Cruveilhier lays some stress upon thr fact that in apoplexy the blood escapes from thmventricle 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 rnorrhage 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 tbe 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 veitehrm 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 expernnents, 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 fcetuses 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 riinents, who states that in general in a subject of adult age and mean size, and in moderate condition, two ounces niay 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 reinarks, 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 vvell as within the cerebro-spinal centres, the time which has elapsed since death must be taken into account. As advancing decom position favours the transudation of fluids through the tissues, it is plain that the longer this period is, the less liquid will be found ; and the earlier after death the investigation takes place, the nearer will be the resemblance of the parts to their condition during life. On the other hand, a very advanced stage of decomposition will favour the developement of liquid, wher ever space may be found for its accumulation. It is, therefore, in vain for the pathologist to attempt to form an opinion respecting the quan tity of the fluid found in the cranio-spinal cavity, unless the inspection have been made at an early period after death.
Practical men are too much in the habit of attributing morbid phenomena of the nervous system to the influence of the pressure of a liquid effusion upon the brain or spinal cord. Many facts tend to shew that in a large pro portion of cases, especially in the adult, the occurrence of an increased quantity of fluid, either around those centres or within the ven tricles, is a result, and that it is probably a result of a conservative kind, consequent upon a morbid change which depresses the general nutrition of those or?.--,ans themselves. We have seen how the universal decay of the tissues, which characterizes old age, favours the increase of the cranio-spinal liquid, when it affects the brain and spinal cord. In examining the bodies of habitual drunkards, patients who die of delirium tremens, or of cirrhose of the liver, the quantity of fluid is always found to be considerable and the brain shrunk. In bed-ridden persons who have ceased to exer cise their faculties for some time, whether for mental or bodily exertion, the sarne pheno mena are witnessed. When there has been much anwmia, as in cases where death has terminated a protracted illness, in phthisis for example, or in persons who have died of hxmorrhage, or after excessive venesection, the nervous centres will be found to be small and the liquid in large quantity. In extreme cases of lead cachexy, in which the nutri tion of the nervous and muscular tissues is materially diminished, I have observed similar appearances. And, when any partial atrophy of either brain or spinal cord has occurred, there will invariably be found, at a point cor responding to it on the exterior of the organ, a local accumulation of fluid occupying a depression on its surface which has been caused by the giving way of the nervous sub stance within.