ANIMAL HEAT 591 



effect is its power of paralyzing the motor inner vation, and 

 so cutting off from the skeletal muscles impulses which in the 

 intact animal would have reached them. The excitation by 

 cold of the cutaneous nerves, or some of them, which in the 

 unpoisoned animal is reflected' along the y motor nerves to the 

 muscles, and causes the increase of metabolism, is now blocked 

 at the end of the motor path ; and the muscles, the great heat- 

 producing tissues, are abandoned to the direct influence of the 

 external temperature (Pfliiger). 



How is it, then, that nervous impulses from the skin produce 

 in the intact animal their effect upon the chemical processes in 

 the muscles ? We know that the heat-production of a muscle 

 is greatly increased when it is caused to contract ; but it has not 

 hitherto been possible by artificial stimulation to demonstrate 

 that any chemical or physical effect is produced in a muscle 

 by excitation of its motor nerve unless as the accompaniment 

 of a mechanical change. When the gastrocnemius of a frog 

 poisoned with not too large a dose of curara is laid on a resistance 

 thermometer (p. 664), and its nerve stimulated from time to 

 time as the curara paralysis deepens, heating of the muscle is 

 observed as long as, and^only as long as, there is any visible 

 contraction. The gaseous metabolism of a rabbit immersed in 

 a bath of constant temperature may sink by as much as 30 to 

 40 per cent, when curara is given. One obvious cause of this 

 is the complete muscular relaxation. And the whole secret of 

 the regulation of the heat-production might be plausibly supposed 

 to lie in the bracing effect of cold upon the skeletal muscles 

 and the relaxing effect of heat. Indeed, in man it has been 

 observed that exposure to moderate cold causes no metabolic 

 increase when shivering is prevented by a strong effort of the 

 will (Loewy). Nevertheless, the explanation is inadequate in 

 the case of small animals, such as guinea-pigs, rabbits, and cats ; 

 for very great changes in the metabolism may be brought about 

 by external cold without any outward token of increased mus- 

 cular activity. In a man also a fall in the external temperature 

 from 23 to 15 C. caused a certain increase in the output of 

 carbon dioxide (from 27-9 to 32-3 grammes per hour), although 

 no shivering was observed. As the temperature of the air is 

 lowered, the point is soon reached at which shivering can no 

 longer be suppressed, and then it is neither practicable nor 

 perhaps very important to distinguish clearly the portion of the 

 increased heat-production associated with the visible muscular 

 contractions and the portion due to quickened muscular met- 

 abolism without contraction. Lefevre found that in man a 

 marked increase in the heat-loss, such as is caused by immersion 

 for a considerable time (one to three hours) in cold water (at a tem- 

 perature of 7 to 15 C.), was accompanied by a great increase 



