FLOW OF BLOOD THROUGH THE ARTERIES 929 



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-corpuscles. 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 capillaries 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 plas- 

 matic 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 off 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 as the products of their metabolism. 

 There is evidence that in some situations the cells forming the capillary 

 wall may be contractile. According to Strieker and others, the cell substance 

 is arranged in strands running from the nuclei around the capillary. By 

 the contraction of these strands the vessel may be narrowed to obliteration. 

 These phenomena have been observed in the nictitating membrane of the 

 frog, but it is doubtful how far they may be extended to 'the other capillary 

 systems of the body. If the contractile power is at all universally present, 

 it must play an important part in determining the amount of blood- flow 

 through the capillaries of an organ, and will doubtless be largely affected 



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