CHAP, iv.] THE VASCULAR MECHANISM. 



167 



but the pressure of the atmosphere is bearing on the fluid in the 

 tubes, and that equally all over. 



FIG. 30. ARTERIAL SCHEME. 



P, unshaded, is an elastic tube to represent the arterial system branching at 

 X and Y, and ending in the region of peripheral resistance, including the capillaries, 

 which are imitated by filling loosely with small pieces of sponge the parts shewn as 

 dilated in the figure. The capillaries are gathered up into the venous system, shaded, 

 which terminates at 0. Water is driven into the arterial system at P by means of 

 an elastic bag-syringe, or any other form of pump. Clamps are placed on the 

 undilated tubes c, c', c". When these clamps are tightened, the only access for the 

 water from the arterial to the venous side is through the dilated parts filled with 

 sponge, which offer a considerable resistance to the flow of fluid through them. 

 When the clamps are unloosed the fluid passes, with much less resistance, through 

 the undilated tubes. Thus by tightening or loosening the clamps the "peripheral" 

 resistance may be increased or diminished at pleasure. 



At A, on the arterial side, and at V, on the venous side, manometers can be 

 attached. At a and v (and also at x and //) by means of clamps, the flow of fluid 

 from an artery and from a vein, under various conditions, may be observed. At Sa, 

 S'a, and Si', sphygmographs may be applied. 



If now, the connecting pieces being freely open, that is to say, 

 the peripheral resistance being very little, we imitate a ventricular 

 beat by the stroke of the pump, we shall observe the following. 

 Almost immediately after the stroke the mercury in the arterial 

 manometer will rise, but will at once fall again, and very shortly 

 afterwards the mercury in the venous tube will in a similar manner 

 rise and fall. If we repeat the strokes with a not too rapid rhythm, 

 each stroke having the same force, and make, as may by a simple 

 contrivance be effected, the two manometers write on the s;\nu> 

 recording surface, we shall obtain curves like those of Fig. 31, 

 A and V. At each stroke of the pump the mercury in the 

 arterial manometer rises, but forthwith falls again to or nearly tu 



