202 LETTERS TO BROTHER JOHN. 



requisite degree of velocity. Under these circum- 

 stances, the blood creeps sluggishly, and, as it 

 were, lazily along the minute vessels composing the 

 elementary tissue : they become gorged ; and this 

 engorgement operates as a still further impediment 

 to the free flow of the blood. 



But there is another most important evil result- 

 ing from this semi-stagnation of blood in the ulti- 

 mate tissue. Arterial blood, when not moving 

 with the due degree of velocity, becomes deterio- 

 rated in its properties : for if you enclose a living 

 artery between two ligatures, the blood so insu- 

 lated assumes the black colour and other proper- 

 ties of venous blood. 



The blood, therefore, when not circulated with 

 sufficient energy through the ultimate tissue, be- 

 comes deteriorated in quality and this, too, pre- 

 cisely where it is of the utmost importance that it 

 should be of the very highest degree of purity : 

 for, as you now know, it is in the ultimate tissue 

 of our organs that all those operations are effected 

 on the blood on which the nutrition of the body 

 depends. The blood, when the circulation is driven 

 on with a due degree of healthy vigour, maintains 

 its vermilion hue and arterial character, not only 

 as far as the extremities of the hairlike arteries, 



