161 



its circulation * The force which carries on the circulation of the blood, 

 along the arteries, is so great, that Nature seems not to have availed her- 

 self of the mechanical advantages which might have facilitated its flow. On 

 the other hand, the power which determines the progression of the venous 

 blood is so feeble, that she has sedulously removed every obstacle which 

 might have impeded its course; and as the relation of the minute to the 

 larger branches, of these to the trunk, is the same as in the arteries, two 

 branches unite to form a vein of greater calibre than each separate ves- 

 sel, but smaller than the two taken together, the blood flows along a space 

 which becomes narrower, the nearer it approaches the heart ; the rapi- 

 dity of its course must, therefore, be progressively increased. 



The veins are almost straight in their course ; at least, they are much 

 less tortuous than the arteries. The force which makes the blood flow 

 along them, is consequently not taken up in straightening these curves ; 

 the anastomoses are, likewise, more frequent, and as the flow of the 

 blood might have been intercepted in the deep seated veins of the limbs, 

 when the muscles, among which these vessels lie, during contraction, 

 compress them by their enlargement and induration, they communicate 

 freely with the superficial veins, towards which the blood is carried, and 

 flows the more readily, as they are not liable to be compressed. It is 

 to be observed, and is to be accounted for on the same principle, that the 

 superficial veins are very large and distinct among the lower orders who 

 are employed in laborious occupations, requiring an almost continual ex- 

 ertion of their limbs. Lastly, the internal part of the veins, like that of 

 the lymphatics, is furnished with valvular folds, formed by the duplica- 

 ture of their epidermoid coat. These valves, which are seldom single, 

 and almost always in pairs, are not found in the minute veins, nor in the 

 great trunks, nor in the veins which bring back the blood from the vis- 

 cera in the great cavities. 



These valves, in falling, close completely the canal of the vessel, de- 

 stroy the continuity of the column of blood returning to the heart, divide 

 it into smaller columns, as numerous as the intervals between the valves, 

 and the height of which is determined by the distance between these 

 folds. So that the power which carries onward the venous blood, and 

 which would be incapable of propelling the whole mass, acts advantage- 

 ously on each of the small portions into which it is divided. 



LXIII. It has been thought, that the principal cause which makes the 

 blood flow into the veins, is the combined action of the heart and arteries; 

 but the impulse from those organs, is lost in the system of capillary ves- 

 sels, and does not extend to the veins. The specific action of their own 

 parietes, aided by auxiliary means, such as the motion of the neighbour- 

 ing arteries, is sufficient to carry the blood on to the heartf. 



* The arteries contain, at all times, nearly the same quantity of blood. The veins 

 are always the seat of plethora, because the blood stag-nates in them more readily; and 

 this condition brings an inflammatory fever (consisting' merely in an increased action of 

 the vascular system, as is expressed by the term ungeioteniqite applied to it by Professor 

 PineV) only when the venous congestion becoming- excessive, the blood passes with 

 difficulty from the arteries into the veins. The heart and the arteries then struggle, 

 with considerable effort, to rid themselves of the fluid which oppresses them, &c. 

 Jiuthor't Note. 



{ In the process of returning the blood to the heart, two causes are principally en- 

 gaged, the most efficient of which is, undoubtedly, the contractile power of the vein$ 

 themselves, AVe are aware that this property is denied to them by many who have 



