384 TEXT-BOOK OF PHYSIOLOGY. 



the remainder of the period, however, inspiration is attended by 

 a rise, expiration by a fall of pressure. The explanation of these 

 results lies in the fact that at the beginning of the inspiration, when 

 the vessels dilate, the blood-flow momentarily slows; the left heart 

 continuing to discharge small volumes into the aorta, the pressure 

 continues to fall. So soon as the left heart begins to be better filled, the 

 pressure at once begins to rise. At the end of an expiration the flow 

 of blood into the left heart continues and the pressure rises, but with 

 the return of the intra-thoracic pressure the vessels diminish in caliber, 

 the volume of blood transmitted by them becomes less, the output 

 of the left heart declines, and the pressure falls. 



