730 REMARKS ON COMPOSITION OF THE BLOOD. 
fluids into solids in the building up the frame. The latter, 
however, would appear to be quite equalised by the recon¬ 
version of the solids into fluids, which is as continuously 
going on. 
The red cells are the chief conductors of oxygen into the 
system, as they are also the conveyors of the carbonic acid 
out of it; and in order to perform these essential offices, it is 
first necessary that they be brought into tolerably close contact 
with the atmospheric air, which is effected by the passage of 
the blood through the lungs. By the act of inspiration the 
atmospheric air is conveyed by the windpipe and bronchial 
tubes into the air-cells of the lungs, where it is only separated 
from the blood itself by the thin walls of the air-cells, and 
those of the capilliary vessels which ramify upon them. The 
capillaries are arranged upon the air-cells after the form of a 
minute network, and they are so closely placed to each other, 
that although the vessels themselves do not exceed the 
1-3000th part of an inch in diameter, the spaces between them 
are considerably less than this. Thus the blood may be said 
to be spread out after the manner of a thin film, and every 
portion of it to be brought freely into contact with the atmo¬ 
spheric air; the delicate intervening tissues offering no real 
impediment to this taking place. The oxygen, being seized 
upon by the red cells of the passing current, is by the onward 
flow and further distribution of the blood carried throughout 
the entire body, and thus reaching the capillaries of the several 
organs and tissues, it here unites with the carbon of the 
system, evolving heat, as has been previously explained. 
In the expiratory act the carbonic acid gas—formed by the 
union of the oxygen and carbon—and the watery vapour— 
the product of the oxygen and hydrogen—are expelled from 
the system, by which means the blood, being first depurated 
and then ^oxygenated by a fresh inspiration of atmospheric 
air, is again fitted for the purposes of life. 
With these interchanges of gases, the blood is likewise well 
known to become altered in its colour, being rendered of a 
bright red hue by the absorption of oxygen while circulating 
through the capillaries of the lungs, and of a dark Modena 
red by that of carbonic acid while in its onward movement 
through the capillaries of the general system. Scarlet- 
colourcd blood is commonly called arterial , as arteries supply 
all parts with the fluid for their support; and dark red blood 
is designated venous , being found within the veins after it has 
served its several purposes. 
In order to explain the phenomenon of this change of 
colour, it is necessary to state that the red matter of the cells 
s rn/ 
