THE FLOW OF BLOOD THROUGH THE ARTERIES 973 



the tonus of the artery under the finger and the resistance in the peripheral 

 branches. 



Perhaps the best-marked condition of the pulse is that known as the 

 ' water-hammer ' pulse, which is observed in cases where the aortic valves 

 are injured or diseased so as to allow of regurgitation into the ventricle. 

 The systolic rise of pressure in the arterial system is followed by an extremely 

 rapid fall, so that towards the end of diastole the pressure in the arteries 

 may be insufficient to keep the arterial system filled. Under such con- 

 ditions, if the arm be held above the head and the wrist of the patient 

 be grasped, the pulse in the arteries of the wrist is felt as a smart blow 

 coinciding with each beat of the hearb. 



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 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 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 the passage 

 only 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 



