DISTRIBUTION OF ARTERIES AND VEINS'. 
217 
stem of a tree; these branches again subdivide into others 
more numerous but smaller, and these again into twigs still 
more numerous and more minute ; until almost every portion 
of the body is so penetrated with them, that not even a 
trifling scratch, cut, or prick, can be made, without wounding 
some one of these small divisions (fig. 120).—The Yenous 
system presents a corresponding distribution, but it is destined 
for an opposite purpose; and we must regard it as commencing 
in the tissues by the minuter canals, which run together like the 
Fig. 120. —Distribution of the smaller Blood-Vessels in the Membrane 
BETWEEN TWO OF THE TOES OF THE HIND FOOT OF THE COMMON FROG; a 
veins ; b b, arteries. 
little rivulets that form the origin of a mighty river, or like the 
smallest fibres of which the roots of a tree are made up. The 
larger canals thus formed gradually unite with each other as 
they approach the heart, towards which they all tend, just as 
the various tributary streams pour their contents into one prin¬ 
cipal channel: and at last all the veins empty into the heart, 
by one or two large trunks, the blood which they have conveyed 
from the several parts of the body; just as all the tributaries 
which have arisen over a wide extent of country, pour into 
the ocean the water they have collected, by one mouth which 
is thus common to all of them. 
247. Although the number of the Arterial branches increases 
