5 io A MANUAL OF PHYSIOLOGY 



of a heat-regulating mechanism may exist in such animals 

 as the frog, the newt, and even the earthworm (Vernon), it is 

 only able to modify to a certain extent the effects of changes 

 of external temperature, not to balance or even override 

 them, as in the homoiothermal animal. The warm-blooded 

 animal loses its heat-regulating power when a dose of curara 

 sufficient to paralyze the voluntary muscles is given. A 

 curarized rabbit, kept alive by artificial respiration, reacts 

 to changes of external temperature like the cold-blooded 

 frog. Now, the only action of curara adequate to account 

 for this effect is its power of paralyzing the motor nerve- 

 endings, 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 meta- 

 bolism, 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. 



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-produc- 

 tion 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. 493), 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 sup- 

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

 muscles and the relaxing effect of heat. And, indeed, in 



