MOVEMENTS OE RESPIRATION ON THE CIRCULATION OE THE BLOOD. 585 
The explanation cannot, however, be regarded as complete unless reference is made to 
other conditions not yet taken into account, i. e. to variations of the velocity of the cir- 
culation, and of the pressure existing in the systemic veins. For although there can be 
no doubt as to the direction in which increase or diminution of the external pressure to 
which the thoracic veins are exposed must affect the quantity of blood which they convey 
to the heart, the degree in which their influence is exercised must depend on the 
fulness of the veins outside of the chest. In healthy animals these two conditions are 
inseparably associated together. The more rapid the circulation, the fuller are the veins 
as compared with the arteries. When the veins are distended and the movement of the 
blood is rapid, the filling of the cavities of the heart, in diastole, takes place in a shorter 
period, while at the same time the contraction of both auricle and ventricle is more 
sudden and effective in consequence of the diminished arterial resistance. In the oppo- 
site case, when the veins are empty and the movement of the blood within them is 
sluggish, the Cavities of the heart fill slowly, and empty themselves imperfectly in con- 
sequence of the excessive arterial resistance. 
This being admitted, it may be readily understood that the effect of inspiration is 
likely to be materially influenced by the relative velocity and tension of the arterial and 
venous circulations. In the one case the right auricle, at the moment of commencing 
diastole, is still full of blood (i. e. when the arterial tension is high, the veins empty and 
the circulation retarded), in the other the right auricle is empty at the end of systole. 
In other words, when the thoracic veins are almost emptied by the heart, at each con- 
traction the effect of thoracic expansion is far greater than when the intrathoracic 
veins, even in their emptiest condition, are much fuller than those that lie outside of the 
chest. 
It being admitted that the expansion of the chest not only aids the filling of the heart 
during diastole, but affords it an abundant supply of blood, the shortening of the diastolic 
period, and the increase of arterial tension may be readily understood. Inasmuch as 
the heart possesses the property of contracting the instant that its walls are dilated with 
blood to the proper degree, it is manifest that the more rapidly the heart fills the shorter 
must be its interval of relaxation, and the more frequent its contractions. It is no less 
obvious that increased pressure must be produced by the same agency ; for, provided that 
the ventricles are well filled with blood at the moment that each systole commences, 
the more frequently they contract the greater will be the quantity of blood forced into 
the systemic arteries, and hence the higher will be the arterial tension. 
I venture to think that the explanation I have offered of the phenomena observed is 
complete and satisfactory, and that it will be found to be consistent with all that has 
been previously ascertained. But I do not deem it the less necessary to pursue the 
investigation further, for by so doing I shall certainly strengthen the basis on which my 
theory is founded, and anticipate objections which might otherwise be made to it. 
Admitting, then, that the influence of the respiratory movements on the heart is partly 
mechanical, I proceed to inquire whether it is not also partly chemical. This may be 
