CH. XXIV.] EFFECT OF RESPIRATION ON CIRCULATION 367 



little later than the inspirator y act, and the fall a little later than 

 the expiratory act. 



It will also be seen that the heart beats more rapidly during the 

 rise of blood-pressure than during the fall. This difference disappears 

 when the vagi are cut. Kespiratory undulations, however, are still 

 present, though not so marked as before ; hence the cardiac variations 

 are not their sole cause. They are chiefly the result of the mechanical 

 conditions dependent on the Jungs and heart with its large vessels 

 being contained within the air-tight thorax. When the capacity of 

 the chest is increased in inspiration, the tension of the lung tissue 

 due to its greater expansion is increased ; hence the difference between 

 the intra-pleural pressure, and that in the lungs (which is atmos- 

 pheric) becomes more marked, for the difference of pressure is to be 

 measured by the elastic force of the lung tending to produce its 

 collapse. If the intra-thoracic pressure is measured, it is found that 

 it varies from 5 to 7 mm. of mercury at the end of expiration to 

 30 at the end of a deep inspiration ; that is to say, from 5 to 7 to 

 30 mm. less than the atmospheric pressure (760 mm. of mercury). 

 The pressure outside the heart and large vessels is correspondingly 

 diminished to the same extent, and produces its main effect (distension) 

 upon the veins because they are never fully distended, and because 

 the pressure within them is low. This increase in the "pressure 

 gradient" (i.e., the rate of fall of pressure) between the intra and 

 extra thoracic great veins results in a proportionately more rapid flow 

 of blood into the thorax, and therefore into the right side of the heart ; 

 for within certain limits the right heart can be easily expanded more 

 fully if a greater supply of blood is provided. Consequently, the 

 output from the right side increases, and thus vid the pulmonary 

 circuit the inflow into the left heart is increased ; in its turn, therefore, 

 the output from the left ventricle rises, and so the aortic pressure is 

 raised. If the aorta and its branches within the thorax were as 

 undistended as the veins and right auricle, this effect would be 

 counteracted, but inasmuch as the aorta and arteries are thick -walled 

 and already over-distended, an increased inflow into them must lead 

 to a further distension, i.e., a further rise of pressure. For we may 

 altogether neglect the change in rate of flow along these vessels due 

 to the change in pressure gradient, not because it is insufficient in 

 itself to produce a distinct change in the flow, if the blood were free 

 to move easily, but because the outflow from the arteries has to take 

 place through a high peripheral resistance, and this small pressure 

 change is not able to exert any appreciable effect in accelerating the 

 flow through such a high peripheral resistance. We must note, too, 

 that the change in pressure gradient would tend to decrease the out- 

 flow, not to increase it. The pressure gradient in arteries and in 

 veins are about equal in magnitude, that in the veins being probably 



