40 LETTERS TO BROTHER JOHN. 



together, and so formed two very large ones, 

 which empty themselves into the heart. One of the 

 grand distinctions, then, between veins and arte- 

 ries, is, that while the arteries arising from the heart 

 are multiplied in number and diminished in size, 

 until they have reached and distributed their blood 

 to the ultimate tissue, the veins arising from the 

 ultimate tissue are constantly becoming diminished 

 in number and increased in size, until they have 

 reached and carried their blood to the heart. 



Another general distinction between arteries and 

 veins is, that arteries possess pulsation; that is, 

 they possess the power of contracting upon (and so 

 propelling) their blood ; and then of recovering 

 their size, and contracting again ; and so on. This 

 alternate contraction and expansion constitute the 

 pulse. The veins are simply and but slightly 

 elastic. The veins, therefore, have no pulse, and 

 consequently little or no power to propel their 

 blood. The blood in the veins is driven onward 

 by various extrinsic circumstances; such as, the 

 contraction of muscles around them, the pulsation 

 of arteries in their neighbourhood, a dependent 

 position, &c. The veins, therefore, have valves, 

 which, when the blood has been squeezed forward, 

 towards the heart, by the adventitious causes just 



