ANIMAL HEAT. 4Ul 



another in the form of contraction known as shivering ; and a third, giving 

 rise to heat as the only important phenomenon. The heat produced by 

 muscles in ordinary or general muscular acts and in repair and growth is a 

 mere incident to activity; but the heat arising during shivering is undoubt- 

 edly a specific product — i. e., the object of the shivering is a production of 

 heat (see p. 433). If the nerve-fibres which convey the impulses tli.it cause 

 shivering be ordinary motor fibres, then these fibres are not only motor film-, 

 but specific thermogenic fibres in so far as they are connected with heat- 

 production by this act. There are also, apparently, fibres which are entirely 

 distinct from the motor fibres, and which convey impulses that give rise to 

 heat-production as a specific product, and even in the entire absence of motor 

 phenomena. Thus, in a curarized animal in which all motor activity of the 

 skeletal muscles is abolished, an enormous increase of heat-production may 

 occur (Reichert) which cannot satisfactorily be explained in any other way 

 than by assuming the existence of such specific thermogenic fibres. Our 

 information at present is, however, so limited that we can do scarcely more 

 than speculate. 



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

 that reflexly affect thermogeuesis 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 centre- 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 thermogeuesis, 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 

 directly upon efferent thermogenic impulses. (2) That injury or excitation of 

 certain parts of the brain is followed by an increase of heat-production. (3) 

 That injury or excitation of certain other parts of the brain is followed by 

 diminished heat-production. (4) That injury of the spinal cord may be fol- 

 lowed by an increase or decrease of heat-production which cannot be entirely 

 accounted for by vaso-motor and other attendant alterations. (5) That after 

 operations upon certain parts of the cerebro-spinal axis their follows an increase 

 or decrease in the quantity of 0O 2 formed, indicating a corresponding effed <>n 

 the heat-producing processes. 



The results of recent calorimetric work show that there are definite regions 

 of the cerebro-spinal axis which are apparently specifically concerned in ther- 

 mogenesis; that the effects of excitation or destruction of each region are more 



