PHYSIOLOGY OF THE NERVOUS SYSTEM. 
experiments show that the brain readily imbibes 
fluid, and that parts in the vicinity of and 
bathed in fluid may present a pseudo-morbid 
softening from such imbibition. The fact, 
thus ascertained, serves to account for the 
more frequent occurrence of softening in the 
fornix and septum lucidum than in other 
parts of the brain. It is obvious that pseudo- 
morbid softening of this kind would occur 
only in parts within the ventricle or in the 
cerebral substance forming their walls, or 
on the surface of the brain itself, and that it 
is less likely to be limited to one side than the 
morbid softening. Now and then, however, in 
cases of general anasarca, where the blood is 
in a very watery condition and much fluid is 
effused, the brain exhibits a softened state from 
the imbibition of this fluid. 
ABNORMAL ANATOMY OF NERVES.—Certain 
nerves are sometimes absent, from a defect in 
the developement of the organ to which they 
eraesoted 5 as the optic nerve, or the olfac- 
tory, when their respective organs are wanting. 
_ The non-developement of the eye will also 
cause a non-developement of the fourth pair 
_and the other orbital nerves which influence the 
movements of the eyeball. 
Inflammation of a nerve rarely occurs idiopa- 
_thically or primarily. Occurring from what- 
_ ever cause, it would be distinguished by hyper- 
xmnia, enlargement, and by deposit of more or 
less of lymph or pus. In the acute inflam- 
‘mation the nerve would be softened ; but in the 
| chronic it would become indurated. Abscess 
_ of a nerve is of very rare occurrence. 
Inflammatory affections of nerves occur chiefly 
im connexion with rheumatic or gouty states of 
the system. Sciatica is, no doubt, an in- 
lammatory affection of the sciatic nerve of the 
“gouty kind. In lumbago probably the mus- 
cular nerves of the lumbar muscles are similarly 
fected. 
| Atrophy is a condition into which nerves 
may fall from disuse or from pressure. In it 
the nerve-fibres shrink, their central axis wastes, 
and in extreme cases disappears entirely, the 
tubular membrane becoming plicated and as- 
Suming the characters of fibrous tissue. The 
herve experiences a great diminution in size, 
‘and the wasting is obvious to the naked eye. 
Hypertrophy.— Whether a nerve becomes 
enlarged when more work is thrown upon it, as 
a muscle does, is as yet quite uncertain. I 
am disposed to think that the nerve-fibres may 
acquire some increase of size; but it seems 
to me impossible that they should become more 
humerous. ‘The number of nerve-fibres in in- 
dual nerves, as that of muscular fibres in 
Muscles, is probably determined at their pri- 
mary developement,* and they undergo no 
change but that of length and thickness subse- 
quently. It would not be difficult, by destroy- 
ing the office of the vagus nerve on one side, 
jto ascertain whether, after the lapse of some 
* A similar law probably prevails with other tis- 
ues, namely, that the number of their proximate 
Plements is determined at primary developement, 
And that in subsequent growth these elements may 
herease in bulk but not in number. 
The morbid states of nerves are few and rare. . 
720G 
time, the other, upon which its function would 
devolve, acquired any increase in the size of its 
nerve-fibres. 
Certain gangliform tumours are formed upon 
nerves, to which the term neuroma has been 
applied. They consist of areolar tissue and of 
nerve-fibres, and seem to be formed by an 
increased developement of the areolar tissue 
between the nerve-fibres. These tumours vary 
considerably in size and number; sometimes 
they are not larger than a filbert or a gooseberry 
sometimes as large as a walnut. In genera 
they are few and limited to one nerve, and their 
size is proportionate to that of the nerve with 
which they are connected. In a few rare cases 
tumours of this kind have been found in im- 
mense numbers scattered over the whole cere- 
bro-spinal system. 
(R. B. Todd. ) 
NERVOUS SYSTEM, Puystotocy or 
THE.—In inquiring into the physiology of the 
nervous system, the first step is to determine 
the vital endowments of nerves and of nervous 
centres. 
When a nerve is laid bare in a living animal, 
and a mechanical or electrical stimulus is ap- 
plied to it, we do not find as in muscle that a 
visible change in the nerve takes place; on the 
contrary, the nerve seems to be uninfluenced 
by the applied stimulus, and the evidence we 
have to the contrary is derived from the con- 
traction of certain muscles, if the nerve be 
muscular, or from indications of pain, if it be 
a nerve of common sensation. 
We infer, then, from the contraction of the 
muscle in the one case, or from the affection of 
the mind in the other, that the application of 
the stimulus has wrought a change in the 
nerve, which, however, is of such a nature as 
not to be discerned by any means of observation 
within our reach. We get, however, excellent 
proof of the excitation of the change in the 
nerve, from the fact that when a ligature is ap- 
plied to a nerve sufficiently tight to produce a 
solution of continuity in the nerve fibres, the 
propagation of the influence of the stimulus 
beyond the ligature is checked. No kind nor 
degree of stimulation of a muscular nerve above 
a ligature so applied is capable of exciting 
muscular contraction. 
The most remarkable feature which we notice 
in the experiment of stimulating a muscular 
nerve, is the instantaneousness with which the 
muscular contraction takes place. Although 
the muscles may be at a considerable distance 
from the point of the nerve to which the stimu-. 
lus is applied, there seems no appreciable 
interval of time between the application of the 
stimulus and the contraction of the muscle. 
And the cessation of the muscular contraction, 
instantly upon the removal of the stimulus, is 
equally conspicuous. 
It would appear, then, that the change in 
the nerve is produced and is propagated along 
the nerve to distant parts, as it were at one and 
the same moment. This rapidity of the pro- 
duction, and the instantaneousness of propaga- 
tion of the change in the nerve, denote that the 
nerve fibres must be the seat of a molecular 
