12 RESPIRATION 



and specially the diaphragm, are, and remain till the interruption 

 is broken by an inspiratory effort, relaxed. This interruption of 

 inspiratory effort came to be interpreted as an apnoea, and appears 

 so if only inspiratory muscular movements are recorded, as, for 

 instance, with the method adopted in Hering's laboratory by 

 Head, 19 in which only the contractions of the diaphragm are re- 

 corded, or with other methods which do not record tonic expira- 

 tory effort. Hence it came to be assumed that there exists what is 

 called "vagus apnoea." The next step was to maintain that all 

 apnoea is in reality vagus apnoea, and this inference was supported 

 by the fact that "apnoea" can still be obtained when the arterial 

 blood is blue owing to air containing a very low percentage of 

 oxygen being breathed, and can also be produced (as Lorrain 

 Smith and I found) by air very rich in CO 2 . It was also affirmed 

 by Brown-Sequard that after the vagi are cut apnoea cannot be 

 produced, though this statement can easily be shown to be com- 

 pletely mistaken. With an efficient apparatus for increasing the 

 ventilation of the lungs apnoea can quite readily be produced 

 after section of the vagi. 



On the other hand, increasingly clear evidence accumulated 

 that apnoea due to over- ventilation of the blood passing through 

 the lungs exists as a matter of fact. The most striking proof of 

 this was afforded by experiments in which Fredericq 20 crossed 

 the circulation of two animals by connecting the vessels in such a 

 way that the respiratory center of each animal was supplied with 

 arterial blood from the other animal. He then found that when 

 excessive artificial respiration was produced in one of the animals 

 apnoea was produced in the other, and when the artificial respira- 

 tion ceased hyperpnoea continued in the animal which had had 

 artificial respiration, since its respiratory center was now receiving 

 blood which was venous owing to the cessation of breathing in the 

 other animal. This hyperpnoea, on the other hand, maintained 

 the apnoea in the other animal, so that one of the animals re- 

 mained apnoeic while the other remained hyperpnoeic. 



This experiment showed clearly the existence of a true "chemi- 

 cal" apnoea; but, as the existence of vagus apnoea was also con- 

 sidered to be firmly established, the existence of both forms of 

 apnoea came to be generally assumed. As regards vagus apnoea the 

 evidence was considered to show that when apnoea is produced by 

 distending the lungs with air or hydrogen it is vagus apnoea that 



19 Head, Journ. of Physiol., X, i and 279, 1889. 

 M Fredericq, Arch, der Physiol., 17, p. 563, 1901. 



