282 CIRCULATION. 



tion the part has to perform. This will be still more appa- 

 rent when we come to consider the course of the blood in the 

 capillaries, and the immense capacity of this system, as com- 

 pared with the arteries or veins. 



The capacity of the capillary system is immense. It is 

 only necessary to consider the prodigious vascularity of the 

 skin, mucous membranes, or muscles, to realize this fact. In 

 injections of these parts, it seems, on microscopic examination, 

 as though they contained nothing but capillaries. In prepa- 

 rations of this kind, the elastic and yielding coats of the capil- 

 laries are distended to their utmost limit. Under some cir- 

 cumstances, in health, they are much distended with blood, 

 as the mucous lining of the alimentary canal during diges- 

 tion, the whole surface presenting a vivid red color, indicat- 

 ing the great richness of the capillary plexus. Various 

 estimates of the capacity of the capillary, as compared with 

 the arterial system, have been made, but they are simply ap- 

 proximative, and there seems to be no means by which an 

 estimate, with any pretentions to accuracy, can be formed. 

 The various estimates which are given are founded upon cal- 

 culations from microscopic examinations of the rapidity of 

 the capillary circulation, as compared with the arteries. In 

 this way Donders estimates the entire capacity of the capil- 

 lary system as 500, and Yierordt as 800 times that of the 

 arterial system. It must be evident to any one who has 

 witnessed the capillary circulation under the microscope, that 

 the conditions under which the animal under examination is 

 placed are liable to interfere with the current of blood ; and 

 the periodical congestion of certain parts, the fugitive flushes 

 of the skin, the condition of the smallest arteries induced by 

 changes of temperature, exercise, etc., make it evident that 

 the current of blood is liable to great variations. It is impos- 

 sible to strictly apply to the capillary circulation in the vari- 

 ous parts of the human subject, observations on the wing of a 

 bat, or the mesentery of a cat. We must consider, then, 



