THE CIRCULATION OF THE BLOOD. 103 
have two sets—arteries and veins, in which the blood 
moves in opposite directions, the former carrying blood 
Jrom a ceutral reservoir or heart, the latter taking it to 
the heart. The walls of these tubes are made of three 
coats, or layers, of tissue, the ar- 
teries being elastic, like rubber, 
and many of the veins being fur- 
nished with valves.” In Verte- 
brates, the great artery coming 
out of the heart is called aorta, 
and the grand venous trunk, en- 
tering the heart on the opposite 
side, is called vena cava. Both 
sets divide and subdivide until 
their branches are finer than hairs ; 
and joining these finest arteries 
and finest veins are intermediate 
microscopic tubes, called capilla- 
ries (in Man about 3,45 of an inch 
in diameter). In these only, so 
thin and delicate are their walls, \i 
does the blood come in contact ‘ 
c a F Fia. 67. — Relation of artery, a, 
with the tissues or the air. vein, b, and capillaries, ¢, as 
In all animals having a special "1" the muscles of a Dos. 
organ for respiration (gills or lungs), there are two sets of 
capillaries, since there are two circulations—the systemic, 
from the heart around the system to the heart again, and 
the pulmonary, from the heart through the respiratory 
organ back to the heart. This double course may be il- 
lustrated by the figure 8. 
There is no true system of blood-vessels below the Star- 
fish. The canals of the Sponge are really on the outside 
of the animal. In the Infusoria and Polyps, the nutritive 
matter mixed with water rises and falls in the body-cavity. 
The simplest provision for the distribution of the products 
rk 
SE ——= 
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2) 
= 
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a 
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