t are very strong. Divided nerves
do not retract.

Nerves are thrown into a state of excitement when stimulated, and are,
therefore, said to possess excitable or irritable properties. The
stimuli may be applied to, or may act upon, any part of the nerve.
Nerves may be paralyzed by continuous pressure being applied. When the
nerves divide into branches, there is never any splitting up of their
ultimate fibers, nor yet is there ever any coalescing of them; they
retain their individuality from their source to their termination.

Nerves which convey impressions to the centers are termed sensory, or
centripetal, and those which transmit stimulus from the centers to
organs of motion are termed motor, or centrifugal. The function of the
nervous system may, therefore, be defined in the simplest terms, as
follows: It is intended to associate the different parts of the body in
such a manner that stimulus applied to one organ may excite or depress
the activity of another.

The brain is that portion of the cerebrospinal axis within the cranium,
which may be divided into four parts--the medulla oblongata, the
cerebellum, the pons Varolii, and the cerebrum--and it is covered by
three membranes, called the meninges. The outer of these membranes, the
dura mater, is a thick, white, fibrous membrane which lines the cavity
of the cranium, forming the internal periosteum of the bones; it is
continuous with the spinal cord to the extremity of the canal. The
second, the arachnoid, is a delicate serous membrane, and loosely
envelops the brain and spinal cord; it forms two layers, having between
them the arachnoid space which contains the cerebrospinal fluid, the use
of which is to protect the spinal cord and brain from pressure. The
third, or inner, the pia mater, is closely adherent to the entire
surface of the brain, but is much thinner and more vascular than when it
reaches the spinal cord, which it also envelops, and is continued to
form the sheaths of the spinal nerves.

The medulla oblongata is the prolongation of the spinal cord, extending
to the pons Varolii. This portion of the brain is very large in the
horse: it is pyramidal in shape, the narrowest part joining the cord.

The pons Varolii is the transverse projection on the base of the brain,
between the medulla oblongata and the peduncles of the cerebrum.

The cerebellum is lodged in the posterior part of the cranial cavity,
immediately above the medulla oblongata; it is glo