148 OUR BODIES AND HOW WE LIVE 



and the cells of the tissues is filled with a clear, watery 

 fluid known as lymph. 



As the blood flows along the capillaries, certain parts of 

 the plasma of the blood filter through their walls into the 

 lymph, and certain parts of the lymph filter in the opposite 

 direction through the walls of the capillaries and mingle 

 with the blood current. A constant interchange of mate- 

 rial is thus going on between the lymph which bathes the 

 tissues and the blood in the capillaries. 



In brief, the lymph acts as a medium of exchange between the 

 blood and the tissues. 



207. The General Course of Circulation. We are now 

 ready to study the circulation as a whole, tracing the course 

 of the blood from a certain point until it returns to the 

 same point. We may conveniently begin with the portion 

 of blood contained at any moment in the right auricle. 



We may perhaps better understand the general course 

 of circulation if we subdivide it into two parts : the pul- 

 monary circulation, or circulation in the lungs, and the 

 systemic, or greater, circulation. 



We must keep clearly in mind, however, that there is 

 but a single circulation in the body. 



208. The Pulmonary, or Lesser, Circulation. Two large 

 veins are busily filling the right auricle with dark (venous) 

 blood, collected from all parts of the body. When the 

 auricle contracts, the blood cannot get back into the great 

 veins because it is flushed forward by the great volume 

 of blood behind it. The door opening into the right 

 ventricle lies open, and the blood flows through it until it 

 is full. 



The ventricle now begins to contract ; the tricuspid valve 

 at once closes and thus prevents the backward flow of 



