d reflecting impulses. It has been found by experiment,
that the same movements will take place if the irritation be applied to
any portion of the body to which the spinal nerves are distributed, thus
giving undoubted evidence that the spinal cord in its entirety is
capable of causing these reflections. Fig. 57 represents the course of
the nervous impulses. The sensory impulse passes upward along the
posterior root, _a_, until it reaches the imbedded gray matter, _b_, of
the cord, by which it is reflected, as a motor impulse, downward along
the anterior root, _c_, to the muscles whence the sensation was
received. This is the reflex action of the spinal cord. There is no
consciousness or sensation connected with this action, and the removal
of the brain and the sympathetic system does not diminish its activity.
Even after death it continues for some time, longer in cold-blooded than
in warm-blooded animals, on account of the difference in temperature,
thus showing this property of the spinal cord. By disease, or the use of
certain poisons, this activity may be greatly augmented, as is
frequently observed in the human subject. A sudden contact with a
different atmosphere may induce these movements. The contraction of the
muscles, or cramp, often experienced by all persons, in stepping into a
cold bath, or emerging from the cozy sitting-room into a chilly December
temperature, are familiar illustrations of reflex movements. It has been
demonstrated that the irritability of the nerves may be impaired or
destroyed, while that of the muscles to which they are distributed
remains unchanged; and that the motor and sensory classes of filaments
may be paralyzed independently of each other.

The reflex actions of the spinal cord have been admirably summed up by
Dr. Dalton, as exerting a general, protective influence over the body,
presiding over the involuntary action of the limbs and trunk, regulating
the action of the sphincters, rectum, and bladder, and, at the same
time, exercising an indirect influence upon the nutritive changes in all
parts of the body to which the spinal filaments are distributed.

THE BRAIN. The brain is a complex organ, which is divided into the
_medulla oblongata_, the _cerebellum_, and the _cerebrum_.

The _medulla oblongata_ is situated just above the spinal cord, and is
continuous with it below, and the brain above. It has distinct functions
which are employed in the preservation and continuance of l