NERVE TERMINATIONS IN LUNG OF RABBIT 125 
represents one of the smaller bronchioles. The most distal point 
to which fibers of the motor type have been followed is renre- 
sented by the small sphincter-like muscle bands at the openings 
into the atria from the alveolar ducts. Slender varicosed fibers 
have been observed to run among the muscle cells of these bands 
and to terminate in relation to them. 
No ganglionic cells have been found beyond the larger bronchi, 
so that the nerve bundles which pass to the smaller air passages, 
must contain, in addition to the few sensory fibers, which can 
be recognized by their large size, also the motor and inhibitory 
fibers. 
Innervation of the pulmonary vessels 
The pulmonary artery and the vessels branching from it have 
a much richer nerve supply than the usual anatomical descrip- 
tion indicates or than might be inferred from the meager and con- 
flicting results obtained in the past by physiological experiment. 
Near the hilum of the lung relatively large nerve trunks 
accompany the larger pulmonary arterial branches. No ganglion 
cells have been encountered anywhere along the nerve trunks 
which accompany these vessels. The source of these nerve fibers 
isreserved for further investigation. It should, however, be stated 
at this point that the smaller branches of the pulmonary artery, 
which lie in close juxtaposition to small bronchi, receive fibers 
from the nerve plexus around the latter. A similar observation 
was made by Berkley (’93) in the gray rat. The recent physio- 
logical results of Carlson and Luckhardt (’20) indicate that 
these fibers are derived in large part, if not entirely, from the 
vagus nerve, at least in Amphibia and reptiles. 
The nerve bundles wind about the blood-vessels, giving off 
individual fibers at more or less irregular intervals. These 
fibers run roughly parallel with the artery, then turn nearly at 
right angles to the longitudinal axis of the latter, and divide 
into several main branches, as illustrated in figure 13. One of 
these rami usually runs back along the artery, proximally, from 
the point of primary division of the fiber, another runs distally 
from this point. These larger branches give off numerous 
