Oil the Composition of the Blood. 
215 
the atmospheric air, which is eff(!cted by the passage of the bh)od 
through the lungs. By the act of inspiration tlie 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 
bv the thin walls of the aii'-cells and those of the capillary vessels 
wliich 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 ■jqVo''^ 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 is consequently brought freely into contact with 
the atmospheric air ; the delicate intervening tissues offering no 
real impediment to this taking place. The oxygen being seized 
upon by the red cells of the ])assing 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 7'eoxygenated by a fi'esh 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 capil- 
laries of the general system. Scarlet-coloured 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 is con- 
tained in their interior, and is designated hwmatine. Mixed with 
this is another fluid, called glohuline, which is closely allied to 
albumen in its chemical composition. With the hamato-globu- 
line are found the salts of iron — to the extent of about 6 per cent. 
— proper to the blood ; so that the contents of the cells may be 
said to be very complex in their nature. 
The alteration of the colour of the blood was until lately be- 
lieved to be due to a chemical change wrought in the iron of the 
Jiosmato-glohuline by the successive influences of oxygen and car- 
