NERVOUS CENTRES. (Humax Anaromy. Tue Cerepro-spinan Fruip.) 641 
the authority of Cruveilhier, that in fifteen sub- 
_ jects in which the latter anatomist found this 
orifice, its margins had the torn appearance in 
every one; “that around the opening, here 
and there, there existed the debris of mem- 
_ branes.”’* 
___ My own opinion is that this orifice does not 
exist naturally, but that it is produced by the 
_ violence to which the brain is subject in its 
emoval, or in the manipulations necessary for 
demonstrating it. It appears to me that the 
fourth ventricle is closed in the same way as 
‘the inferior horn of the lateral ventricles, namely, 
by the reflection of its proper membrane from 
its floor on to the adjacent pia mater. This 
embrane is so extremely delicate that the 
ightest traction upon it is sufficient to dis- 
rb its connexions. Its existence may be 
2st proved by the resistance which a probe 
pushed into the fourth ventricle from above 
experiences at its inferior extremity, a resist- 
ance, however, which a little force can over- 
come. Or, if the fourth ventricle be opened 
from the side, by a vertical section of the 
median lobe of the cerebellum some dis- 
nce to one side of the median plane, and 
if this be done on a brain previous to its re- 
moval from the body, or on one which has been 
removed with great caution, so as to occasion 
the least possible disturbance to the parts, it 
ll be found that the ventricle is closed below 
y the reflection of its proper membrane upon 
@ pia mater. There can be no doubt that 
uid driven against this membrane with force 
Would easily rupture it, whether from with- 
hn the ventricle or from the sub-arachnoid 
‘Space.t 
_Itis plain that if there be a direct commu- 
tion between the fluid in the ventricles and 
in the sub-arachnoid cavity at the inferior 
emity of the fourth ventricle, it must take 
ce through an opening in that portion of the 
mater which ascends into the fourth ven- 
icle to form the choroid plexus. But it is 
bt necessary to have recourse to such a sup- 
sition to account for the transmissibility of 
from one cavity to the other, for the 
Mater is evidently hygrometric, and will 
lily admit of the passage of fluid through 
by endosmose, and it is highly probable 
, if any interchange of fluid takes place 
the intra-ventricular cavity and the 
‘sub-arachnoid space, it is accomplished through 
‘the influence of endosmose and exosmose, 
etiected not merely by the pia mater at the 
in extremity of the fourth ventricle, but 
likewise by that at the inferior cornua of the 
jateral ventricles, and perhaps also by that of 
| the third ventricle, at the velum interpositum. 
id it is worthy of remark, as tending to con- 
firm this opinion, (which, so far as I am aware, 
has not previously been suggested.) that at 
each of these situations there is a conflux (to 
eiedie’s phrase) of the sub-arachnoid 
uid. 
_* Martin St. Ange. Sur les membranes du 
cerveau et de la moelle epiniere. 
| Be the description of the fourth ventricle fur- 
on, . 
VOL. IIIf. 
Cruveilhier lays some stress upon the fact that 
in apoplexy the blood escapes from the-ventricle 
into the sub-arachnoid space. For my own part, 
I would say that:this occurrence takes place as 
often, if not more frequently, at the inferior cor- 
nua of the lateral ventricles, as at the fourth ven- 
tricle. And therefore, if such a fact be used as 
an argument in favour of the direct communica- 
tion of the latter with the sub-arachnoid space, it 
ought equally to lead to the supposition of the 
existence of similar orifices at the former situa- 
tions, the absence of which may be easily proved. 
Moreover it may be stated that blood sometimes 
extravasates into the arachnoid sac, breaking 
through the arachnoid membrane; it is, there- 
fore, less difficult to conceive its bursting the 
pia mater, which is evidently more porous, and 
is the seat of those vessels from which the he- 
morrhage comes, a morbid condition of which 
is the frequent precursor of the apoplectic 
attack. 
The best way of obtaining the sub-arachnoid 
fluid with a view to form an estimate of its 
quantity, is to open the dura mater and arach- 
noid in the lumbar region of the spine, having 
previously, by means of a trephine, made a 
small perforation in the skull, so as to allow the 
pressure of the atmosphere to bear upon the 
cranial contents. “ If,” says Cotunnius, “ you 
open the vertebre of the loins before the head 
is touched, and cut the enclosed tube of the 
dura mater, a great quantity of water will burst 
out, and after all this spontaneous flux of water 
is spent, if you lift up the head, and shake it 
toward the aperture, a more plentiful stream 
will burst out, as if a new fountain was un- 
locked. In these experiments, which I made 
on the bodies of near twenty adults, and which 
I repeated at different times, I could draw off 
freely from the hollow of the spine four and 
sometimes even five ounces of water: I com- 
monly found it very clear in such subjects, 
although it sometimes inclined a little to a 
yellow colour; but in foetuses strangled in 
difficult labour, little as it was, I observed 
it to be always red and opaque.’””* 
The estimate of the quantity of sub-arach- 
noid fluid here assigned by Cotunnius exceeds 
that which Majendie deduces from his expe- _ 
riments, who states that in general in a subject 
of adult age and mean size, and in moderate 
condition, two ounces may be regarded as the 
minimum quantity. Much depends upon the 
age and size of the subject and the state of nu- 
trition of the nervous centres. In children the 
quantity is very small; in old age, when the 
brain and spinal cord have shrunk considerably, 
the quantity is large. In examining the bodies 
of the aged poor, as Majendie remarks, eight, 
ten, or twelve ounces of fluid may be obtained 
from the cranio-spinal cavity, according as 
there is a greater or less degree of atrophy of 
the brain. 
In judging of the quantity of fluid around 
as well as within the cerebro-spinal centres, 
* From an English translation of Cotunnius’s 
essay, entitled, A Treatise on the Nervous Sciatica, 
or Nervons Hip Gout, translated by Henry Crantz,» 
London, 1775. | 
2T 
