538 
DE. SANDEESON ON THE INFLUENCE EXEECISED BY THE 
contractions became more frequent. This effect usually lasted for several seconds, 
during which the mercurial column attained an elevation which sometimes equalled and 
sometimes exceeded that which existed before apnceawas produced; in the latter case it 
soon relapsed to the normal level. Observations of this nature were made on each occa- 
sion. The tracing on Plate XXV. fig. 5 was obtained on the 8th of June. After about 
three minutes of apnoea, a slight inflation was made, twenty-two minutes having elapsed 
since the injection of the solution. The arterial pressure had sunk from 5 inches to 3. 
The interval between inflation and its effect was six seconds. The increased action of the 
heart lasted fourteen seconds, after which the mercury subsided to its former level. Again, 
on the 2nd of June (Plate XXV. fig. 6), twelve minutes after the injection of woorara, 
artificial respiration was discontinued for four minutes, at the end of which time the 
arterial pressure had sunk to 3 inches. The effect followed the cause at an interval of 
seven seconds, and lasted for about ten seconds. About half a minute after, the clock- 
work was stopped, and the animal was again deprived of air for four minutes. The 
pressure having again sunk to 3 inches, the inflation was repeated with a perfectly similar 
result. An observation of the same nature was made in 1865. Apnoea had existed for 
four minutes, the injection of woorara having been made one hour and ten minutes previ- 
ously. The usual mechanical effect accompanied the inflation of the chest, but no more 
permanent elevation of arterial pressure occurred until about eight seconds later. It is 
to be noted that the effects described above could only be obtained when the arterial 
pressure had been considerably reduced by apnoea ; for if the inflation of the chest had 
been discontinued for shorter periods no material increase of tension was produced by 
resuming it. 
5. As in all the observations recorded in the preceding paragraph the interval between 
each inflation and its effect was too long to admit of any mechanical explanation, the 
phenomenon can only be referred to the chemical action of the injected air on the cir- 
culating blood. To test this I repeated the experiment on two occasions, with this 
difference, that hydrogen was substituted for air. In each case the result was the same. 
An observation of this nature was recorded on the 2nd of June ; artificial respiration 
had been suspended until the arterial pressure had sunk to 3'4 inches. The chest was 
then fully inflated with hydrogen, when it was observed that the mercury, instead of 
rising at the sixth or seventh second, remained at the same level. Fourteen seconds after, 
a very slight injection of air was made, which was followed by the usual result. Still 
later the lungs were fully inflated with air, in consequence of which the pressure rose 
to 6 inches. 
6. It is obvious that if artificial respiration exercises any mechanical effect at all on 
the circulation, that effect must be not only different from that produced by the natural 
movements, but of an opposite nature. For when air is injected it is evident that the 
expansion of the thoracic veins must be diminished, just as it is increased by natural 
inspiration. I have had several opportunities of observing that this is actually the case ; 
i. e. that the invariable effect of inflation is to induce an immediate but very slight 
