THE RESPIRATORY SYSTEM. 401 
etc., with a pressure equal to that of the atmosphere. It follows 
that the deeper the inspiration the greater the difference be- 
tween the intra-thoracic and the atmospheric pressure. Even 
in expiration, except when forced, the intra-thoracic pressure 
. remains less, for the same reason. 
These conditions must have an influence on the heart and 
blood-vessels. Bearing in mind that the pressure without is 
practically constant and always greater than that within the 
thorax, the conditions are favorable to the flow of blood toward 
the heart. As in inspiration, the pressure on the great veins 
and the heart is diminished, and, as these organs are not rigid, 
they tend to expand within the thorax, thus favoring an on- 
ward flow. But the opposite effect would follow as regards the 
large arteries. Their expansion must tend to withdraw blood. 
During expiration the conditions are reversed. The effects on 
the great veins can be observed by laying them bare in the 
neck of an animal, when it may be seen that during inspiration 
they become partially collapsed, and refilled during expiration. 
In consequence of the marked thickness of the coats of the 
great arteries, the effect of changes in intra-thoracic pressure 
must be slight. The comparatively thin-walled auricles act 
somewhat as the veins, and it is likely that the increase of 
pressure during expiration must favor, so far as it goes, the 
cardiac systole. 
More blood, then, entering the right side of the heart dur- 
ing inspiration, more will be thrown into the systemic circula- 
tion, unless it be retained in the lungs, and, unless the effect be 
counteracted, the arterial pressure will rise, and, as all the con- 
ditions are reversed during expiration, we look for and find 
exactly opposite results. The lungs themselves, however, must 
be taken into the account. During inspiration room is pro- 
vided for an increased quantity of blood, the resistance to its 
flow is lessened, hence more blood reaches the left side of the 
heart. The “immediate effect would be, notwithstanding, some 
diminution in the quantity flowing to the left heart, in conse- 
quence of the sudden widening of the pulmonary vessels, the 
reverse of which would follow during expiration; hence the 
period of highest intra-thoracic pressure is after the onset of 
the expiratory act. During inspiration the descent of the dia- 
phragm compressing the abdominal. organs is thought to force 
on blood from the abdominal veins into the thoracic vena cava. 
That the respiratory movements do exert in some way a 
pronounced effect on the circulation the student may demon- 
26 
