NERVOUS SYSTEM. (Nenve.) 
from each other, and are extremely variable in 
shape and size. Two conditions appear to 
_ favour the production of this varicose state of 
__ the nerve-tube,—namely, a feeble power of re- 
sistance in the tubular membrane, and, se- 
condly, perhaps, a semi-fluid state of the con- 
tained nervous pulp; and hence we find that 
some nerve-tubes are much more prone to 
become varicose than others. In the nerves 
_ of pure sense the tubes are very delicate in 
_ Structure and very apt to exhibit this change of 
form, and in the brain and spinal cord they 
exhibit the same tendency. Ehrenberg sup- 
posed formerly that these varicosities were 
natural and existed during life, and that they 
afforded a valuable morphological character of 
the nerves of pure sense and the cerebro-spinal 
centres. But many circumstances favour the 
Opinion that the varicosities are accidental; 
thus, the very irregularities above noticed in 
their shape, size, form, and number on a single 
tube are not likely to occur in the natural 
State. Moreover, in a piece of the brain or 
Spinal cord not much pressed nor torn, the 
ones may be distinctly seen: even in 
the manipulated specimens the varicose tubes 
form only a small portion of the whole. And 
in those nerves whose fibres are not prone 
to become varicose, such as muscular nerves, 
may be made so by firm pressure and 
fiolence in manipulation. In the nerve-tubes 
‘of young animals, in whom the tissues are more 
tender and contain more abundant water, these 
' ges are also very apt to take place. 
_ A cerebro-spinal nerve, then, consists of a 
_ Congeries of fascicles or bundles of the nerve- 
‘fibres or nerve-tubes (and we shall use these 
‘terms synonymously) above described, enve- 
| and bound together by fibrous mem- 
brane, the nerve-sheath. The nerve-tubes lie 
Side by side, parallel, and sometimes have 
@ wavy course within the general sheath (fig. 
331). The relation of the nerve-tubes to each 
‘ 
| 
Fig. 331. 
Diagram to illustrate the wavy course of the nerve 
tubes within the neurilemma. 
other is simply that of juxta-position. All 
observers, from Fontana down to those of 
the present day, agree in denying the ex- 
istence of any inosculation or anastomosis 
between the fibres in vertebrate animals; 
and it seems almost certain that this complete 
isolation of the nerve-tubes is not limited 
to those of the nerves, properly so called, but 
may be observed in the nervous centres also. 
When a piece of nerve is examined on a dark 
ground, as an opaque object, with an object 
glass of a quarter of an inch focus, the disposi- 
tion and relation of its component tubules are 
more beautifully seen than by any other mode 
of examination. The primitive fibres present 
the appearance of a series of transparent tubes, 
containing an exquisitely delicate, soft, pearly- 
white material. 
VOL. III. 
593 
In point of size the nerve tubey present 
considerable variety even in the same trunk, 
while they maintain an identity of structure. 
The smallest tubes have very delicate walls, 
and are more easily rendered varicose than the 
larger ones. The following table gives a state- 
ment of the results of the admeasurement of 
the cerebro-spinal tubules in Man and other 
Vertebrata. 
Man, and other Mammalia, from yJ35 to g455 
of an inch. 
Birds, sfyp tO sqyp Of an inch. 
Reptiles, Frog, 355 tO sig5 Of an inch. 
Fish, Eel, y4j3 of an inch. 
Codfish, optic nerve, g$, of an inch. 
It has already been remarked that no such 
thing as subdivision or branching of the pri- 
mitive tubules takes place in the cerebro-spinal 
nerves of the vertebrate series. Whatever be 
the connection which each primitive tubule 
forms with the nervous centre, or with the 
textures to which it is distributed at its pe- 
riphery, it passes from one point to the other 
without any change, save perhaps in size, and 
without any communication with neighbour- 
ing tubules, beyond simple juxta-position, or 
investment by a common sheath. This fact 
was recognized by Fontana, whose description 
of the structure of nerve, although drawn up 
from observations made at a great disadvan- 
tage through the imperfection of his instru- 
ments, corresponds in all essential particulars 
with modern observations.* And as there is 
the same absence of subdivision in the con- 
tinuations of these nerve-tubes in the nervous 
centres, we may fairly infer that each point on 
the periphery which is in contact with a nerve- 
tube, is, as it were, represented by that same 
nerve-tube in the centre. 
The structure of the cerebro-spinal nerve 
admits of an obvious comparison with that 
of the striped muscle. Both are composed 
of bundles of fibres, united by a sheath, 
which also passes between the bundles, 
and is a nidus for the support of the nu- 
trient vessels. Both admit of being subdi- 
vided into primitive fibres, which are very 
analogous in structure. The primitive fibre of 
muscle (primitive fasciculus of some authors) 
consists of the true muscular tissue, or sarcous 
elements contained in a transparent sheath of 
homogeneous elastic membrane called sarco- 
lemma by Mr. Bowman. The peculiar mor- 
phological characters of the primitive fibre 
depend upon the arrangement of the sarcous 
particles within this transparent tube; and to 
this arrangement is due any further subdivision 
of which the primitive muscular fibre may be 
susceptible. So is it with the primitive nerve- 
fibre: its tubular membrane is strictly analo- 
gous in structure and other characters with the 
sarcolemma. It contains the elements of the 
true nervous tissue or neurine, and this ad- 
mits of a certain subdivision which may be ren- 
dered more apparent under the influence of re- 
agents, and which is variously interpreted by 
different observers, and has been compared to 
* Fontana sur le venin de la vipere. 
2a 
