272 EXPERIMENTAL INVESTIGATION OF THE ACTION OF MEDICINES. 



to it by the veins. In order to make our schema complete, we 

 must connect its two ends by tying them into a Wadder or large 

 thin caoutchouc bag (such as is used, after inflation, as a toy for 

 chilclreu), so that the air shall pass into it from tlie nozzle and 

 be sucked out of it by the elastic ball. This will represent the 

 veins. If we then repeat the experiment just described, we 

 shall llnd that, wdien we begin to work the ball and stretch the 

 elastic bag representing the arteries, the bladder, representing 

 the veins, becomes empty and collapsed; and just in proportion 

 as we fill the bag do we empty tlie bladder. If we now stop, 

 the air will gradually escape from the bag to tlie bladder, till 

 both are equally filled as they were at first. 



Circulation in the Living Body. — The phenomena of the cir- 

 culation in the lieart and vessels are very much the same as in 

 the spray-producer. When the heart stands still (as when the 

 vagus is strongly galvanised), the blood flows from the arteries- 

 into the veins till they are nearly full and the pressure inside 

 both is about the same. If the heart now begin to beat, it 

 forces blood into the elastic aorta and arteries at each systole^ 

 and distends them, just like the elastic bag of tlie spray- 

 producer ; while at the same time it takes blood from the veins,. 

 and they become empty in proportion as the arteries become 

 full. At every diastole, the elasticity of the distended aorta 

 causes it to contract on the blood it contains, and keeps it 

 .flowing on through the capillaries till another systole occurs. 

 During the diastole, the heart is completely shut off from the- 

 aorta by the sigmoid valves (just as the ball of the schema was- 

 shut off from the elastic bag), and the blood is kept flowing 

 during this time by the elastic contraction cf the aorta and 

 large arteries. In general, the diastole is longer than the 

 systole ; so that for the greater part the circulation is carried on 

 by the elasticity of the arteries, and not directly by the heart.. 

 The arteries become distended by the heart, just as the elastic 

 bag was by the ball, and press more and more on the blood in 

 them (so that it would spout higher and higher, if one of them 

 w^ere cut), till they are able during the diastole to press the 

 same amount of blood through the capillaries into the veins as 

 had been pumped into them during the systole. The more 



