sec. 270). It is injury to this
respiratory center which proves fatal in cases of broken neck.

From this nerve center there is sent out to the nerves that supply the
diaphragm and other muscles of breathing, a force which stimulates them to
regular contraction. This breathing center is affected by the condition of
the blood. It is stimulated by an excess of carbon dioxid in the blood,
and is quieted by the presence of oxygen.

Experiment 108. _To locate the lungs_. Mark out the boundaries of
the lungs by "sounding" them; that is, by _percussion_, as it is called.
This means to put the forefinger of the left hand across the chest or
back, and to give it a quick, sharp rap with two or three fingers. Note
where it sounds hollow, resonant. This experiment can be done by the
student with only imperfect success, until practice brings some skill.

Experiment 109. Borrow a stethoscope, and listen to the respiration
over the chest on the right side. This is known as _auscultation_. Note
the difference of the sounds in inspiration and in expiration. Do not
confuse the heart sounds with those of respiration. The respiratory
murmurs may be heard fairly well by applying the ear flat to the chest,
with only one garment interposed.

Experiment 110. Get a sheep's lungs, with the windpipe attached.
Ask for the heart and lungs all in one mass. Take pains to examine the
specimen first, and accept only a good one. Parts are apt to be hastily
snipped or mangled. Examine the windpipe. Note the horseshoe-shaped
rings of cartilage in front, which serve to keep it open.

Experiment 111. Examine one bronchus, carefully dissecting away the
lung tissue with curved scissors. Follow along until small branches of
the bronchial tubes are reached. Take time for the dissection, and save
the specimen in dilute alcohol. Put pieces of the lung tissue in a basin
of water, and note that they float.

The labored breathing of suffocation and of lung diseases is due to the
excessive stimulation of this center, caused by the excess of carbon
dioxid in the blood. Various mental influences from the brain itself, as
the emotions of alarm or joy or distress, modify the action of the
respiratory center.

Again, nerves of sensation on the surface of the body convey influences to
this nerve center and lead to its stimulation, resulting in a vigorous
breathing movement. Thus a dash of cold water on the face or neck of a