes we 

 are led to the conclusion that two corresponding sets of nerve-fibres con- 

 trol them, and it seems probable that the katabolic processes which give rise to 

 muscular contraction and its accompanying heat-production are due to im- 

 pulses carried to the muscles by motor nerves, while those specifically con- 

 cerned in the production of heat are transmitted by nerve-fibres of an entirely 

 different character, possibly those fibres subserving muscular tone. Upon this 

 hypothesis the latter fibres might be designated as specific thermogenic fibres 

 in other words, they are specifically engaged in conveying impulses from the 

 nerve-centres to the muscles, bringing about katabolic changes which have for 

 their especial object the production of heat. According to another hypothesis 

 both muscular contraction and muscular tone are subserved by the motor nerves, 

 whether or not contraction results being a question of intensity of the impulses. 



Our knowledge of the character of the afferent fibres which carry impulses 

 that reflexly affect thermogenesis is very unsatisfactory. There can be no 

 doubt that sensory impulses arise in various parts of the organism, especially in 

 the skin, which exercise important influences upon the heat-producing pro- 

 cesses. Thus, cooling the skin reflexly excites heat-production, which cannot 

 be attributed to indirect influences upon other functions, but whether or not 

 there exist specific afferent thermogenic fibres is not known. It is possible that 

 the temperature nerves of the skin, the cold and the heat nerves, may be 

 responsible for reflex excitation or depression of heat-production. 



The Thermogenic Centres. The existence of specific thermogenic centres has 

 for many years been conceded, but it has only been recently that hypothesis 

 has given place to fact. The most important results of recent research may be 

 generalized as follows: (1) That the irritation of the skin by heat or cold is 

 followed by marked changes in thermogenesis, which effects are to a certain 

 extent entirely independent of vasomotor and other incidental changes, and 

 which, therefore, are due in part to an increase of heat-production dependent 



1 Therapeutic Gazette, 1889, p. 155. Ibid., 1891, p. 151. 



