CIRCULATING SYSTEM. 357 



Although these two hearts, the pulmonic and systemic, 

 are united in quadrupeds and birds, it is otherwise with many 

 genera of molluscous animals, in which they are removed to 

 a considerable distance from each other, and exhibit apart 

 the functions of the two systems of circulating vessels. 



The arteries observe usually a very tortuous course. In 

 dividing into branches, there may, in general, be observed, 

 the continuation of a principal stem. These branches fre- 

 quently communicate with each other, or anastomose. The 

 number of branches, or the quantity of blood which they 

 convey to any particular part, may be considered as propor- 

 tional to the quantity of action which it performs, whatever 

 be the structure of the organ. Mr CARLISLE * has observed, 

 that in the limbs of slow moving quadrupeds, the trunk of 

 the artery subdivides into a number of parallel branches, 

 the sum of whose areas is much greater than the trunk 

 from which they have proceeded. By this arrangement^ 

 the rapidity of the circulation must be diminished, proba- 

 bly to make it correspond with the slow but continued ex- 

 ertion of the muscles, to which such parallel arteries are 

 distributed. 



The arteries terminate in those minute twigs which have 

 been termed capillary vessels. In these, the blood flows 

 in such minute quantity, and their coats are so thin and 

 transparent, that they appear colourless. These capillary 

 vessels open into the extremities or radicles of the vein, so 

 as to form a continuous circulation. This, however, is not 

 universally the case. For these capillaries, in some cases, 

 open into cavities, into which they pour a watery liquor. 

 When exercising this function, of whose mechanical or 

 chemical arrangements we know nothing, they have been 



Phil. Trans. 1800, p. 98, and 1804, p. 17, 



