A "HEAD OF PRESSURE" in 



on very careful measurements made in London by Dr 

 Thomas Lewis. 1 The muscular wall of the left ventricle 

 varies in thickness according to the state of contraction 

 and the part of the wall which is measured, but we may 

 look upon half an inch as a common amount. The 

 problem of grouping tens of thousands of contractile 

 cylinders in the wall of a pump, so that each of them can 

 exert its full power in forcing the blood from the pump 

 chamber, taxed the ingenuity of Nature to its utmost. 

 The difficulty is at once manifest when we take in our 

 hand a hollow thick- walled india-rubber ball filled with 

 water and try to empty it by compressing the thick wall 

 with our fingers. The compressing fingers represent the 

 muscular wall or engine of the ventricle ; we find it 

 impossible to squeeze the ball uniformly, and thus force 

 out its contents by obliterating its cavity. Yet Nature, 

 using a ventricular chamber which is half an inch thick 

 or more in some parts, succeeds in bringing the .walls 

 uniformly inwards so that the cavity is obliterated and 

 practically the whole pumpful of blood discharged. 

 This she has succeeded in doing by arranging layers of 

 muscle cylinders in overlapping spirals, with the result 

 that when they contract they move inwards, much in the 

 same way as the segments of the iris diaphragm, used by 

 photographers, move inwards on each other until only a 

 pin-hole aperture is left. Thus the left ventricle is a very 

 remarkable kind of pump — one with walls built out of 

 the engine which actually sets it in motion, and in this 

 way drives the blood onwards. 



There is one defect which all force-pumps have : their 

 outflow is jerky or intermittent. Engineers have over- 

 come this defect by adopting a very simple contrivance. 

 The outflow pipe (fig. 29) leads to a closed chamber 

 which contains air in its upper part. The pipe leading 

 from the air chamber is fitted with a stopcock so that 

 the water may not flow away from the air chamber as 

 quickly as it enters from the pump. Each stroke not 

 only forces a pumpful into the air chamber, but also com- 



1 Heart, 19 14, vol. v. p. 367. 



