298 THE CIRCULATION. 



nal portions of the current, and are sometimes momentarily arrested ; 

 apparently adhering for a few seconds to the internal surface of the 

 vessel. Whenever the current is obstructed or retarded in any 

 manner, the white globules accumulate in the affected portion, and 

 become more numerous there in proportion to the red. 



It is during the capillary circulation that the blood serves for 

 the nutrition of the vascular organs. Its fluid portions slowly 

 transude through the walls of the vessels, and are absorbed by the 

 tissues in such proportion as is 'requisite for their nourishment. 

 The saline substances enter at once into the composition of the 

 surrounding parts, generally without undergoing any change. The 

 phosphate of lime, for example, is taken up in large quantity by 

 the bones and cartilages, and in smaller quantity by the softer parts ; 

 while the chlorides of sodium and potassium, the carbonates, sul- 

 phates, &c., are appropriated in special proportions by the different 

 tissues, according to the quantity necessary for their organization. 

 The albuminous ingredients of the blood, on the other hand, are 

 not only absorbed in a similar manner by the animal tissues, but at 

 the same tim o transformed by catalysis, and converted into new 

 materials, characteristic of the different tissues. In this way are 

 produced the musculine of the muscles, the osteine of the bones, the 

 cartilagine of the cartilages, &c. &c. It is probable that this trans- 

 formation does not take place in the interior of the vessels them- 

 selves ; but that the organic ingredients of the blood are absorbed 

 by the tissues, and at the same moment converted into new mate- 

 rials, by contact with their substance. The blood in this way fur- 

 nishes, directly or indirectly, all the materials necessary for the 

 nutrition of the body. 



The physical conditions which influence the movement of the 

 blood in the capillaries, are somewhat different from those which 

 regulate the arterial and venous circulations. We must remember 

 that, as the arteries pass from the heart outward, they subdivide and 

 ramify to such an extent that the surface of the arterial walls is 

 very much increased in proportion to the quantity of blood which 

 they contain. It is on this account that the arterial pulsation is so 

 much equalized at a distance from the heart, since the influence of 

 the elasticity of the arterial coats is thus constantly increased from 

 within outward. But as these vessels finally reach the confines 

 of the arterial system, having already been very much increased 

 in number and diminished in size, they suddenly break up into 



