CIRCULATION OF THE BLOOD 9 



This is only a very rough description of the circulation 

 of the blood. It leaves out of account some very im- 

 portant factors, such, for instance, as the movements of 

 the chest wall during breathing, and in general the effect 

 of respiration on the circulation. It leaves out of account 

 also the fact, which may be gathered from the diagram 

 (Plate I.), that the veins from certain organs, as, for 

 example, the intestine, break up into capillaries for a 

 second time before returning to the heart. Bearing these 

 omissions in mind, however, we may turn to a discussion 

 of the process by which it was discovered. This discovery 

 was the work of an Englishman, William Harvey. Before 

 we can understand his work we must learn something of 

 what men believed before his time concerning the action 

 of the heart and blood-vessels. 



