CIECULATION IN THE BLOOD-VESSELS. 335 



diameter, with single rows of blood-particles flowing in the 

 centre of a stream of colourless, transparent blood-liquid 

 liquor sanguinis. The current is tardy near the walls of the 

 vessels, and rapid in the middle. It has been estimated that 

 blood advances in the capillaries at the rate of nearly two 

 inches per minute. 



By the coalescence of the capillaries veins are formed in 

 which the pulsations are wanting; the flow is more equal 

 and less rapid than in the arteries; more rapid and less 

 equal than in the capillaries. The pressure from behind 

 still pushes on the column of blood, which is supported in its 

 progress from bearing against the current of the circulation 

 by the valves. Any pressure on the veins which is especi- 

 ally active during the contraction of muscles favours the on- 

 ward flow, and in the vicinity of the chest, where the veins 

 are held rigid and open by surrounding membranes or 

 fasciae, the aspiratory force exerted by the opening of the 

 chest tends to suck the blood towards the heart. One method 

 of witnessing this suction power is by opening the veins of 

 the neck low down ; a gurgling sound succeeds the infliction 

 of the wound, and, instead of blood flowing, air is drawn 

 in, and destroys the animal This is a caution against 

 approaching too near the chest in the practice of vene- 

 section. 



The forces which are in exercise to produce a regular flow 

 of blood are, therefore : 



1. Contraction of the heart 



2. Vis a terr/o. 



3. Capillary attraction affecting the circulation locally. 



4. Muscular exertion. 



5. Aspiratory force, or vis a f route. 



Support by the valves, so as to prevent regurgitation, and 

 the regulating force exerted by the elastic and contractile 



