64 PHYSIOLOGY OF THE HUMAN BODY. 
Respiration. 
The function of Respiration is performed in the lungs with common 
air, Air is composed of about 1 part of a gas called oxygen,! and 4 parts 
of another, nitrogen,? with an 
exceedingly small quantity of a 
third, carbonic acid. The change 
effected on air by respiration is 
the giving off one of these gases, 
oxygen, and receiving another, 
carbonic acid, in its place. 
The chief difference between 
arterial and venous blood is, that 
the former contains oxygen, while 
the latter contains carbonic acid; 
and also that when the venous 
blood is sent from the heart to 
the lungs, its errand is simply to. 
discharge this carbonic acid, and _ 
receive in exchange a supply of 
oxygen, with which it returns to 
the heart, ready to be sent off 
again throughout the body. It is clear then that in the lungs there must 
be (1) a provision for the blood coming in contact with the air; (2) an 
arrangement by which the blood in contact with the air shall be con- 
tinually changing ; and (3) an arrangement by which the air in contact 
with the blood shall be continually changing. 
Let us consider, first, the provision for bringing the blood into contact 
with the air. It consists generally in having an immense extent of internal 
surface, covered by a complete network of minute vessels, through which 
the blood to be acted upon flows, separated from the air which has been 
inhaled only by an extremely thin membrane, so that the effect is the same 
as if the two were in actual contact. But how is this immense extent of 
surface obtained in a small compass ; for the lungs are of moderate size? 
It is accomplished by the peculiar structure of the lungs. When the 
main air-tube or windpipe descends to the level of the lungs, it divides 
into two branches, called the bronchi [plural of Latin bronchus, the wind- 
pipe], one going to each lung (B, fig. 57). These again divide and subdivide 
into smaller tubes, calléd bronchial tubes, as b, b. They subdivide in 
this way, until at length an immense number of minute tubes is formed, 

1 From Greek oxys, acid, and gennad, to produce. © 
2 From Greek nitron, soda, and gennad, to produce, 
