FLOW OF BLOOD THROUGH THE ARTERIES 1047 



THE CIRCULATION THROUGH THE CAPILLARIES 

 The capillary circulation is most easily studied by examining 

 under the microscope the tongue of the frog or the web of the frog's 

 foot. Under a power of about 150 to 180 diameters a network of 

 vessels is seen, consisting of small arteries, capillaries, and veins. The 

 direction of flow in the arteries is opposite to that in the veins. In 

 the capillaries the flow is from arteries to veins, though, on account 

 of the reticular arrangement of these vessels, the direction of the 

 stream through them is not quite constant and may occasionally be 

 reversed. The flow of blood in the arteries is rapid, whereas in the 

 veins it is generally possible to distinguish the individual blood-cor- 

 puscles. Through the capillaries the flow is very inconstant. If a 

 group of capillaries be watched for some time the blood may at first 

 hurry through a number of them with great rapidity ; the flow then 

 becomes slower and then may quicken up to a moderate pace again. 

 These variations in the capillary flow are probably associated with 

 spontaneous alterations in the condition of contraction of the small 

 arteries supplying the group of capillaries. It is easy to observe that 

 the arterial flow is pulsatile, the pulsation disappearing in the capil- 

 laries and veins. Another difference between the circulation in these 

 three kinds of vessels is to be found in the condition of the peripheral 

 zone. In the arteries the blood -stream is divided into two parts, the 

 peripheral stream about '01 mm. wide, consisting only of colourless 

 plasma with occasionally a stray leucocyte and an axial stream, in 

 which all the red blood-corpuscles are being hurried along. In the veins 

 there is a similar peripheral plasmatic zone, but here we find regularly 

 scattered leucocytes which travel rather more slowly than the axial 

 stream of red corpuscles. The formation of this axial zone is purely 

 mechanical, and may be imitated in any fluid containing in suspension 

 particles whose specific gravity is somewhat higher than that of the 

 fluid. In the capillaries there is no separation of the two zones, since 

 the lumen of these vessels as a rule allows only the passage of one or 

 two corpuscles abreast, so that they are everywhere in contact with 

 the wall. The corpuscles are evidently elastic structures, and may be 

 seen to bend if they impinge on the dividing point of two capillaries 

 before they are finally swept ofE by the stream into one or the 

 other. 



The capillary wall is composed of a single layer of elongated 

 flattened cells which present little resistance to the passage through 

 them by diffusion of dissolved substances, such as sugar, salts, oxygen, 

 or carbon dioxide. In this way the tissue-cells obtain oxygen from the 

 red blood-corpuscles and nutriment from the plasma, and give off 

 to the circulating blood carbon dioxide and other effete substances 



