CIRCULATION OF THE BLOOD 5 



This time its course must be through one particular 

 and designated organ, the lung. In this second journey 

 our particle of blood again passes through an artery 

 which divides up, becoming smaller and smaller as it 

 does so. The division takes place on this occasion in 

 the lung. At last, still in the lung, a capillary network 

 is again reached. Passing through these capillaries 

 the particle goes on until it reaches again a small vein 

 which passes into a larger and larger vein. Through 

 these the particle goes on until it reaches the heart once 

 more. Finally it arrives at the actual cavity of the heart 

 from which it started. 



To get back to its starting-point, our particle of blood 

 has thus to go through the journey, heart-cavity, artery, 

 capillary network, vein, at least twice. There are thus 

 two main capillary circulations. One of them passes 

 through the lung, and is called the pulmonary or lesser 

 circulation. The vessels of the other capillary circula- 

 tion, which we began by describing in greater detail, 

 penetrate almost every part of the body, and this 

 circulation is spoken of as the systemic or greater 

 circulation. 



Each of these is spoken of as a circulation, not because 

 the movement is actually in a geometrical circle, or any 

 figure resembling a circle, but because a circle is a figure 

 that ends at the point at which it started. You must 

 nevertheless remember that the word circulation should 

 not, in absolute strictness, be applied to either of these 

 by itself, since a particle of blood cannot get back to the 

 actual cavity of the heart from which it started unless it 

 passes through both circulations. There are thus, strictly 

 speaking, not two circulations, but only one circulation. 



Let us now turn to examine the different parts of 

 the circulation somewhat more minutely, beginning with 

 the heart itself. The heart is a very complicated structure. 



