Orr/ans of the Animal Body: their Forms and Uses. 
35 
In the act of breathing, air from without, comparatively pure, 
passes into the impure air in the lungs, and some of it remains, 
mixing with the air which is already there ; so that when the 
breathing-out occurs, the air which comes from the nostrils or 
mouth contains less oxygen and more carbonic acid than the 
air which was inspired. The process is very simple, and not, 
from a mechanical point of view, very perfect in its results ; it 
really amounts to this: the air in the lungs is always getting 
noxious matter from the blood, and always receiving at short 
intervals, many hundreds of times a day, a little good air from 
outside to mix with the bad air within, and sending forth 
much of the good air along with a little of the bad. The air 
in the lungs is never pure, never indeed very much the better for 
the little pure air which it gets ; but it is bound, for the life 
of the animal, not to get any worse. A certain standard of 
impurity is allowed, but it must not be exceeded. 
Fig. 22. — TJie Heart and principal vessels, left face. 
a, right ventricle ; I, left ventricle ; c, right auricle j <?, left auricle ; e, arlery going to the lungs ; 
/, veins coming from the lungs ; g, anterior largi> artery (aorla) ; q, posterior large artery (aorta) ; 
r, large vein bringing blood from tore parts of body (anterior vena cava) ; v, posterior cava bring'ns 
blood from hinder parts of body ; y, thoracic duct. 
D 2 
