64 CIRCULATION. 



ment in arterioles is more rapid than in veins of corre- 

 sponding size ; that in the veins the coloured corpuscles are 

 carried along by the axial stream, the leucocytes tending 

 towards the vascular wall ; that when a tissue is injured 

 the capillaries begin first to widen and then to leak, the 

 plasma and leucocytes passing out in succession, and that 

 more intense injury produces stasis and extravasation of 

 the coloured disks. 



Circulation in the Liver, Kidneys, and Spleen. The dif- 

 ference between the pressure which exists in the trunk of 

 the portal vein and that of the hepatic vein probably does 

 not exceed half an inch. Consequently the blood-stream 

 through the liver is extremely slow. In the kidneys, the 

 blood enters the glomeruli at high pressure and with rapid 

 motion ; in the capillaries of the convoluted tubes the 

 motion is slow, and the pressure that of the venous system. 

 In the spleen, the quantity of blood contained in the 

 organ at one time, and, consequently, its bulk are subject 

 to very great variations ; these are mainly due to the 

 action of the muscular fibres contained in the capsule and 

 framework. 



Venous Circulation. The capacity of the venous system 

 much exceeds that of the arterial, and is sufficient for the 

 reception of the whole of the circulating blood. The mean 

 lateral pressure in the venous system, though much inferior 

 to that which exists in the arteries, is dependent on it and 

 varies with it. It is greater in the capillary veins than in 

 the venous trunks : this difference is the chief cause of the 

 venous blood-stream. It is greater during inspiration than 

 during expiration. This difference is more marked in the 

 intra-thoracic veins (where during inspiration the lateral 

 pressure sinks below that of the atmosphere) than in others : 

 it manifests itself in the respiratory movements of the brain, 

 and other similar phenomena. The venous blood-stream 

 is promoted by intermittent external pressure, hindered by 

 continuous pressure. The venous pressure, and, conse- 



