592 
DE. SANDERSON ON THE INFLUENCE EXERCISED BY THE 
beyond their normal capacity admit of no further amplification. On the other hand, the 
marked rise of pressure which accompanies expiration, is no doubt, as Ludwig says, due 
to the compression of the air contained in the air-passages produced by the sudden con- 
traction of the chest. It resembles the elevation of arterial pressure, which has been 
already described as the immediate mechanical result of sudden inflation of the lungs. 
Similar effects always accompany violent expiratory movements. 
Conclusions. 
1. The force and frequency of the contractions of the heart may be influenced either 
by variations in the intrathoracic pressure — viz. the pressure to which its own surface or 
that of the intrathoracic blood-vessels is exposed — or by variations in the chemical state 
of the circulating blood. 
2. In natural breathing the influence exercised on the heart by each expansion of the 
chest is entirely mechanical. This may be inferred from its being no longer observed 
when the mechanism is altered, as in artificial respiration, as well as from the shortness 
of the interval by which the effect is separated from the cause. 
[This conclusion is strongly confirmed by observations which I have made in the course 
of my experiments on apncea, in which it was found that in animals asphyxiated by the 
continued inhalation of a limited quantity of air from a bladder, the normal relation 
between the thoracic movements and the arterial pressure and frequency of the pulse 
remains unaltered.] 
3. The degree in which this influence is exercised varies according to the state of the 
circulation. It is greatest when the systemic veins are full, the circulation rapid, and 
the arterial pressure low. Under the opposite conditions it can scarcely be recognized. 
4. In tranquil breathing the influence exercised by variations of air-pressure in the 
bronchial tubes and vesicles of the lung (amounting to about 0"‘02 of mercury) (Donders) 
is so slight as to be inappreciable ; and even when the extent of variation is much in- 
creased by narrowing the aperture through which air passes in and out of the chest (as 
in the experiments related in Observation XII.), no effect is observed which can be attri- 
buted to the impeded influx and efflux of air. 
5. In forcible breathing the effects of variations of air-pressure may be recognized. 
This is particularly the case as regards violent expulsive movements ; for whereas the 
effect of normal expiration is not appreciable, violent expiration is always accompanied 
by a simultaneous increase of arterial pressure, as e. g. in animals in which the vagi have 
been divided. In this case the elastic contractility of the lung being expended in ext 
pelling the air contained in the chest, the negative pressure on the external surface of 
the great arteries is correspondingly diminished, and hence the positive pressure against 
their internal surfaces is increased. 
6. The increased action of the heart which results from chemical changes produced 
in the circulating fluid by exposure to air resembles the mechanical effect of inspiration ; 
both being indicated by increase of arterial tension and acceleration of the pulse. The 
