470 PHYSIOLOGY CHAP. 



exercised by work upon the respiration of the muscular tissues 

 and the combustion which takes place within them is enormous. 

 To form an idea of it we must consider the variations per tiin6 

 unit of the quantity of carbonic acid given off in different states 

 of the muscle. According to Gad, the same individual gave off in 

 one minute from the lungs : 



During sleep 0'38 grms. of C0 2 



During the waking state and horizontal position . 0*57 



In walking 1-42 



In more rapid walking 2O3 



In climbing 3'83 



It will be noted that in the work of climbing ten times 

 as much carbonic acid is eliminated as in sleep. The need 

 of breathing, therefore, increases proportionately, and is amply 

 satisfied by the dyspnoea which provides for the due elimination of 

 the excess C0 2 formed, and absorption of the excess 2 consumed. 



Analysis of the blood gases of animals which are dyspnoeic in 

 consequence of muscular work show, however, that boj/h carbonic 

 acid and oxygen are present there in normal quantities. Accord- 

 ing to the elegant researches of Geppert and Zimtz (1888), the 

 oxygen is somewhat increased and the carbonic acid considerably 

 diminished below the normal. There is thus a certain adaptation 

 between the need of air and the pulmonary ventilation, but it is 

 not strictly commensurate with the chemical requirements of our 

 tissues. The dyspnoea of muscular work exceeds the limit of strict 

 necessity, i.e. there is a superfluous increase of the respiratory 

 activity which cannot be explained either by the increase of C0 2 , 

 or by the diminution of 2 in the blood. Since the effect of 

 energetic muscular work is to diminish the alkalinity of the blood, 

 it has been supposed (in order to account for the increased activity 

 of the respiratory centres) that the muscles during their activity 

 develop an acid product of consumption, different from carbonic 

 acid, which is capable of exciting the respiratory centres (Curt 

 Seehman). It is possible that this product may be lactic acid, 

 which is developed and poured out into the blood by the muscles 

 during their activity (Spiro), and which is found, e.g., in the urine 

 of soldiers after a long and fatiguing march, or gymnastic 

 exercises (Colasanti). 



But in explaining the superfluous increase of respiratory 

 activity in the dyspnoea of muscular fatigue, we must also take 

 into consideration the increased temperature of the blood, which 

 is necessarily associated with the increased combustion of the 

 muscular tissues, and which in itself is capable of provoking a 

 dyspnoeic acceleration of respiratory rhythm to which the name of 

 tachypnoea, polypnoea, or thermal dyspnoea, characterised by very 

 rapid and superficial respirations, with increased tone of the 

 inspiratory centres, has been given. 



