MOVEMENTS OE RESPIRATION ON THE CIRCULATION OE THE BLOOD. 583 
between the thoracic movements and those of the heart do not depend on mechanical 
conditions peculiar to the mode of experiment ; and they furnish additional proof that 
it is not affected by the abnormal psychical conditions of the animal, which in this in- 
stance was throughout under the influence of morphia. It may therefore be assumed 
that in the dog, so long as the respiratory passage is sufficiently open to allow of the 
entrance of air into the chest, the act of inspiration is invariably followed in normal 
breathing by increase of tension and shortening of the diastolic interval, i. e. acceleration 
of the heart’s action*. It remains to be considered by what instrumentality this influence 
is exercised. The facts indicate that the result consists in an alteration of the mode of 
contraction of the heart. That part therefore of the nervous system which presides over 
the movements of that organ must be concerned in its production. But the effect may 
be brought about either by agencies which are entirely mechanical, i. e. altered relations 
between the pressures existing in different parts of the circulation, or may be also more 
or less due to changes in the chemical state of the circulating fluid. For this reason 
the proper course seems to be, first to determine to what extent the increased activity 
of the heart which follows each inspiration may be accounted for as a mechanical effect 
of the expansion of the chest. Then, even if it be found that the whole of the observed 
phenomena may be thus explained, it will still be open to question how far the chemical 
consequences of each respiration may be also concerned in their production. 
The effect of the respiratory movements on the arterial pressure stands in relation to 
the fact, demonstrated by Donders, that all the organs contained in the chest are kept 
when its walls are at rest (as e. g. after death) in a state of distension, so that the mass of 
the thoracic viscera has constantly a tendency to shrink to a smaller volume than that of 
the cavity in which they are contained. As all of these organs possess elasticity, they must 
necessarily all participate in any expansion of the whole mass, but inasmuch as they resist 
expansion in very different degrees, their participation is unequal. Of the four principal 
kinds of organs contained in the chest, viz. the lungs, veins, arteries, and heart, the 
arteries and heart (when contracted) are by far least capable of distension, for they are 
already distended by an internal pressure equal to that of 5 to 7 inches of mercury. 
Consequently in inspiration the arteries and contracting heart take little or no part in 
the amplification of the chest ; so that the increase of bulk produced by dilatation of 
the thorax is for the most part divided between the lungs, the great veins, and the heart 
when in a state of relaxation. The actual ratio between the resistance to expansion of 
the arteries and that of the veins may be inferred, from what we know of the relative 
tension of the blood in the two systems of vessels, to be about 20:1. 
In ordinary inspiration with free access of air two effects are produced. The tension 
of the air contained in the respiratory cavity is reduced, and the resistance to expansion 
of the lung is increased. These two conditions exercise a similar influence on the 
* If, however, the communication between the chest and the atmosphere is completely closed, the relation 
is reversed. The variations of blood-pressure in the arteries then become coincident in time and of similar 
extent with those of air-pressure in the thoracic cavity. This will he shown in my paper on apnoea. 
