1 
- 
‘and tissues and the nervous centres. 
NERVOUS SYSTEM. 
no means free from them, and they will be 
employed in the course of this article. 
uch are the only subdivisions of the nervous 
system which anatomy appears to warrant. 
Others have been proposed; but as they are 
founded upon physiological opinions which are 
as yet hypothetical, it is unnecessary to discuss 
them at present. 
Having thus given a brief and general account 
of the nervous system and of nervous matter, 
we proceed to consider the anatomy and phy- 
siology of this system under the following 
divisions:—I. THE GENERAL ANATOMY OF 
NERVES. IJ. THE COMPARATIVE ANATOMY 
OF THE NERVOUS SYSTEM OR ITS DISPOSI- 
TION THROUGHOUT THE ANIMAL KINGDOM. 
Hil. Tue anaroMy OF THE NERVOUS CEN- 
TRES, THE GANGLIA, BRAIN, AND SPINAL 
corp. IV. aND LaSTLY, THE OFFICE OF 
‘THE NERVOUS SYSTEM AND THE FUNCTIONS 
OF ITS VARIOUS PARTS. 
NERVE.—(vevgor, nervus; Germ. nerve ; 
Fr. nerf.) The nerves perform the internuncial 
office in the nervous system by maintaining 
communications between the various organs 
They are 
bundles of threads of various size, surrounded 
by sheaths of membrane, with more or less of 
areolar tissue interposed. 
The nerves of the cerebro-spinal system and 
of the great sympathetic exhibit such different 
characters as regards their anatomy, that they 
may be examined separately. 
bro-spinal nerves—In examining a cere- 
bro-spinal nerve, we find it invested by a sheath 
of membrane, which has adherent to its inner 
surface thin layers of areolar tissue which pass, 
like so many partitions, between the threads or 
fibres of which the nerve is composed. This 
sheath is commonly called the neurilemma ; it 
_ is analogous to the sheath which surrounds mus- 
cles. Its office is chiefly mechanical, namely, 
that of binding the constituent fibrillz and fas- 
cicles of the nerve together, so as to protect 
them and to support the delicate plexus of 
capillary bloodvessels from which they derive 
their nutriment. 
The neurilemma is composed of fibres of the 
white fibrous kind. It exhibits to the naked 
eye the appearance of a fibrous membrane, 
white and almost silvery; and its microscopic 
characters are those of the white fibrous ele- 
ment, although not presenting much appearance 
of wavy fibres. The fibres are, for the most 
part, parallel to the axis of the nerve; but 
re are some which cross the nerve at right 
angles, or appear to pass spirally round it. The 
septa between the secondary bundles of the 
nerves seem to consist of a less perfect fibrous 
tissue, containing the remains of numerous 
cytoblasts. A yellow fibrous tissue of the 
finest kind exists here in very small quantity. 
The bloodvessels are distributed upon the 
external investing sheath and upon the septa. 
In some of the large nerves, the sciatic for 
example, these may be often seen minutely 
injected with blood. They are disposed some- 
what similarly to those of muscles, running 
(NervE.) 591 
peeiel to the fibres and fascicles of the nerve. 
he capillaries are among the small st in the 
body: they form oblong meshes of considerable 
length, completed at long intervals by vessels 
which cross the fibres of the nerve more or less 
in the transverse direction. Henle assigns to 
them, when empty, a diameter not exceeding 
sdoth of an inch. These bloodvessels are 
generally derived from neighbouring arterial 
branches; sometimes a special vessel accom- 
panies a nervous trunk, and even perforates it, 
passing along its central axis, as is well known 
to descriptive anatomists in the sciatic and the 
optic nerves. 
After the external part of the neurilemma 
has been dissected off, the nerve may be torn by 
needles and divided into secondary bundles and 
fibres. The ultimate fibre can then be readily 
distinguished by the aid of the microscope, from 
its being incapable of further subdivision by 
mechanical means ; and an accurate knowledge 
of its structure is of the utmost importance to 
the formation of correct opinions respecting the 
actions of the nervous system. 
The ultimate (or, as it is also called, the pri- 
mitive) nervous fibre, is a tube composed of a 
fine transparent homogeneous membrane, in a 
great degree resembling the sarcolemma of 
muscle. It is elastic, like that membrane, per- 
fectly homogeneous, and, according to Schwann, 
in young nerves has the nuclei of cells connected 
with it. Itmay be called the tubular membrane 
of nerve (a, fig. 329). The contents of this tube 
consist in a soft, semifluid, whitish, pulpy sub- 
stance, which is readily pressed out of its cut 
extremity. In the nerve that is quite fresh, 
having been taken from an animal just dead, 
this pulpy matter is quite transparent and appa- 
rently homogeneous. The tube membrane pre- 
sents the appearance of a delicate line, resem- 
Fig. 329. 
Nerve tubes altered by re-agents. 
a, tube altered by water. The light external line 
is the tubular membrane; the dark inner double- 
edged one, broken here and there, is the white 
substance of Schwann, 6, shows the change pro- 
duced by the action of ether on the nerve-tube of 
the common eel ; several oil globules have coalesced 
in the interior, and others have accumulated around 
the exterior of the tube.’ 
