476 PHYSIOLOGY CHAP. 



We have seen that when artificial respiration with the bellows 

 is performed on any animal, the lungs being rhythmically dilated 

 in proportion as the normal limits are transcended, the animal, 

 owing to the auto-regulation set up by the vagus, reacts to each 

 insufflation by a respiratory movement. After a few insufflations, 

 however, these reactions diminish, and soon cease altogether. If 

 artificial respiration is now suspended, the animal remains for a 

 few seconds, half a minute at the outside, without breathing, it 

 has become apnoeic. To Rosen thai this phenomenon of apnoea 

 appeared to be an experiment-urn crucis in favour of the doctrine 

 which subordinates the respiratory movements to the degree of 

 venosity of the blood. When there is a relative augmentation of 

 C0 2 , and a relative diminution of O 2 , there is in the time unit 

 a corresponding augmentation in the respiratory capacity. The 

 latter then depends upon, and is in strict ratio with, the venosity 

 of the blood. The vagi, according to Eosenthal, only distribute 

 the work of the respiratory muscles in various ways, since it 

 remains approximately the same after section of these nerves. 



But the forced apnoea of artificial respiration is by no means 

 so simple a phenomenon as was assumed by Rosenthal ; indeed 

 it is highly complex. It can be easily demonstrated that it 

 depends not so much on the diminished venosity of the blood, as 

 on an inhibition or reflex paralysis of the rhythmical activity of 

 the centres, determined by mechanical excitation of the centripetal 

 pulmonary branches of the vagi. Brown -Sequard (1877) was the 

 first who brought forward this opinion, founding it on the fact 

 (subsequently confirmed by all experimenters) that apnoea is 

 entirely absent or lasts for a few seconds only, when forced 

 respiration is employed after section of the vagi. It is therefore 

 conditional on the integrity of those nerves. 



On the other hand, there are not wanting facts which show 

 that the diminished venosity of the blood is of secondary importance 

 in determining apnoea. In 1865 Thiry noticed that he was able to 

 produce apnoea even when air mixed with half its bulk of some 

 indifferent gas, such as hydrogen, was insufflated. It was sub- 

 sequently found by Fredericq, Gad, and Knoll that in order to 

 produce apnoea, it is not necessary to insufflate with pure air, but 

 that provided the vagi are intact, repeated insufflation with the 

 same air (which becomes more and more charged with carbonic acid 

 and poorer and poorer in oxygen) suffices to produce the phenomenon. 

 Knoll further observed that after prolonged pulmonary ventilation, 

 the apnoeic state is persistent even when the blood becomes blackish, 

 i.e., has assumed the character of asphyxial blood. On exposing 

 the heart in a rabbit, by removal of the sternum, without opening 

 the pleura (which is possible in this animal owing to the persist- 

 ence and bulk of the thymus) and inducing apnoea by energetic 

 artificial respiration, Gad observed that the right auricle preserves 



