THE BLOOD AND ITS CIRCULATION. 343 



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 coagula- 

 ting quality. It is also of importance that the greater por- 

 tion of the veins should be situated near the surface, in or- 

 der 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 absorb- 

 ents ; thus these various purposes are effected by the rela- 

 tive 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 pressure. 



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 hy- 

 drostatics, "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 sup- 

 port that in the veins. This effect is greatly assisted by 

 the action of the valves in supporting the column of blood. 

 The blood thus supported and propelled by the arteries, as- 

 sisted by atmospheric pressure, must go somewhere, as the 

 valves prevent return ; it goes, therefore, where alone a va- 

 cancy is afforded, and that is in the right auricle of the 

 heart, which has just propelled its contents into the ventri- 

 cle. To these several forces may be added a power of suc- 

 tion the heart possesses whenever the chest is enlarged in 

 respiration. 



The manner in which the chyle is mixed up with the 

 blood, so that its color quickly disappears, is worthy of par- 

 ticular notice. It is owing, indeed, to the great agitation 

 the blood receives, and to the irregularity of the heart's in- 

 ternal surface. When the auricles contract, their contents 

 are, in a great measure, discharged into the ventricles, but a 

 portion is thrown back into the veins, which constitutes 

 what is called the venous pulse, and may sometimes be seen 

 in the jugular veins. In like manner, when the ventricles 

 contract a portion of their contents is thrown back into the 



