CH. xiv. DOUBLE CIRCULATION OF THE BLOOD. in 



LEFT 



The blood has now made one round, during which its 

 colour has been changed from a bright scarlet to a dark purple, 

 in consequence of its having parted with oxygen and gained 

 carbonic acid in its passage through the different parts of the 

 body. But it does not stop here. 

 It escapes through a valve down 

 into the lower chamber c ; out of 

 the right top corner of which it 

 starts again in the direction of 

 arrows 8 and 9, and passes through 

 the lungs, returning by the lung- 

 veins, or pulmonary veins as they 

 are called, in the direction of 

 arrow 10, back into the left top 

 chamber of the heart d. During 

 this second journey it is acted 

 upon by the air in the lungs and 

 rendered pure and bright again by 

 taking in fresh oxygen. It then 

 passes down from d into the 

 chamber a, from which it first 

 started, and the whole round 

 begins again. The first journey 

 of the blood round the whole 

 body is called the general circula- 

 tion, and the second journey through the lungs is called the 

 pulmonary circulation ; when Harvey had traced these two 

 journeys he had proved the double circulation of the blood. 



Although this discovery as stated here appears very 

 simple, yet it took Harvey nineteen years to trace the blood 

 through all the channels of the body, before he felt quite 

 certain that he had hit upon the truth. Meanwhile he had 

 returned to London, and had been made physician at St 



FIG. 13. 



Diagram of Heart and Blood-vessels 

 seen from the front. 



a c, Lower chambers of the heart, 

 called ventricles, b d, Upper 

 chambers of the heart, called 

 auricles. The arrows and num- 

 bers show the course of the 

 blood. 



