DISEASES OF THE NERVOUS SYSTEM. 211 
dense areolar tissue, and inclosed in a membranous sheath—the neuri- 
lemma. Nerve fibers possess no elasticity, but 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 cerebro-spinal axis within the cra- 
nium, which may.be divided into four parts—the medulla oblongata, 
the cerebellum, the pons Varolii, and the cerebrum—and it is cov- 
ered 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, hav- 
ing between them the arachnoid space which contains the cerebro- 
spinal fluid, the use of which is to protect the spinal cord and brain 
from pressure. The third, or inner, the pia mater, is closely adher- 
ent 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 i in the horse: ib i is pyramidal i in shape, the narrowest part join- 
ing the cord. * 
The pons Varolii is the transverse projection on the base of the 
brain, between the medulla oblongata and the pecans of the cere- 
brum. 
The cerebellum is ledeed3 in the posterior part of the cranial fe, 
immediately above the medulla oblongata; it is globular or ellip- 
tical in shape, the transverse diameter being greatest. The body 
of the cerebellum is composed of gray matter externally and of 
white matter in the center, The cerebellum has the function of co- 
