584 
DE. SANDEESON ON THE INFLUENCE EXEECISED BY THE 
thoracic veins and heart. The former, however, is of little importance, for its operation 
is inconsiderable in itself, and is moreover confined to the period during which the 
thorax is actually expanding. It is to the latter condition (the resistance offered by 
the contractility of the lung to expansion) that the effect of inspiration is mainly refer- 
able. If the veins, like the lungs, contained air, and communicated freely with the atmo- 
sphere, they would evidently expand as rapidly. Actually their expansion is much slower, 
so that during the act of inspiration the relation between their expansibility and that of 
the lung is altered, the proportion of the thoracic space occupied by the lungs being 
increased, that occupied by the veins being diminished. Correspondingly the resistance 
to dilatation of the lungs, as compared with the resistance offered by the mass of intra- 
thoracic organs, is increased, that of the veins diminished. If the chest continues ex- 
panded the balance between the two resistances is gradually restored, that is to say, the 
veins fill with blood until their distension attains the same proportion to that of the 
lungs which it possessed before inspiration. Hence it follows that the repletion of the 
veins produced by inspiration varies in degree according to the length of the period 
during which the expansion of the chest continues, so that by a short inspiration, how- 
ever deep it may be, scarcely any effect will be produced on the circulation. As during 
diastole the cavities of the heart are affected by the movements of the thorax, in pre- 
cisely the same manner and probably in about the same degree, the preceding conside- 
rations are as applicable to them as to the veins, mutatis mutandis. 
In expiration a slight increase of tension of the air contained in the air-passages takes 
place. But if the efflux of air is free and unrestrained, this influence is so inconsider- 
able as to be without influence on the thoracic organs. The only way in which expira- 
tion can materially affect the circulation is by diminishing the capacity of the thorax. 
In regular breathing its effect must be always equal to that of inspiration ; for whatever 
increase of the calibre of the veins results from the expansion of the chest, must be 
reduced when it collapses. 
But if in any expiratory act more air is expelled by the forcible contraction of the 
expiratory muscles than has previously been inhaled, the capacity of the veins will be 
thereby reduced in a degree proportional to the diminution of the capacity of the chest 
itself. Thus, if it were possible for the chest to be so contracted by the action of the 
expiratory muscles as to allow the lungs to collapse to a bulk equal to that which they 
assume when left to themselves, their tendency to contract would be in abeyance and 
their distending influence on other organs contained in the chest would no longer be 
exercised. Similarly, in all less degrees of contraction, the distension of these organs 
must be proportionably diminished. In other words, the difference between the pressure 
to which the thoracic veins are exposed and that of the atmosphere (the so-called negative 
pressure), varies with the volume of the thoracic cavity provided that the air-passages are 
open. 
The preceding considerations lead to the conclusion that the dilatation of the chest in 
inspiration aids the expansion of the heart during the diastole, and of the thoracic veins. 
