THE CIRCULATION OF THE BLOOD. 5^ 



nor have they any pulsating j^o^^ei'- They have, however, the 

 assistance of other agents in propelhng the blood to its destina- 

 tion. The greater number of them possess valves, which admit 

 the blood to pass in one direction, but effectually prevent its 

 passing in any other. It was, indeed, from I'cflecting on the 

 structure and necessary office of these valves, that led the 

 immortal Harvey to discover the circulation of the blood. 

 Another circumstance peculiar to the veins is their situation, 

 being mostly near the surfoce of the body, whilst the arteries 

 are generally deep seated. The wisdom of this provision is 

 evident : it is well known that in wounds it is readily ascer- 

 tained if an artery be wounded by the jet of blood that ensues, 

 and which, even from an artery of small size, is very consider- 

 able, and the danger of death from bleeding is often great in 

 consequence of the force with which the blood is thrown into 

 these vessels. Now such being the danger attending the divi- 

 sion of arteries, it was necessary to remove them as much as 

 possible from the risk of injury, and accordingly they are almost 

 invariably deep seated, and when they do approach the surface 

 it is in parts least likely to be injured. Thus round these im- 

 portant vessels nature throws a thick muscular covering, and 

 protects the whole by a mantle so sensitive as to give warning to 

 the least attack. The veins, however, do not require this care ; 

 in them the circulation is languid, and their wounds are com- 

 paratively unimportant and unattended with danger, for the 

 blood generally stops, without assistance, from its coagulating 

 quality. It is also of importance that the greater portion of the 

 veins should be situated near the surface, in order to receive the 

 influence of the atmospheric pressure, which greatly assists the 

 motion of the blood ; and it has also been found that veins 

 possess a power of absorption in common with a particular order 

 of vessels called the absorbents ; thus these various purposes are 

 effected by the relative position of the veins and arteries. 



The structure of the veins is very different from that of the 

 arteries ; for whilst the latter are thick, elastic, and composed 

 of three coats, the former are thin, inelastic, and composed only 

 of two coverings. But although thin, they are yet capable of 

 affording great resistance to pi-essure ; for Dr. Hales found that 

 the jugular vein of a marc required a force equal to a column of 

 water 144 feet high to burst it. 



We have seen that the blood is sent to all parts of the body 

 by the action of the heart and arteries, but what is the cause of 

 its return? First in Importance is the law of liydrostatics, "that 

 all fluids support their level." Thus the same law by which 

 springs arise, and streams are produced, and rivers flow towards 

 the sea, is brought to bear in the living system, and enables the 

 blood in the arteries to support that in the veins. This effect 



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