128 O. LARSELL 
repeatedly emphasized the fact that the pulmonary arterial 
branches are located ‘‘as near as possible to the bronchi” which 
they accompany, while the corresponding veins are as far 
removed as the architectural plan of the lung will permit. 
It will be observed that the nerve fibers are present all along 
the larger arterial branch represented, as well as in the various 
divisions of this branch. The definite relationship to the smooth 
muscle bands of the arterial walls is not so clearly indicated in 
all parts of the figure, but this is due to the variations of different 
portions of the arterial wall with respect to the plane of section. 
That these fibers terminate in relation to the smooth muscle 
cells may be seen from figure 15, which pictures a portion of an 
arteriole under high magnification. It will be noted that the 
nerve fiber, which lies in the adventitious layer of the arteriole, 
gives off lateral branches at short intervals. These branches 
subdivide and send terminal twigs to the individual smooth 
muscle cells of the muscular layer of the arteriole. These 
terminal twigs end as small knobs, apparently on the surface of 
the muscle cell and usually near the nucleus. In the figure, 
which was sketched from a relatively thick section, the smooth 
muscle cells are represented at various angles. Some were cut 
transversely, some obliquely, and others curve upward on one 
side of the arteriole, pass across the field and curve downward 
on the opposite side. 
In some of my preparations there are indications of a very 
delicate nerve plexus on the blood capillaries which are located 
in the walls of the air sacs and the atria. Extremely fine vari- 
cosed fibers are present which both in their staming reaction 
and in their general appearance bear a closer resemblance to 
nerve fibers, aside from their small size, than to elastic fibers. 
No indication of nerve cells in connection with this possible 
plexus was observed, such as have been described by Dogiel 
(98) and by Prentiss (04). 
Attention has been already called to Ponzio’s figure 9, which 
pictures a similar network on a blood capillary in the lung. The 
further study of these fibers appears to the writer to present a 
separate problem, which must be left for further investigation. 
