monly affect the tissues of the same kind oc- 
curring in other parts of the body. 
Inflammatory affections of the dura mater are 
_ exceedingly rare, and occur chiefly in connexion 
with wounds or injuries of the spine, or in 
extension of disease from the bones. Occa- 
_ sionally but very rarely we find osseous or 
cartilaginous deposits upon it, which are most 
obvious on its arachnoid surface. Blood is 
sometimes, but not frequently, effused externally 
to it, and effusions of serous fluid are still more 
rare. Such effusions, from the usually depen- 
dent position of the spinal canal, and from the 
large venous plexus which exists around the 
dura mater, are very likely to be pseudo-morbid, 
resulting from the gravitation of the fluids after 
_ death. Cancerous or fungoid tumours may 
_ originate from the dura mater, or may arise 
externally and grow to it afterwards. Tuber- 
eles may form between the dura mater and its 
; arachnoid lining. 
When a deficiency of more or less of the 
_ posterior osseous wall of the spinal canal occurs, 
we find a corresponding dilatation of the dura 
_ mater and arachnoid sac, which, being filled 
~ with water, causes an external tumour, consti- 
tuting what has been called Hydrorachis, the 
_ consequence of spina bifida. These tumours 
"are altogether dependent on the congenital 
Biliaperfection of the bones of the spine, and 
_ whatever peculiar disposition of the spinal cord 
_ or its nerves may be found within them is due 
"to an arrest of or a disturbance in the process of 
_ developement of those parts. The details of the 
| anatomy of these tumours will be found in the 
article Spine. 
Whe arachnoid —The spinal arachnoid exhi- 
bits marksof the inflammatory process more fre- 
i ee Saitedura mater. Butin neithermem- 
_ brane does this state of disease occur often, ex- 
_ €ept as a complication of injury or of a morbid 
_ State of other parts,either of the vertebral column 
. Or of the spinal cord itself. The signs of an 
inflamed state of this membrane are lymph 
_ @ffused on its free surface, recent, or indurated 
_ €ausing more or lessthickening. Adhesions be- 
_ tween corresponding parts of the two arach- 
‘noid layers are also a good indication of a 
previously existing inflammation. But care 
Must be taken not to mistake the adhesion, 
which is often found between points of these 
membranes, for inflammatory adhesion. The 
former occurs in minute points, and is probably 
a result of drying of the membranes at the 
points of contact; the latter is always ac- 
companied by the formation of new matter 
which forms the medium of union between the 
layers. 
Cartilaginous spots are by no means unfre- 
quently found on the arachnoid membrane, 
chiefly in connection with its viscerallayer. They 
are generally small detached lamine thoroughly 
incorporated with themembrane,rarely exceeding 
in size the flat surface of a split pea, more fre- 
quently much smaller. They generally occur on 
the posterior surface of the arachnoid. I have 
seen such deposits in cases where there were no 
previous symptoms to denote any affection of 
the central nervous system, and I am disposed 
VOL. III. 
i 
é 
f 
NERVOUS SYSTEM. (Nervous Cenrres. AsNormMat Anatom ¥.) 
713 
to believe that such deposits, separated as they 
must be from the surface of the cord, are not 
likely to occasion much if any irritation to that 
organ. They are found mostly in the dorsal and 
lumbar regions. Sometimes spots of bone, of 
similar size and shape, are found scattered over 
the membrane. 
The pia mater.—The spinal pia mater being | 
the seat of the vascular ramifications which con- 
tribute to the nutrition of the cord, is also sub- 
ject to congestions often depending on causes 
quite extraneous to the spinal canal or cord. 
Thus the congestions which are produced in 
animals drowned, or asphyxiated in any other 
way, exist in the vessels of this membrane. The 
most frequent cause of congestion in these 
vessels, however, is the position of the corpse 
after death. After deaths, preceded by violent 
convulsions, there is always congestion of the 
vessels of the pia mater. But this congestion 
must be regarded as a consequence, not asa 
cause of the convulsions. The holding of the 
breath, which accompanies continued convul- 
sions, gives rise to a very general congestion of 
the venous system. 
When the congestion is very considerable, it 
may occasion rupture of bloodvessels and 
effusion of blood into the sub-arachnoid cavity. 
This constitutes a form of spinal apoplexy, which 
is apt to follow concussion of the cord, caused 
by a fall or by a blow inflicted upon the back. 
It may follow any of those diseases which are 
accompanied by convulsions—tetanus, hydro- 
phobia, epilepsy, cerebral apoplexy. 
Inflammatory affections of the niembranes, 
deposits of tubercle or other foreign matter 
which may cause induration of the cord, have 
their primary seat among the vessels of the pia 
mater. Inflammation of the membranes is 
more apt to occur among children than in 
adults. 
Abnormal anatomy of the spinal cord.—The 
absence of this organ (amyelia ) occurs chiefly 
in anencephalous foetuses. In such cases the 
posterior wall of the spinal canal is often defi- 
cient, and the canal is occupied by a reddish, 
vascular pulpy substance. It is a question 
whether the absence of the cord, in such cases, 
is to be attributed to a real defect of its deve- 
lopement or to its destruction ‘while yet in a 
very delicate semi-fluid state, by the formation 
of a dropsical effusion, either around it or in 
the canal or ventricle which exists in it at an 
early period of its developement. This latter 
explanation is rendered probable by the fact 
that all the recorded cases are of foetuses which 
had reached an advanced period of intra-uterine 
developement; and in some of them move- 
ments had been distinctly felt by the mothers, 
which could not have taken place with a com- 
bined or definite character without the existence 
of the cord. And in some of the records it is 
affirmed that the children lived some hours and 
exhibited movements and even signs of sensa- 
tion, or at least of excitability to stimuli. 
Such phenomena, if true, leave us no alterna- 
tive but to suppose that the whole cord could 
not have been absent—some portion must have 
existed as the centre of these movements, 
22 
