104 
COMPARATIVE ZOOLOGY. 
have two sets— arteries and veins , in which the blood 
moves in opposite directions, the former carrying blood 
from a central reservoir or heart , 
the latter taking it to the heart. 
In the Vertebrates, the walls of 
these tubes are made of three 
coats, or layers, of tissue, the arte¬ 
ries being elastic, like rubber, and 
many of the veins being furnished 
with valves. 58 The great artery 
coming out of the heart is called 
aorta , and the grand venous trunk, 
entering 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 ^oVo of an inch 
in diameter). 59 In these only, so 
thin and delicate are their walls, 
does the blood come in contact with the tissues or the air. 
In those Vertebrates which have 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 th z pulmonary, from the heart through the res¬ 
piratory organ back to the heart. This double course may 
be illustrated by the figure 8. In gill-bearing animals there 
are capillaries in the gills, but not a double circulation. 
There is no true system of blood-vessels below the 
Star-fish. The simplest provision for the distribution of 
the products of digestion is shown by the Jelly-fish, whose 
stomach sends off radiating tubes (Fig. 196), through which 
the digested food passes directly to the various parts of 
- Relation of artery, a, 
&, and capillaries, c, as 
seen in the muscles of a Dog. 
