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 Carxiste* 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 vesscls. 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. 
