592 
bling, when perfectly fresh and unaltered by 
re-agents, the margin of an oil globule. When 
the nerve-tube has been treated with water, or 
has been allowed to remain a little time ona 
piece of glass, we observe within the tubular 
membrane a double-edged layer of a whitish 
material of different refracting power from either 
that which occupies the centre of the nerve- 
tube or the tubular membrane itself. The later 
after death the nerve is examined, the more dis- 
tinct does this inner layer become. The addi- 
tion of water, alcohol, and other re-agents always 
renders it more evident, and seems to destroy 
the apparent homogeneousness of the pulpy 
contents of the nerve tube. This layer within 
the tubular membrane is that which, according 
to Schwann, gives to the nerve-tubes their white 
colour; it is therefore called by him the white 
substance. Within this and occupying the 
centre of the tube is a transparent, somewhat 
flattened, band, which is extremely delicate, and 
in which it seems impossible to recognize any 
more definite structure. 
Thus Remak and others describe three dis- 
tinct parts in the nerve fibre :—1, the outer in- 
vesting membrane, tubular membrane; 2, an 
inner layer of membrane (the white substance 
of Schwann) lying immediately within the first; 
3, a central substance of nervous matter, called 
flattened band by Remak, and supposed by 
him to consist of several filaments, or the aris- 
cylinder of Rosenthal and Purkinje. 
It is evident that the contained matter of the 
nerve-tube is extremely soft: it yields under very 
slight pressure, and may be readily made to pass 
from one part of the tube to another. When 
pressed out of the nerve tube, it is apt to assume 
the appearance and form of globules varying in 
shape and size, which are easily distinguished 
from the true nervous globules by the absence 
of nucleus. Firm pressure will also completely 
empty the tubular membrane, and thus afford 
us a good opportunity of examining its struc- 
ture, which has always appeared to present the 
same homogeneousness as the sarcolemma of 
muscles to which we have compared it. Some 
observers, however, admit a complexity of struc- 
ture in this tubular membrane; an ap nce 
of longitudinal fibres has been noti by Va- 
lentin and Rosenthal, and the former describes 
a fibrous arrangement, as of oblique fibres wind- 
ing in opposite directions, surrounding the tube.* 
The addition of water causes the contents 
of the tubular membrane to separate from the 
inner surface of the tube, owing to a shrivelling 
or coagulation which it excites in the nervous 
pulp. Alcohol produces a similar effect, but 
occasions a more perfect coagulation of the soft 
nervous matter; and it is particularly worthy of 
observation, that the complication of structure 
remarked by various observers and above de- 
scribed, in the tubular membrane as well as in 
its contents, is never seen in the perfectly fresh 
nerve, but is always rendered visible by keepi 
or by the influence of various re-agents. And 
* The spiral arrangement of fibres described by 
Barry is attributed by him to the membranous layer 
which forms within the tubular membrane, the white 
substance of Schwaun, 
NERVOUS SYSTEM. (Nenve.) 
this fact may well excite a doubt as to the re. 
ality of the complex structure of the nervy 
tube, as described in the preceding paragraph 
In a word, the real structure of the primit 
nerve fibre appears to be a tube composed 
homogeneous membrane, containing a delice 
soft, pulpy, semi-fluid and nedu 
or nervous substance, which is readily disturb 
by manipulation, and altered by the addition | 
the simplest substances—even water. The tu 
when quite fresh are perfectly cylindrical ; br 
pressure or separation alters them in shape lik 
wise, probably by disturbing the position 
the nervous oregon pushing more than is ni 
into one part, and consequently dimini 
the bulk of the contents of anode part 
the latter will consequently collapse, and 
former become enlarged, distended, and eve 
varicose. ( Fig. 330.) The margins of nerve 
tubes that have been separated, for this reaso 
constantly appear wavy, and at other tim 
distinct swellings or enlargements form in th 
course of the fibre, separated by constricte 
portions. These swellings sometimes occu 
one side of the tube only: in shape they: 
globular or ovoidal, and more frequently involv 
the whole tube ; they exist at irregular intery: 
Fig. 330. 
= 
A, Nerve tubes becoming varicose at thi 
trance into the spinal cord. Ata, b, ¢ the 
diminution of the thickness of the wall is 
B, a single nerve tube, cylindrical at one 
varicose in the rest, (From Valentin.) 
