egarded as one body.

Where the thalami come together and touch or unite on the median line,
the junction is called a commissure (commiss. med.) and the space
between them where they do not touch is called the third ventricle
(ventric. III), which, like the lateral ventricles, may also hold a
little serum. It is unnecessary to consider the small parts above the
thalami, the choroid plexus of blood vessels, the fornix or strip of
nerve membrane, and the septum lucidum or delicate fibres under the
corpus callosum.

Beginning at the bottom of the figure, we observe the medulla
oblongata rising from the spinal cord to reach the cerebrum. Behind
this we see the cerebellum divided on the median line, and thus
presenting where it is divided the appearance called _arbor vitae_,
from its resemblance to the leaf of that evergreen.

As the fibres of the medulla oblongata ascend they pass between the
cerebellum and the _pons Varolii_ (bridge of Varolius) mingling with
its substance. The pons or bridge (for if the brain were laid on its
upper surface the pons would appear like a bridge over the river
represented by the medulla oblongata) is the commissure or connecting
body of the cerebellum, as the corpus callosum is of the cerebrum.
When the head is held erect the fibres of the pons arch forward from
the interior of the cerebellum on one side across the median line to
the other side, so that a straight line through from the right to the
left ear would pierce its lower portion. It looks toward the front,
corresponding with the upper jaw, just below the nostrils, through
which region it may be reached for experiment.

My experiments upon the brain of man show that the pons on each side
of the median line is the commanding head of the respiratory impulse,
and in marking the organ of respiration on my busts, it is located
around the mouth from the nose to the chin. When this region
(especially its lower portion) is prominent it indicates active
respiration and a forcible voice. Hence there is a great contrast in
the vocal power of two such heads as are shown in the adjoining
figure. This discovery has been verified by the pathological
researches of Dr. J. B. Coste, published at Paris, 1857.

[Illustration]

Following the line of the ascending fibres, after passing through the
pons they continue expanding and plunge into the thalamus and corpus
striatum. Their first appearance above the pons (marked in the
engraving by the word