228 



THE CIRCULATION OF THE BLOOD. [CH. xix. 



and Leeuwenhoek, seven years later, saw the circulation in the 

 lung of the frog. 



We can now proceed to study some of the principles on which 

 the circulation depends : 



The simplest possible way in which we could represent the 

 circulatory system is shown in fig. 231 A. Here there is a closed 

 ring containing fluid, and upon one point of the tube is an 

 enlargement (H) which will correspond to the heart. It is 

 obvious that if such a ring made of an ordinary Higginson's 

 syringe and a tube were placed upon the table, there would be no 

 movement of the fluid in it ; in order to make the fluid move 

 there must be a difference of pressure between different parts of 

 the fluid, and this difference of pressure is caused in the fluid by 

 the pressure on it of the heart walls. If, for instance, one takes 

 the syringe in one's hand and squeezes it, one imitates a con- 



Fig. 231. Simple schema of the circulation. 



traction of the heart : if the syringe has no valves, the fluid 

 would pass out of each end of it in' the direction of the two 

 arrows placed outside the ring. When the pressure on the 

 syringe is relaxed (this would correspond to the interval between 

 the heart beats), the fluid would return into the heart again in 

 the direction of the two arrows placed inside the ring. This, 

 however, would be merely a to and fro movement, not a circula- 

 tion. Fig. 231 B shows how this to and fro movement could, by 

 the presence of valves, be converted into a circulation ; when the 

 heart contracts the fluid could pass only in the direction of the 

 outer arrow ; when the heart relaxes it coiild pass only in the 

 direction of the inner arrow ; the direction of both arrows is 

 the same, and so if the contraction and relaxation of the heart 

 are repeated often enough the fluid will move round and round 

 within the tubular ring. 



The main factor in the circulation is difference of pressure. 



