490 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



The Thermogenic Tissues. — Almost if not every tissue of the body may be 

 regarded as being a heat-producing structure. The very tact that oxidative 

 processes lie at the bottom of all forms of vital activity, and that heat-produc- 

 tion is a concomitant of oxidation, leads inevitably to the conclusion that as 

 Ion- as cells possess life they must produce heat. There arc, however, certain 

 of the bodily structures, especially the skeletal muscles and the gland-, which 

 are exceptionally active as heat-producers. Indeed, in the case of the skeletal 

 muscles the heat-producing processes are of such a character as to justify the 

 belief' that with them therniogendsis is a specific function, because heat is pro- 

 duced not merely as an incidental product of activity but as a specific product. 

 When a muscle contracts, heat is evolved as an incident of the performance of 

 work, and when it is at rest heat is produced not only as an incident of growth 

 and repair but as the result of a specific act. This latter is proved by the fact 

 that when the muscles have been in a state of prolonged rest, when the chemi- 

 cal changes concerned in growth and in repair of waste are practically inactive, 

 heat-production continues to a marked degree. Moreover, the quantity which 

 is produced varies with the immediate needs of the economy and bears a 

 reciprocal relationship to the quantity of heat formed in other structures, 1 

 and is regulated apparently by specific nerve-centres. 



When the muscles are contracting less than one-fifth of the energy appears 

 as work, and more than four-fifths as heat. The contractions of the heart also 

 furnish an appreciable percentage of heat as an accompaniment of contraction; 

 and considerable heat is formed indirectly by the resistance offered by the 

 the blood-vessel walls to the blood current. Indeed, the entire work of the 

 heart becomes converted into heat, representing approximately 5 to 10 per 

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

 tin- activity of various structures during a state of muscular quiet is doubtless 

 small compared with the quantity produced by the muscles. 



The Thermogenic Nerves <ni</ ( 'entres. — 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 beat as do muscles with their nerves intact. Injuries to certain 

 part- of the cerebro-s pinal 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 centre.- and muscles is destroyed. 



Thermogenic Nerves. — Specific ther genie nerve-fibres have not as yet 



been i-olated, although the researches of Kemp, 2 Reichert, 3 Schultz, 4 and 

 others indicate that such fibre- exist. In the skeletal muscles probably 

 three independent kinds of processes go on which produce heat, one subser- 

 vient to the contraction- of the mii-dcs. as observed in locomotion, etc. ; 



1 Riibner: Sitzungsberichte d. kimigl. Bayer. Akad. der Wi&semchaft, 1885, Heft 4. 



Therapeutic Oazette, L889, |>. 1 •">">. 

 3 Ibid., 1891, p. 151. 

 1 Schultze: Archiv fur experimentelle Pathologic und Pharmakologie, 1899, Bd. 43, S. 193. 



