486 



PHYSIOLOGY 



CHAP. 



apnoea is produced suddenly or with a short decrease ; but the 

 resumption of respiration is preceded almost invariably by general 

 movements of the animal, without the gentle and regular increase 

 that always succeeds the apnoea produced by air (Fig. 224, B). 

 Often the return occurs with periodic respiration, which soon dies 

 away, and is replaced by the ordinary rhythm (Fig. 224, C). 



(/) After section of the vagi, complete and persistent apnoea 

 can no longer be obtained in birds, either by inflation with air, or 



FIG. 222. Effects of continuous pulmonary ventilation in fowls. (Luciani tincl Bordcni.) A, 

 Incomplete apnoea after ventilation commencing at V; B, gradual return to normal respira- 

 tion, after insufflation ceases, at <" 



with oxygen, no matter at what pressure the ventilation is 

 effected. Eespiration becomes weakened to a very marked extent 

 in pigeons, and to a less degree in turkeys and fowls (Fig. 225, A, 

 B, C). If the vagi are divided during apnoea, it will usually persist 

 for a certain time, owing perhaps to the effect of operative 

 traumatism. Sometimes, however, the respiratory movements are 

 reinstated immediately after section of one vagus (Fig. 226). 



FIG. 223. Effect of passing a small amount of CO* through the respiratory passages, during 

 apnoea from continuous ventilation, in turkeys. (Luciani and Bordoni.) 



Most of the interpretations of respiratory rhythm suggested 

 from Kosenthal onwards (Pfliiger, Hering, Kosenbach, Burkart, 

 Marckwald/Gad, etc.) start with the fundamental concept that 

 not only nutrition, but also functional activity, is maintained 

 in the respiratory centres by the blood circulating in them, 

 which, when normally constituted, acts as an external stimulus, 

 i.e. it contains stimulating factors such as carbonic acid or the 

 other products of tissue consumption. This general theory is 

 always based on the apnoea which inevitably sets in when 



