400 THE HUMAN BOJ)T. 



again by oxygen; hence if the heart has not yet quite 

 stopped beating, artificial respiration, kept up patiently, 

 should be employed for the restoration of persons poisoned 

 by carbon monoxide. 



The Phenomena of Asphyxia. As soon as the oxygen 

 in the blood falls below the normal amount the breathing 

 becomes hurried and deeper, and the extraordinary muscles 

 of respiration are called into activity. The dyspnoea be- 

 comes more and more marked, and this is especially the 

 case with the expirations which, almost or quite passively 

 performed in natural breathing, become violently muscular. 

 At last nearly all the muscles in the Body are set at work; 

 the rhythmic character of the respiratory acts is lost, and 

 general convulsions occur, but, on the whole, the contrac- 

 tions of the expiratory muscles are more violent than those 

 of the inspiratory. Thus undue want of oxygen at first 

 merely brings about an increased activity of the respiratory 

 centre, and especially of its expiratory division which is not 

 excited in normal breathing. Then it stimulates other por- 

 tions (the convulsive centre) of the medulla oblongata also, 

 and gives rise to violent and irregular muscular spasms. 

 That the convulsions are due to excitation of nerve-centres 

 in the medulla (and not, as might be supposed, to poisoning 

 of the muscles by the extremely venous blood) is shown by 

 the facts (1) that they do not occur in the trunk of an animal 

 when the spinal cord has been divided in the neck so as to 

 cut off the muscles from the medulla; and (2) that they still 

 occur if (the spinal cord remaining undivided) all the 

 parts of the brain in front of the medulla have been re- 

 moved. 



The violent excitation of the nerve-centres soon exhausts 

 them, and all the more readily since their oxygen supply 

 (which they like all other tissues need in order to continue 

 their activity) is cut off. The convulsions therefore gradu- 

 ally cease, and the animal becomes calm again, save for an 

 occasional act of breathing when the oxygen want becomes 

 so great as to cause efficient stimulation even of the dying 

 respiratory centre: these final movements are inspira- 

 tions and, becoming less and less frequent, at last cease, 



