NERVOUS CENTRES. (Human Anatomy. Tue Sprnat Corp.) 
fibres of the antero-lateral columns separate 
most quickly from it. 
The direction of these roots is almost vertical, 
and their length within the canal of the dura 
Mater is very considerable. The aggregate of 
them forms the cauda equina. 
I have not observed that the situation in 
which the ganglia of the sacral nerves are 
formed is different from others. They are 
contained, as elsewhere, in sheaths of dura 
mater, and lie in the sacral foramina, sur- 
‘rounded by fat, and from the looseness of their 
“connection with the walls of those foramina 
5 may be very easily detached. 
___ I cannot confirm Cruveilhier’s statement that 
the anterior and posterior roots in the sacral 
region together form the ganglia. 
_ The roots of the sacral nerves gradually di- 
Minish in size, so that the lowest are smaller 
than any others which emerge from the spinal 
In connecting the peculiar anatomical cha- 
facters of the spinal nerves in the various re- 
= with their physiological action, some 
‘interesting points are presented to our notice. 
_ The great size of the cervical nerves is quite 
‘in conformity with the exalted vital actions of 
the upper extremities. And the predominance 
of the posterior over the anterior roots, both 
positive, and as compared with other regions, 
corresponds with the great developement of 
Sensation in the upper limbs. 
The posterior root of the second cervical 
nerve, as has been noticed by Longet, is con- 
siderably larger than the anterior, as 3:1; and 
it is from this source that the occipital and 
mastoid nerves, the sensitive nerves of the 
“integument in the occipital region, derive their 
ents. 
In the dorsal region the almost equality of 
‘the anterior and posterior roots and the small 
“size of both is consistent with the absence of 
any great degree of developement either of the 
" Sensitive or motor power. Or if, as there is 
“some reason for believing, many of the move- 
ts in that segment of the body which is 
‘supplied from this region of the cord be of 
the excito-motory kind, then we might suppose 
each posterior root contains an excitor fila- 
‘Ment for each motor one in the anterior root; 
and if the sensitive fibres are superadded to the 
former (allowance being made for the smaller 
‘size of sensitive fibres,) the slight predominance 
of the posterior root may be accounted for. 
Lastly, the increased muscular activity of 
the lower extremities and their greater sensibi- 
lity as compared with the dorsal segment, ren- 
ders necessary the increase of size which the 
Toots of the lumbar and sacral nerves experi- 
ence. And it may be conjectured that the 
predominance of the posterior roots has refer- 
ence to the exalted sensibility of some parts 
of the lower limbs. 
One of the most important problems in the 
anatomy of the spinal cord is to determine the 
ia relation which the roots of the nerves 
bear to the columns of the cord and to the grey 
Matter. As far as coarse dissection enables me 
to determine, I would venture to make the fol 
659 
lowing statement, founded upon my own ob- 
servations. 
The anterior roots derive their(>bres wholly 
from the antero-lateral columns. Of these fibres 
it is probable that some are continuous with the 
longitudinal fibres of the cord, and that others 
pass into the grey matter. This, however, is 
very difficult, if it be possible, of demonstra- 
tion by the ordinary modes of dissection. The 
posterior roots adhere to the posterior part of 
the antero-lateral columns, and derive their 
fibres chiefly from that source. I have never, 
in numerous dissections, seen any thing to in- 
duce me to believe that the posterior columns 
contribute to the formation of the posterior 
roots. If they do, it must be by few and ex- 
tremely delicate fibres. It seems highly pro- 
bable (although the demonstration of the fact is 
attended with great difficulty) that the fibres of 
the posterior roots have a similar disposition to 
that described for the anterior, and that some 
pass into the posterior horn of the grey matter, 
and others are continuous with the longitudinal 
fibres. 
Various conflicting statements have been 
made by the anatomists who have written 
upon the spinal cord, with regard to the actual 
connection of the roots of the nerves with the 
pepe substance of the organ. Nor is this to 
e wondered at, when we consider the great 
delicacy of the investigation. It is very easy 
to trace any set of filaments to the pia mater; 
but after they have passed beyond that covering, 
the nervous fibres lose their main support and 
their bond of union, and they separate from 
each other. Their exquisite delicacy and mi- 
croscopic size render any further dissection of 
them extremely difficult. Mr. Grainger, in 
his excellent treatise on the spinal cord,* re- 
commends certain precautions which I have 
adopted with advantage. The cord should be 
examined immediately after death, as the delay 
even of a few hours increases the softness of 
the medullary substance. Great advantage is 
derived from placing the cord, immediately 
after its removal, in a very weak mixture of 
alcohol and water, as by these means firmness 
is given to the parts without rendering them 
crisp and brittle, as happens if strong alcohol 
be used. The parts should be dissected with 
very fine instruments under water. “I have 
met with most success,” says Mr. Grainger, 
“ by dividing the pia mater at the median fis- 
sure, and very cautiously raising it as far as the 
lateral furrow, leaving its connection with the 
fibres of the nerves intact; it is then necessary 
to open either the anterior or posterior lateral 
fissure aceording to the root examined, at a 
little distance above the exact place where the 
nerve which is to be dissected is attached to 
the cord, when by cautiously proceeding to 
open the fissure, the threads which dip into the 
grey matter are perceived.” Mr. Grainger re- 
commends the adoption of a similar mode of 
dissection for the cranial nerves, care being 
taken in every case not to disturb the connec- 
* Observations on the Structure and Functions 
of the Spinal Cord, p. 37. Lond. 1837. 
2Urs 
