372 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the distance to be traversed by the blood while fulfilling these functions; 

 and explain the importance of the comparatively slow rate at which it will be 

 found to move through that short distance. The histological study of a typ- 

 ical capillary (see Fig. 95) shows that its thin wall is composed of a single 

 layer only of living flat endothelial cells set edge to edge in close contact ; and 

 that the edges of the cells are united by a small quantity of the so-called 

 cement-substance. If the capillary be traced in either anatomical direction, 

 the wall of the vessel is seen to become less thin and more complex, till it 

 merges into that of a typical arteriole or venule, the walls of which are still 

 delicate, though less so than that of a capillary. That the capillary walls are 

 so thin and soft, and are made of living cells, are very important facts as 

 regards the relations between blood and tissue. It is of great importance 

 for the variation of the blood-supply to a part that they are also distensible, 

 elastic, and possibly contractile. 



Direct Observation of the Flow in the Small Vessels. The capillary 

 flow is visible under the compound microscope, best by transmitted light, in 

 the transparent parts of both warm-blooded and cold-blooded animals. It is 

 important that the phenomena ^observed in the latter should be compared with 

 observations upon the higher animals ; but the fundamental facts can be most 

 fruitfully studied in the frog, tadpole, or fish, inasmuch as no special arrange- 

 ments are needed to maintain the temperature of the exposed parts of these 

 animals. Moreover, their large oval and nucleated red blood-corpuscles are 

 well fitted to indicate the forces to which they are subjected. The capillary 

 movement, therefore, will be described as seen in the frog ; it being under- 

 stood that the phenomena are similar in the other vertebrates. In the 

 frog the movement may be studied in the lung, the mesentery, the urinary 

 bladder, the tongue, or the web between the toes. During such study the 

 proper wall of the living capillary is hardly to be seen, but only the line on 

 each side which marks the profile of its cavity. Even the proper walls of 

 the transparent arterioles and veuules are but vaguely indicated. The plasma 

 of the blood, too, has so nearly the same index of refraction as the tissues, 

 that it remains invisible. It is only the red corpuscles and leucocytes that 

 are conspicuous ; and when one speaks of seeing the blood in motion, he means, 

 strictly speaking, that he sees the moving corpuscles, and can make out the 

 calibre of the vessels in which they move. The observer uses as low a power 

 of the microscope as will suffice, and takes first a general survey of the minute 

 arteries, veins, and capillaries of the part he is studying, noting their form, 

 size, and connections. In the arteries and veins he sees that the size of the 

 vessels is ample in comparison with that of the corpuscles ; that, in the veins, 

 the movement of the blood is steady, but in the arteries accelerated and 

 retarded, with a rhythm corresponding to that of the heart's beat. In the 

 veins, moreover, the individual red corpuscles can be distinguished, while in 

 the arteries they cannot, as at all times they shoot past the eye too swiftly. 

 The fundamental observation now is verified that the blood is incessantly 

 moving out of the arteries, through the capillaries, into the veins. 



