111 
atria, and arising from each atrium, two to five large irregular shaped 
cavities, the air-sacs. The atria are to be distinguished on the one 
hand from the terminal bronchi by the absence of smooth muscle fibres 
and the change which takes place in the epithelium; that of the 
terminal bronchi being cuboidal, while that of the atria is flat pave- 
ment epithelium. On the other hand the atria are to be distinguished 
from the air-sacs by their smaller size and by there having from three 
to six openings leading out from them, while the air-sac has only 
a single opening, that by which it communicates with its atrium. 
The artery follows closely the terminal bronchus until it ends in 
the center of the lobule. It then passes distal to the terminal bronchus 
and divides into as many branches as there are atria. Frequently the 
most dependent of the atria and its air-sacs receives its supply of 
blood from a branch which leaves the main artery just as it is entering 
the lobule; in this case the branch corresponds in size to that of the 
branches which are given off within the lobule. 
We thus see that the artery occupies a central position in regard 
to the lobule. The veins on the other hand occupy a peripheral 
position both as regards the lobule and the air-sacs. In number the 
veins exceed those of the arteries. 
There is one important exception to the rule that the artery is 
central and the veins peripheral, and it has an important relation to 
the course of the lymphatics. Arising one from either side of the 
distal extremity of the terminal bronchus, and receiving blood from 
the final ending of the bronchial artery and also from the adjoining 
air-sacs, are two short but good sized veins which pass by the shortest 
route to the nearest venous trunk. The veins belonging to the lobules 
which lie directly beneath the pleura extend up over the air-sacs and 
form an extensive network under the pleura’). It is only exception- 
ally that an artery passes out of the lobule and appears under the 
pleura. 
I have been able to show by reconstruction methods that the 
same rule holds true for a bronchus of the second order with its 
branches, as for a terminal bronchus and the lobule to which it gives 
origin; namely, the artery is central and the veins peripheral. Here 
we also find that at the point where branches are given off from the 
main bronchus two veins arise, one on either side. These veins are 
2) ScHÄFER in Quaın’s Anatomy 10th Edition states that this net- 
work is derived from branches of the bronchial artery. I can not under- 
stand how he has fallen into this error, 
