ii THE THERMIC ECONOMY OF THE ORGANISM 81 



all birds, pulmonary transpiration is the chief means of defence 

 against heat. 



Ackermann (1867) called attention to the fact, already known 

 to Hippocrates, that pulmonary ventilation can cool the blood. 

 When the external temperature rises, respiration becomes more 

 frequent and more rapid (heat tachypnoea or polypnoea), cools the 

 lungs, and increases the loss of heat owing to the greater discharge 

 of water-vapour from the lungs. These researches were resumed 

 and carried further by Fick, Goldstein, Gad, and more especially 

 by Richet (see Vol. I. Chap. XIII. p. 470), who in 1885 proved by 

 numerous experiments the heat-regulating function of tachypnoea 

 in dogs, which do not sweat. 



Tachypnoea is generally a reflex phenomenon independent of 

 the vagus nerves and caused by stimulation of the cutaneous 

 nerves. Richet observed in a small dog that respiration, after 

 section of the vagus nerves, became very slow and difficult, five 

 respirations per minute. After being kept for several hours in 

 an incubator, the temperature of the animal remained unaltered 

 (39'1), and it breathed 120 times a minute. Thus after section 

 of the vagus nerves, the dog could maintain the normal temperature 

 in the same way that a healthy dog exposed to the sun in summer 

 begins to pant in a few minutes. Tachypnoea not only keeps the 

 temperature normal, but even at times, according to Richet, 

 causes it to fall slightly, " comme si la regulation par production 

 de froid avait depasse le but et produit plus de froid qu'il n'e'tait 

 necessaire." l 



Heat tachypnoea is central in its origin when it is due not to an 

 excessive rise in the external temperature, but to an abnormal 

 increase in the thermogenic processes of the organism, as in fever. 

 It can be experimentally produced in various ways: by general 

 tetanisation with induced electric currents ; by the injection of 

 toxic substances which produce a rise in the temperature of- the 

 animal by causing a rapid increase of combustion in the muscles. 

 Richet found that whenever the bodily temperature of dogs rose 

 above 41 '7, the frequency of breathing invariably increased 

 enormously ; it may rise from 80 to 400 respirations per minute. 

 The dog in such circumstances breathes with its mouth wide open 

 and its tongue hanging out. 



The following experiment made by Richet seems to me to 

 afford conclusive proof of the efficacy of tachypnoea in regulating 

 the temperature of dogs. If two dogs be exposed to the sun, one 

 being normal, the other curarised, artificial respiration unvarying in 

 rhythm being carried out on the latter, it will be found that the 

 temperature of the normal dog will not rise ; on the contrary, it 

 will fall slightly after a few hours, owing to the over-cooling 



1 " As if the regulation by the production of cold had surpassed its aim and 

 produced more cooling than was necessary." 



VOL. V G 



