634 ANIMAL PHYSIOLOGY. 
It may be regarded as the most complicated nerve-trunk in 
the body, and the distribution of its fibers is of the most exten- 
sive character. Following our classification efferent and 
afferent, we recognize: 
1. Efferent fibers, which are motor to an extensive tract in 
the respiratory and alimentary regions, 
Thus the constrictors of the pharynx, certain muscles of the 
palate, the cesophagus, the stomach, and the intestine, receive 
an abundant supply from this source. By the laryngeal nerves, 
probably derived originally from the spinal accessory, the mus- 
cles of the larynx are innervated. The muscles of the trachea, 
bronchi, etc., are also supplied by the pneumogastric. It is 
probable that vaso-motor fibers derived from the sympathetic 
run in branches of the vagus. The relations of this nerve to 
the heart and lungs have already been explained. 
2. Afferent Fibers.—It may be said that afferent impulses 
from all the regions to which efferent fibers are supplied pass 
inward by the vagus. One of the widest tracts in the body for 
afferent impulses giving rise to reflexes is connected with the 
nerve-centers by the branches of this nerve, as evidenced by 
the many well-known phenomena of this character referable 
to the pharynx, larynx, lungs, stomach, etc., as vomiting, sneez- 
ing, coughing, etc. This nerve plays some important part in 
secretion, no doubt, but what that is has not been as yet well 
established. 
Pathological._Section of both vagi, as might be expected, 
leads to death, which may take place from a combination of 
pathological changes, the factors in which vary a good deal 
with the class of animals the subject of experiment. Thus, the 
heart in some animals (dog) beats with great rapidity and 
tends to exhaust itself. In birds especially is fatty degenera- 
tion of heart, stomach, intestines, etc., liable to follow. 
Paralysis of the muscles of the larynx renders breathing 
laborious. From loss of sensibility food accumulates in the 
pharynx and finds its way into the larynx, favoring, if not 
actually exciting, inflammation of the air-passages. 
But it is not to be forgotten that upon the views we advo- 
cate as to the constant influence of the nervous system over 
all parts of the bodily metabolism, it is plain that after section 
of the trunk of a nerve with fibers of such wide distribution 
and varied functions the most profound changes in so-called 
nutrition must be expected, as well as the more obvious func- 
tional derangements; or, to put it otherwise, the results that 
