NERVOUS SYSTEM. (Nerve.) 
lexus, and so returning to the spinal cord. 
he nervous arch which is thus formed must 
evidently establish a communication between 
the cervical region of the spinal cord and that 
portion of the medulla oblongata whence the 
ninth nerve appears to arise. 
We find in connexion with the optic nerve a 
remarkable example of this kind of anastomosis, 
which, as in the instance just mentioned, serves 
more to connect different portions of the ner- 
vous centre than to associate particular nerves. 
In the optic tracts of man three series of fibres 
may be distinguished, one which passes to the 
retina of the same side, another series which 
goes to the retina of the opposite side, decus- 
Sating with the corresponding fibres from that 
side, and a third which passes from right to left, 
being apparently identified with or fused into 
one another at what is called the commissure, 
_and forming a series of nervous arches, which 
Serve to connect the opposite sides of the brain. 
These arches are convex towards the eyes and 
concave towards the brain. In the mole, in 
which I have failed to discover an optic nerve, 
this commissural band exists alone, the other 
two series of fibres being absent. Mr. Mayo 
has given a representation of these three sets of 
fibres belonging to the human chiasma in his 
| admirable plates of the brain. 
___ Volkmann gives an account of several anas- 
__ tomoses of this kind which he distinguishes by. 
___ the expression “verschmelzungen,” to which that 
_ of “ fusion” appears sufficiently to correspond. 
The fibres of one nerve appear as if they had been 
fased into those of an adjacent one, and thus 
_ feturn to some part of the cerebro-spinal centre 
different from that at which it had emerged. 
_ The instances cited by Volkmann are as follows : 
4. In the calf he has found an anastomosis 
nm the fourth pair of nerves and the first 
branch of the fifth pair, forming an arch from 
_the convexity of which several branches passed 
off in a peripheral direction. By far the greater 
_ part of these appeared, on microscopic exami- 
Nation, to receive their fibres from the fourth ; 
- while those fibres of the fifth which contributed 
to the formation of the nervous arch, passed 
centripetally to the brain, bound up in the 
Sheath of the fourth nerve. 2. A similar ner- 
vous arch is found very generally among 
Mmammifera between the second or third cer- 
vical nerve and the accessory. Certain fibres, 
when traced from the former nerve, appeared 
to to the centre in the sheath of the latter. 
_ Thisanastomosis Volkmann found in the human 
Subject, and in horses, dogs, calves, and cats.* 
Another example of this kind of anastomosis 
has been described by Gerber, but I am not 
aware whether his statements have been con- 
firmed by other observers. This consists of 
one or more simple loops contained in one 
and the same neurilemma. Certain primitive 
fibres emerge from and return to the nervous 
centre, forming a loop, with convexity directed 
s the periphery, without connecting 
| themselves with any peripheral texture or going 
‘ond the nerve-sheath. Gerber has desig- 
* Miiller’s Archiv. 
595 
nated these loops nervi nervorum, frt_n a sup- 
posed rather fanciful analogy to the vasa vasorum. 
Plexuses.—The plexuses are nervous anasto- 
moses of the most complicated and extensive 
kind. Those which are connected with the 
spinal nerves are found in the neck, the 
axilla, the loins, and the sacral region, and are 
well described by anatomists. There are also 
plexuses connected with the fifth nerve, the 
portio dura of the seventh, the glosso-pharyn- 
geal, and the par vagum. Each plexus is 
formed by the breaking up of a certain number 
of nervous trunks, the subdivisions of which 
unite together to form secondary nerves, and these 
again, by further interchange of fibres, give rise 
to nerves which emerge from the plexus, and 
consequently in their construction may derive 
their fibres from several of the trunks that 
enter the plexus. 
The object of the various kinds of anasto- 
mosis of nerves above enumerated appears to 
be to associate together nervous fibres con- 
nected with different parts of the brain or 
spinal cord. Thus nerve-tubes of different pro- 
perties or endowments become united together 
in one sheath, forming compound nerves; and 
certain sets of muscles, instead of receiving 
their nerves from a very limited portion of the 
cerebro-spinal centre, are supplied from a con- 
siderable extent of that centre, and each muscle 
may probably receive nerves which arise in dif- 
ferent and distant parts of the spinal cord or 
brain, an arrangement whereby remote parts of 
those centres may be brought into connection 
with neighbouring muscles or other parts, or 
even with a single muscle. 
Origin of nerves—The connexion of a nerve 
with the nervous centre is called by descriptive 
anatomists its origin. The determination of 
the exact nature of this connexion is of the 
last importance to the adoption of a correct 
theory of nervous action. Yet but little is 
known upon this subject. The fibres of the 
nerves are continuous with some of those 
fibres of the centre, in passing into which they 
experience considerable diminution of size 
and perhaps some change of texture, (see 
Jig. 330, A,) as evinced by their much greater 
tendency to become varicose under mecha- 
nical means than we generally find in the 
nerves themselves. Thus far we may con- 
fidently assert, that every nerve at its central 
extremity forms a connexion with grey matter. 
This fact, proved by anatomy to be constant 
and universal, may be considered as a law of 
the morphology of nerve which has the most 
important bearing upon its physiological action. 
What is the precise nature of the connexion 
between the two kinds of nervous matter in 
the centres has not yet been determined. We 
can see the white nerve-tubes passing between 
the elements of the grey matter and the vascular 
plexus, in the meshes of which they are depo- 
sited; but whether they form any continuity of 
substance with those elements, or simply come 
into contact with them,has yet to be demonstrated. 
We shall recur to this interesting and important 
question in the article Nervous Centres. 
Termination of nerves.—Under this term 
292 
