552 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



merits, but after the insertion of the tubes, by means of which the gas was 

 carried directly to the smaller bronchi, the characteristic action of the CO 2 was 

 no longer observed. From the results of these experiments we may con- 

 clude that the marked increase in the depth of the respiratory movements 

 in CO 2 -dyspnoea is due in part to the irritation of the sensory nerve-fibres of 

 the mucous membrane of the larger bronchi. 



The form of dyspnoea due to muscular activity is owing to the action upon 

 the respiratory centre of certain substances which are formed in the muscles 

 during contraction and are given to the blood. Muscular activity, as is well 

 known, is accompanied by an increase in the rate and depth of the respiratory 

 movements, and when the exercise is violent more or less marked dyspnoea 

 may occur. Some physiologists have been led to the belief that the respiratory 

 centre is connected directly or indirectly with the muscles by means of afferent 

 nerve-fibres which convey impulses to the centre and thus excite it to activity; 

 while others have regarded a diminution of O and an increase of CO 2 in the 

 blood as the cause, the active muscles rapidly consuming the O in the blood 

 and giving off CO 2 in great abundance ; but Geppert and Zuntz * have clearly 

 shown that neither of these theories is tenable, and that the respiratory excita- 

 tion is due to products of muscular activity which are given to the blood and 

 which act as powerful excitants to the respiratory centre. The precise nature 

 of the bodies is unknown, but it is probable that they are of an acid character, 

 for Lehmann 2 found that there was a distinct lessening of the alkalinity of 

 the blood after muscular exercise. It is likely that the bodies are broken up 

 in the system, because the results of Loewy's 3 investigations indicate that they 

 are not removed by the kidneys. 



Cardiac and hemorrhagic dyspnoeas are chiefly due to the deficiency in the 

 supply of O the former, to the poor supply of blood due to the enfeebled action 

 of the heart; and the latter, both to this and to the reduced quantity of blood 

 (haemoglobin). All circumstances which enfeeble the circulation or lessen the 

 quantity of haemoglobin therefore tend to cause dyspnoea; hence individuals 

 with heart troubles or weakened by disease or with certain forms of anaemia 

 are apt to suffer from dyspnoea upon the least exertion. 



All circumstances which interfere with the interchange of O and the 

 elimination of CO 2 in the lungs are favorable to the production of dyspnoea, 

 as in pneumonia, pulmonary tuberculosis, growths of the larnyx, abdominal 

 tumors, etc., especially so upon exertion. 



Asphyxia is literally a state of pulselessness, but the term is now used to 

 express a series of phenomena caused by the deprivation of air, as by placing 

 an animal in a closed chamber of moderate size. These phenomena may be 

 divided into three stages: the first is one of hyperpnoea; the second, of 

 developing dyspnoea, and finally of convulsions ; and the third, of collapse. 

 During the first stage the inspiratory portion of the respiratory centre 

 especially is excited, the respirations being increased in frequency and depth. 



1 Pfluger's Archiv f. Physiologic, 1888, vol. 42, p. 189. " Ibid., p. 284. 



3 Ibid., p. 281. 



