BLOOD VESSELS 4G7 



Definite figures for the velocity of the lymph stream 

 cannot be given. 



Disturbance of any of the factors which govern the rate 

 of fiow will bring about alterations in the velocity of the 

 blood in arteries, capillaries, and veins. 



2. Special Characters of Blood Flow. 



(a) Arteries. — This may be investigated by a hcemodromo- 

 graph, which consists of a paddle suspended in a box. By 

 means of a tube at each end the box is inserted into the 

 course of an artery, and with each acceleration of the flow 

 the paddle is pressed forward. The movements of the other 

 end of the paddle are recorded through a tambour on a 

 cylinder. 



The flow of blood in an artery is rhythmically accelerated 

 with each ventricular systole. This is due to the pulse wave. 

 As the wave of high pressure passes along the vessels, the 

 blood tends to flow first forwards and then backwards from 

 it — so that in front of the wave there is an acceleration of 

 the stream and behind it a retardation, just as occurs in a 

 wave at sea. 



(b) Capillaries. — In the capillaries the flow is uniform, 

 unless when in excessive dilatation of the arterioles the pulse 

 wave is propagated to them. 



(c) Veins. — In most veins the flow is uniform, but in the 

 great veins near the heart it undergoes acceleration — 



\st, with each diastole of auricle and of ventricle (p. 

 444) ; 



^nd, with each inspiration (p. 447). 



In all vessels, the blood in the centre of the stream moves 

 more rapidly than that at the periphery on account of the 

 friction between the blood and the vessels. This rapid 

 " axial " and slow " peripheral " stream is well seen in a 

 small vessel placed under the microscope. The erythrocytes 

 are chiefly carried in the axial stream, while the leucocytes 

 are more confined to the peripheral stream, where they may 



