THE CAPILLARY CIRCULATION. 3CU. 



steady pressure. Under this pressure, the blood passes through the 

 capillary vessels; and it is then carried backward to the heart 

 through the veins, assisted by the action of the muscles and the 

 respiratory movements of the chest. 



At the same time there are certain phenomena which are very 

 important in this respect, and which show that various local in- 

 fluences will either excite or retard the capillary circulation in par- 

 ticular parts, independently of the heart's action. The pallor or 

 suffusion of the face under mental emotion, the congestion of the 

 mucous membranes during the digestive process, the local and de- 

 fined redness produced in the skin by an irritating application, are 

 all instances of this sort. These phenomena are usually explained 

 by the contraction or dilatation of the smaller arteries immediately 

 supplying the part with blood, under the influence of nervous 

 action. As we know that the smaller arteries are in fact provided 

 with organic muscular fibres, this may undoubtedly have something 

 to do with the local variations of the capillary circulation ; but the 

 precise manner in which these effects are produced is at present 

 unknown. 



The rapidity of the circulation in the capillary vessels is much 

 less than in the arteries or the veins. It may be measured, with a 

 tolerable approach to accuracy, during the microscopic examination 

 of transparent and vascular tissues, as, for example, the web of the 

 frog's foot, or the mesentery of the rat. The results obtained in 

 this way by different observers (Valentine, Weber, Volkmann, &c.) 

 show that the rate of movement of the blood through the capil- 

 laries is rather less than one-thirtieth of an inch per second ; or not 

 quite two inches per minute. Since the rapidity of the current, as 

 we have mentioned above, must be in inverse ratio to the entire 

 calibre of the vessels through which it moves, it follows that the 

 united calibre of all the capillaries of the body must be from 350 to 

 4uO times greater than that of the arteries. It must not be sup- 

 posed from this, however, that the whole quantity of blood contained 

 in the capillaries at any one time is so much greater than that in 

 the arteries ; since, although the united calibre of the capillaries is 

 very large, their length is very small. The effect of the anatomical 

 structure of the capillary system is, therefore, merely to disseminate 

 a comparatively small quantity of blood over a very large space, so 

 that the chemico-physiological reactions, necessary to nutrition, may 

 take place with promptitude and energy. For the same reason, 

 although the rate of movement of the blood in these vessels is very 



