84 CIRCULATION OF THE BLOOD. 



serous membranes, tendons, etc. In none of the tissues do we find capillaries penetrat- 

 ing the anatomical elements, as the ultimate muscular or nervous fibres. Some tissues 

 receive no blood, at least they contain no vessels which are capable of carrying red 

 blood, and are nourished by imbibition of the nutrient plasma of the circulating fluid. 

 Examples of these, which are called extra-vascular tissues, are cartilage, nails, and 

 hair. 



The foregoing anatomical sketch gives an idea of how near the blood is brought to 

 the tissues in the capillary system, and how, once conveyed there by the arteries, and the 

 supply regulated by the action of the muscular coat of the smaller vessels, the blood is 

 distributed for the purposes of nutrition, secretion, absorption, exhalation, or whatever 

 function the part has to perform. This will be still more apparent when we come to 

 consider the course of the blood in the capillaries and the immense capacity of this sys- 

 tem, as compared with the arteries or veins. 



The capacity of the capillary system is immense. It is only necessary to consider 

 the great vascularity of the skin, mucous membranes, or muscles, to realize this fact. 

 In injections of these parts, it seems, on microscopical examination, as though they con- 

 tained nothing but capillaries. In preparations of this kind, the elastic and yielding 

 coats of the capillaries are distended to their utmost limit. Under some circumstances, 

 in health, they are largely distended with blood, as the mucous lining of the alimentary 

 canal during digestion, the whole surface presenting a vivid-red color, indicating the 

 great richness of the capillary plexus. Various estimates of the capacity of the capil- 

 lary, as compared with the arterial system, have been made, but they are simply approxi- 

 mative, and there seems to be no means by which an estimate, with any pretensions to 

 accuracy, can be formed. The various estimates which are given are founded upon cal- 

 culations from microscopical examinations of the rapidity of the capillary circulation as 

 compared with the circulation in the arteries. In this way, it has been estimated that 

 the entire capacity of the capillary system is from five hundred to eight hundred times 

 that of the arterial system. It must be evident to any one who has witnessed the capil- 

 lary 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 periodi- 

 cal 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 impossible to strictly apply to 

 the capillary circulation in the various parts of the human subject observations on the 

 wing of a bat or the mesentery of a cat. "We must consider, then, these estimates as 

 mere suppositions, and they are given for what they are worth. 



Phenomena of the Capillary Circulation. The most convenient situation for the 

 practical study of the capillary circulation is the tongue or the web of the frog. Here 

 may be studied, not only the movement of the blood in the true capillaries, but the cir- 

 culation in the smallest arteries and veins, the variations in caliber of these vessels, 

 especially the arterioles, by the action of their muscular tunic, and, indeed, the action 

 of vessels of considerable size. This has been a most valuable means of studying the 

 circulation in the capillaries as contrasted with the flow in the small arteries and veins, and 

 the only one, indeed, which could give us any definite idea of the action of these vessels. 



The magnificent spectacle of the capillary circulation was first observed by Malpighi, 

 in the lungs, and afterward by Leeuwenhoek, Spallanzani, Haller, Cowper, and others, 

 in other parts. We see the great arterial rivers, in which the blood flows with wonder- 

 ful rapidity, branching and subdividing, until the circulating fluid is brought to the net- 

 work of fine capillaries, where the corpuscles dart along one by one. The blood is then 

 collected by the veins and carried in great currents to the heart. This exhibition, to 

 the student of Nature, is of inexpressible grandeur ; and our admiration is not dimin- 

 ished when we come to study the phenomena in detail. We find here a subject as inter- 



