694 ANIMAL HEAT 



effect is its power of paralyzing the motor innervation, 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 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 aban- 

 doned 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. 783), 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 muscular 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 

 metabolism 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 in 

 the production of heat, so that the axillary temperature fell com- 

 paratively little e.g., only i C. during a stay of three hours in a 

 bath at 15 C. With short periods of immersion, a characteristic 



