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 life. It has
been termed the "vital knot," owing to the fact that the brain may be
removed and the cord injured and still the heart and lungs will continue
to perform their functions, until the medulla oblongata is destroyed.

The arrangement of the white and gray matter of the medulla oblongata is
similar to that of the spinal cord; that is to say, the white matter is
external and the gray internal; whereas in the cerebellum and cerebrum