54 RESPIRATION 



and found that the respiratory response to increased muscular 

 work was the same as before, but was entirely absent if the circu- 

 lation from the working muscles was interrupted. Similarly they 

 found that severance of the nervous connection between the lungs 

 and the center did not affect the response. Lorrain Smith and I 

 found, similarly, that when air containing about 20 per cent of 

 CO 2 was supplied to a rabbit there was no difference in the time 

 required for the onset of hyperpnoea after the vagi were cut. 



No definite anatomical group of nerve cells has been defined at 

 the position occupied by the respiratory center; and the exact 

 meaning which ought to be attached to the expression "respira- 

 tory center" is still doubtful. It seems pretty clear, however, that 

 the center is at about the position which is sensitive to the chem- 

 ical respiratory stimuli. To judge from analogy the sensitive 

 elements are probably not the bodies of nerve cells, but end- 

 organs or arborizations. The central paths for the innervation of 

 inspiratory and expiratory movements must also be different, but 

 in what sense the center itself is double is still obscure. Its excita- 

 tion by chemical stimuli depends more upon the character of the 

 blood supplied to it than on substances generated by its own local 

 metabolism. Thus the temporary diminution of blood supply in 

 fainting does not produce the same prompt effect on the center as 

 changes in the arterial blood owing to imperfect aeration in the 

 lungs. In this respect the center is very well suited to fulfill the 

 function of taking a part in controlling the quality of the general 

 arterial blood supply of the body. The amount of arterial blood 

 supplied is controlled in other ways. 



Like other parts of the central nervous system, the respiratory 

 center can easily be fatigued; and, as will be explained later, 

 fatigue of the respiratory center is of great importance in practical 

 medicine. Fatigue of respiration was recently studied by Davies, 

 Priestley, and myself, and its phenomena described in the paper 

 already referred to. The fatigue was produced by breathing 

 against a resistance, the breathing being also increased at the 

 same time, if necessar)', by muscular exertion. The resistance was 

 produced by cotton wool in the manner already described. 



So long as the center is functioning normally it responds to the 

 resistance, in the manner indicated above, by producing a constant 

 slow and deep type of breathing. When, however, the resistance 

 is excessive and continued for some time, the breathing becomes 

 progressively shallower and more frequent. At the same time the 

 alveolar ventilation becomes less and less effective, until at last 



