124 LETTERS TO BROTHER JOHN. 



The blood, therefore, having fulfilled its func- 

 tion, quits the arteries, and enters the veins. 



I have said, that when the arteries cease to be 

 arteries, and become veins, they bend back upon 

 themselves. The veins, therefore, in their passage 

 towards the heart, run alongside the arteries, and 

 parallel with them ; and wherever you find an 

 artery bringing arterial blood from the heart, you 

 will also find, by the side of it, and enclosed in the 

 same sheath with it, a vein carrying back venous 

 blood to the heart. Thus the several streams of 

 venous and arterial blood pass each other on the 

 road, as it were, like two trains of carriages 

 moving side by side, but in contrary directions 

 the one train going out, the other returning home. 

 As the terminations of arteries form the be- 

 ginnings of veins, it follows that the number of the 

 veins, at their commencement, is equal to the 

 number of the arteries. But these numerous 

 minute veins, as they travel towards the heart, are 

 every now and then uniting, to form larger ones : 

 consequently, the streams of venous blood, as 

 they approach the heart, are constantly becoming 

 larger and larger also; and thus the whole quantity 

 of venous blood is eventually collected into two 

 large veins, which empty themselves into the right 



