360 D. OGATA AND SWALE VINCENT 



creased respiration never occurs. Thus it would not be un- 

 reasonable to assume that figure 2 shows real vasomotor reflexes, 

 even though weak, not complicated by the increased respiratory- 

 movements, while figure 1 represents the vasomotor reflex 

 masked by the effects of increased respiration. 



It is often very difficult or almost impossible to obtain any 

 rise of blood-pressure when the respiratory movements are very 

 violent. In those cases a marked fall is the only result of cen- 

 tral stimulation of afferent fibers. 



How these increased respiratory movements affect the blood- 

 pressure was very carefully investigated by Vincent and Cam- 

 eron. After pointing out several possible causes, they came to 

 the conclusion that this fall is due to direct mechanical interfer- 

 ence with the heart's action and with the return of the blood to 

 the heart. 



In order to confirm this theory, we opened the thorax in the 

 middle line as did Vincent and Cameron, and found that the falls 

 disappeared. The contrast is clearly shown in figures 3 and 4. 



In these two cases the same nerve of the same animal was 

 stimulated with the same strength of stimulus, in figure 3 in the 

 intact animal and in figure 4 with thorax open. 



In addition to opening the thorax we cut both vagi, both 

 phrenici, and as many intercostals as possible on both sides, 

 without obtaining very different results from those obtained by 

 merely opening the thorax. 



In a very few cases a marked fall of a similar character to that 

 due to the increased respiratory movements, was observed in 

 animals with thorax wide open. It was, however, soon dis- 

 covered that this fall was produced by compression of the in- 

 ferior vena cava by the heart which became more freely movable 

 than before through opening the thorax. The heart fell back 

 upon the soft-walled vein, and thus diminished the flow of blood 

 to the right heart. 



But it is certainly true that by means of almost pure vaso- 

 motor reflex, i.e., without any or with very little increase of 

 respiratory movements one can obtain a marked fall preceded 

 by a rise, as shown in figure 5. 



