PHYSIOLOGY OF THE CIRCULATION. 85 



secretion and excretion. But as the capillaries, or 

 intermediate vessels, are generally considered to 

 exercise a still more important influence in the 

 performance of these functions, it becomes neces- 

 sary to consider how far the preceding views are 

 applicable to this part of the circulating system. 

 And on considering the endless varieties that are 

 observed to prevail in different organs of the body, 

 in respect to the relative length, number, arrange- 

 ment, and connexion of these vessels with the 

 adjacent arteries and veins, the following are those 

 conclusions which appear to derive the greatest 

 amount of support from the knowledge already 

 possessed on the subject. Wherever, in an arbor- 

 escent arrangement of vessels, the blood flows from 

 a main branch into numerous collateral twigs, it 

 must encounter more or less obstruction during its 

 passage through the latter, and will consequently 

 continue to exert a certain amount of lateral pressure 

 until it has traversed the most contracted portion of 

 the tubes. In those organs where a peculiar dispo- 

 sition of the vessels impedes the flow of blood from 

 the capillaries into the veins, the same effect, viz. an 

 increase in the lateral pressure of the capillary 

 streams, must follow the diminished rate of discharge 

 from the capillaries thus induced. The causes of 

 this partial obstruction may be seated either in the 

 capillaries themselves, or in the adjacent veins. In 

 some organs, as in the Malpighian tufts of the kidney, 

 their own arrangement is a considerable source of 

 obstruction to the blood traversing the former vessels. 

 And in other parts, a tortuous course of the neigh- 



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