MOVEMENTS OF RESPIRATION ON THE CIRCULATION OF THE BLOOD. 589 
increase of pressure. Thus towards the close of my first experiment, August 11th, when 
the animal had been for a long time under the influence of the poison, and the pulse 
had increased in frequency, it was found that if artificial respiration was vigorously per- 
formed at regular intervals, a tracing could be obtained of the character shown in 
Plate XXV. fig. 1, in which it is seen that each injection of air is accompanied by a 
simultaneous but very slight elevation of the arterial lever, which, however, was not 
associated with acceleration of the heart’s contractions. This experiment was repeated 
under similar conditions on the 8th of June, when a corresponding but not so marked 
mechanical effect was observed. Ten minutes afterwards both pneumogastrics were 
divided in the neck. The arterial pressure at once rose to 9 inches, and the pulse attained 
a frequency of 240 per minute. The mercurial column remained at the same height for 
several minutes, during which the tracing (Plate XXV. fig. 2) was taken. Here, as in 
fig. 1, it is seen that the diminution of arterial pressure produced by each inflation is 
not accompanied with any acceleration of the heart’s contractions*. 
The facts related in the preceding paragraphs afford no answer to the question whether 
the phenomena observed are due to the direct influence of aerated blood on the heart 
itself, or to its indirect influence through the nervous centres over the rhythmical move- 
ments of the heart. Their principal significance in relation to the present inquiry consists 
in their affording ground for the inference that, whereas the effects of the thoracic 
movements in ordinary inspiration are almost immediate, a considerable time is required 
for the production of those which are due to chemical changes in the circulating blood. 
4. Experiments as to the relation between the arterial pressure and the thoracic 
movements after section of the pneumogastric nerves. 
It is the opinion of Ludwig (Lehrbuch der Physiologie des Menschen, Bd. ii. p. 163) 
that the increased frequency of the contractions of the heart which follows ordinary 
inspiration is in part owing to excitation of the pneumogastric nerves. This view he 
supports on the ground that, although an increase of arterial tension corresponding 
exactly to the increased expansion of the thorax is observed to accompany the expiratory 
act in animals in which the vagus has been divided on each side, this effect; is not asso- 
ciated with any acceleration of the pulse. I have already endeavoured to show that the 
ordinary respiratory variations of frequency of the heart’s action admit of a more simple 
explanation ; it is, however, not the less necessary to investigate the facts in question, 
which certainly at first sight seem to suggest such an inference as that drawn from them 
by Ludwig. 
It is well known that the immediate effect of section of the vagi in the neck is to 
diminish the frequency of respiration, and to accelerate the contractions of the heart. 
There are, however, several points relating to these changes which must be referred to 
here in their bearing on our present inquiry. The mode of respiration, after section, is 
* From the extreme frequency of the contractions of the heart the oscillations of the arterial lever are not 
distinguishable in the engraving, although they are 'well defined in the original. 
4 l 2 
