228 THE CIRCULATION OF THE BLOOD [CH. XIX. 



circulatory system is shown in fig. 239 A. Here there is a closed 

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

 ment (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 contraction 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. 



FIG. 239. Simple schema of the circulation. 



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

 tion. Fig. 239 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 could 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. In 

 general terms fluid flows from where the pressure is high to where it 

 is lower. This difference of pressure is produced in the first instance 

 by the contraction of the heart, but we shall find in our study of the 

 vessels that some of this pressure is stored up in the elastic arterial 

 walls, and keeps up the circulation during the periods that the heart 

 is resting. 



Coming to different groups in the animal kingdom we may take 

 the crayfish or the lobster as instances of animals which possess a 



