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NERVE TERMINATIONS IN LUNG OF RABBIT 115 
ing reflex contraction of the bronchial musculature, constricting 
the smaller air passages still further, and possibly also initiating 
reflex coughing to expel the object. 
The terminations at the openings into the _ respiratory 
portion of the bronchial tree might likewise serve the purpose of 
guarding against the entrance of foreign objects into the atria 
and air sacs by initiating reflex constriction of the small sphincter- 
like muscle bands at the openings into the atria. The difference 
in form, however, of these endings, which appear better adapted 
to react to pressure stimuli than to touch, suggests the possibility 
that such terminations may be stimulated rather by the partial 
collapse of the lung during expiration. When the lung is dis- 
tended the alveolar ducts are doubtless spread apart at relatively 
greater angles with each other than are the other subdivisions 
of the bronchial tree. On expiration they probably approach 
relatively closer together, thus affording sufficient pressure to 
stimulate the terminations in question. 
The most distal point at which nerve endings have been found 
in the epithelium is just inside the atria as they are entered 
from the alveolar ducts. These terminations (fig. 7) are of 
small size and are covered by a delicate capsule, formed in part 
by the squamous epithelium, beneath which they lie in the wall 
of the atrium. The nerve fibers leading to these terminations 
are branched, as shown in the figure, so that each fiber probably 
terminates in a number of sensory endings. On entering the 
capsule the fiber divides into several branches, which in turn 
subdivide into small twigs. The latter terminate in small knobs. 
All the branches have typical varicosities. The whole struc- 
ture has a flattened appearance, roughly elliptical in outline. 
It appears possible that this type of termination may be 
influenced by stretching of the atrial walls during inspira- 
tion, and may represent the nerve terminations which many 
physiologists have maintained are responsible for the inhibition 
of the inspiratory movements and excitation of expiration. 
Retzius (93) found fibers and terminal branches in a human 
foetus of 15 em. reaching to ‘‘the necks of the alveoli,”’ as a 
rule, with an occasional fiber spreading over the rounded alveoli. 
