RESPIRATION. 65 
each of which opens into a small sac, a little wider than itself. Each of 
these, again, is partitioned off into a number of cells. Into the smallest of 
these cells the air has access, and it is on their walls that the network of 
capillaries is spread, in which the 
blood comes in contact with it. 
When the venous blood has re- 
turned to the heart, it is propelled 
from the right ventricle into the 
pulmonary artery. It must be 
remembered that this artery con- |4 
tains true venous blood. This ; 
artery soon divides into two, and | 
sends a branch to each lung (g, % 
fig. 54); and these ramify so as to 
send a minute artery along with 
each of the minute air-tubes, till 
they reach the lung-sacs already 
mentioned. The blood now dis- 
tributes itself into capillaries on 
the walls of the cells and of the 
adjoining air-passages. The blood 
having in this manner come in Fig. 57. 
contact with the air, and thus been 
rendered fit for being again sent out through the body, now passes into 
the first roots of the veins (so called, although they contain true arterial 
blood), which unite and reunite to form the four pulmonary veins that 
pour the blood, thus arterialised, into the left auricle of the heart, e, e’, 
fig. 54. 
The action of the lungs is very much like what takes place on squeezing 
a hollow india-rubber ball in the hand, and allowing it to expand 
alternately. By a peculiar arrangement of the ribs, and by the con- 
traction of the diaphragm (the floor of the chest, as it were, which, when 
uncontracted, rises up into the chest with a convex surface), the capacity 
of the chest may be very much increased. Suppose, then, the chest to 
be expanded ; the lungs, being of a spongy nature, are also free to expand, 
and immediately the air rushes in through the windpipe, and fills them, 
just as happens when the ball is held in the open hand. But now the 
ribs fall, the muscles of the diaphragm relax, and the chest is contracted 
so as to squeeze the air out of the lungs, as happens when the hand is 
closed on the ball. The muscles that contract to cause these various move- 
ments act independently of the will, so that the action is kept up during 
sleep, which it could not be if it were voluntary. 

E 
