)

[Fig. 36]

Fig. 36--*Terminal air sacs.* The two large sacs are infundibula; the small
divisions are alveoli. (Enlarged.)

*The Alveoli.*--The alveoli, or air cells, are the small divisions of the
infundibula (Fig. 36). They are each about one one-hundredth of an inch
(1/4 mm.) in diameter, being formed by the infolding of the infundibular
wall. This wall, which has for its framework a thin layer of elastic
connective tissue, supports a dense network of capillaries (Fig. 37), and
is lined by a single layer of cells placed edge to edge. By this
arrangement the air within the alveoli is brought very near a large
surface of blood, and the exchange of gases between the air and the blood
is made possible. It is at the alveoli that the oxygen passes from the air
into the blood, and the carbon dioxide passes from the blood into the air.
At no place in the lungs, however, do the air and the blood come in direct
contact. Their exchanges must in all cases take place through the
capillary walls and the layer of cells lining the alveoli.

[Fig. 37]

Fig. 37--*Inner lung surface (magnified)*, the blood vessels injected with
coloring matter. The small pits are alveoli, and the vessels in their
walls are chiefly capillaries.

[Fig. 38]

Fig. 38.--*Diagram to show the double movement of air and blood through the
lungs.* The blood leaves the heart by the pulmonary artery and returns by
the pulmonary veins. The air enters and leaves the lungs by the same
system of tubes.

[Fig. 39]

Fig. 39--*Diagram to show air and blood movements in a terminal air sac.*
While the air moves into and from the space within the sac, the blood
circulates through the sac walls.

*Blood Supply to the Lungs.*--To accomplish the purposes of respiration,
not only the air, but the blood also, must be passed into and from the
lungs. The chief artery conveying blood to the lungs is the _pulmonary
artery_. This starts at the right ventricle and by its branches conveys
blood to the capillaries surrounding the alveoli in all parts of the
lungs. The branches of the pulmonary artery lie alongside of, and divide
similarly to, the bronchial tubes. At the places where the finest
divisions of the air tubes enter the infund