MOVEMENTS OE EESPIRATION ON THE CIRCULATION OE THE BLOOD. 587 
of August, when immediately after the thoracic movements had ceased, air was injected 
for some time at regular intervals, at first of ten seconds, and subsequently of fifteen 
seconds. In both cases each inflation was followed by an increase of arterial tension, 
and an acceleration of the pulse. The result of one of these experiments is shown in 
Plate XXIY. fig. 6, the interval between each inflation and its successor being fifteen 
seconds. 
2. It soon, however, appeared that the relation observed on this occasion was not con- 
stant ; for even in the same animal it was subsequently found that it could no longer be 
traced when the interval was shortened to five seconds ; the variations of arterial pres- 
sure and of pulse-frequency in this case resembled those previously seen, but it was only 
every third stroke of the bellows that appeared to be effectual (see Plate XXV. fig. 4). 
A similar effect had been previously observed in the experiment of the 11th of August. 
Again, it was repeatedly noticed that even when artificial respiration was entirely dis- 
continued, rhythmical variations in the force and frequency of the heart’s action mani- 
fested themselves. This is well seen in Plate XXY. fig. 3, taken about an hour after the 
solution had been injected. Similar undulations were observed during apnoea in all the 
animals experimented upon, the intervals varying from five to fifteen seconds. 
3. The results stated in the preceding paragraphs seem to show (1) that there is a 
marked tendency to periodical variations in the activity of the heart of animals under 
the influence of woorara, and (2) that these variations are for the most part independent 
of external agencies ; and it seems not improbable that the apparent relation observed 
between the artificial thoracic movements and the fluctuations of the mercurial column 
in certain cases, may be due to the mechanical stimulation of the heart by the sudden 
inflation of the chest. However this may be, subsequent observations show that a much 
more marked and constant influence is exercised by the injection of air into the chest 
under other circumstances. In my experiment of June 2nd artificial respiration was 
commenced about a minute and a half after the natural breathing had ceased, and the 
inflations were continued at intervals of ten seconds, just as in the experiment of the 
previous year. About five seconds after each stroke, as is well seen in the tracing 
(Plate XXIY. fig. 5), the arterial pressure rose. Here the length of the interval between 
the two events at once suggested that the relation between them could not possibly be 
mechanical. 
4. In other observations which were made after several minutes’ discontinuance of 
artificial respiration this was still more evident. In all the animals I found that the 
heart was much less affected by the privation of air than in the normal state. Thus the 
inflations could be suspended for three or four minutes without making any material 
alteration in the character of the oscillations of the mercurial column; but when apnoea 
was still further prolonged the arterial pressure gradually subsided from 5 or 6 inches 
to 2 or 3. If under these circumstances air was injected no immediate effect was pro- 
duced, excepting a slight elevation of the arterial lever simultaneous with the inflation. 
After the expiration of six or seven seconds the pressure began to rise, while the heart’s 
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