author's schema op the circulation. 271 



'cock so as to present some resistance to the escape of air, and 

 then compress the india-rubber ball, very little air will issue 

 from the stopcock even while we are squeezing the ball ; the 

 greater part of it goes to distend the bag ; and, when we cease 

 to compress the ball, no air at all comes out from the stopcock. 

 At the next squeeze, the bag becomes a little more distended ; 

 and a little air issues from the stopcock, not only while we are 

 compressing the ball, but even when we relax our grasp. At 

 each squeeze of the ball, the elastic bag becomes tighter, 

 till it is so tense and contracts so strongly on the air inside, 

 that it can press all the extra amount of air forced into it when 

 the ball was compressed, out through the stopcock, during the 

 time when the ball is relaxed. AVhen this is the case, every 

 time we squeeze the ball we see the bag become a little fuller, 

 and air issue more quickly from the nozzle. At each relaxation, 

 while the ball is refilling, the bag gets a little slacker, and the 

 air passes out of the nozzle a little more slowly, but never stops 

 entirely. During the time the ball is filling, the valves between 

 it and the bag and nozzle are closed, and cut it oft' from any 

 connexion with them. All this time, then, the stream of air 

 from the nozzle must be entirely independent of the ball ; it is 

 produced by the contraction of the elastic bag, and by it alone. 

 The bag may be stretched, and the tension of its walls increased 

 in consequence, in two ways : first, by working the ball more 

 quickly ; second, by lessening the opening of the nozzle, and 

 thus hindering the passage of air through it. One trial will, I 

 think, be enough to show you how much easier it is to alter the 

 pressure by changing the size of the nozzle than by any alteration 

 in the working of the ball, and thus convince you that altera- 

 tions in blood-pressure probably depend much more on altera- 

 tions in the lumen of the small arteries than on changes in the 

 action of the heart. 



But our schema, as it at present exists, is not a perfect repre- 

 sentation of the heart and vessels ; for it draws its air from an 

 inexhaustible reservoir, the atmosphere, and is not obliged each 

 time to use that amount alone which it had previously driven 

 tlirough the nozzle ; while the heart can only use the blood 

 which has been forced by it through the capillaries and returned 



