On the other hand an increase in the quan tity of the nervous substance, or an enlarge ment of the brain or spinal cord, consequent on an undue injection of their bloodvessels, is invariably accompanied with a diminution in the quantity of this fluid or with the total ab sence of it. In hypertrophy of the brain no fluid is found in the subarachnoid space, and very little or none in the ventricles. Irt cases of tumour of the brain encroaching upon the cra nial cavity, we find no fluid; and the same is observed where chronic inflammation of the brain has given rise to a new deposit which increases the bulk and the density of the cra nial contents. In all cases where a considera ble quantity of fluid has accumulated within the ventricles, that upon the surface is either greatly diminished or entirely disappears. In the ordinary hydrocephalus internus of chil dren fluid is never found on the exterior of the brain.
\Vhen an arrest in the developement of any portion of the cerebro-spinal axis has taken place, the space which ought to be occupied by the organ of imperfect growth is filled by liquid. In examining the heads of idiots m c ahvavs find a considerable quantity of sub aradmoid fluid, either general, or partial if a portion only of the brain be deficient. 0 if any portion of the wall of the cmnio-spinal cavity be defective, the contained viscus i protected by the accumulation of an inclease quantity of liquid in the situation of the de ficiency. Ilence the explanation of those watery tumours which occur over various re gions of the spine, in cases of spina bifida, i which the accumulation of water is .avoure by the absence of the resisting osseous wall the spine for a greater or less extent. An similar tumours are found projecting from th cranium, being occasioned by a protrusion the cranial meninges through a congenital aper ture, containing fluid and sometimes a portio of the encephalon itself.
Enough has been said to show, that the pre ternatural increase of this fluid should ii general be regarded as secondary to and con sequent upon'the diminished size of the cereb spinal centre itself, and that it has most p bably little or nothing to do with the mantles tation of peculiar symptoms during life in th great majority of instances. Whatever be th immediate cause of the shrinking of the cerebro spinal centre or of any portion of it, the increas of the fluid goes on pari passu, and in quantity duly proportionate to the decrease of bulk, so that it is in the highest degree impr bable that, in such cases as I have enumerate the nervous centre experiences any increase degree of pressure beyond that which it bears the normal state. If, however, the fluid, eith
within or without the brain, were to increa while that organ itself either preserved th same bulk or became enlarged It is plain dr it must experience an increased' degree of con pression. which doubtless would produce s rious symptoms. This very rarely happen according to my experience, as regards t subarachnoid fluid on the exterior of the brain we more frequently meet with an increase the fluid within the ventricles, and, in sualt cases, we shall find evidence of the compres sion in a manifestly greater firmness and density of its structure, and in this fact, that the lateml ventricles, when laid open by a ho rizontal section, do not collapse, as in the ordinary state of the brain, but remain quite patulous, owing to the firmness and density of their walls. And this patulous state of the ventricles may be regarded as a good indication that the fluid, collected in them, had for some time occasioned a preternatural amount of pressure.* Majendie infers, and as it appears to me with justice, that the cerebro-spinal fluid is secreted from the vessels of the pia mater. Ile states that, when a portion of the pia meter is exposed in a living animal, " an attentive eye may observe the transpiration of a liquid which evaporates, it is true, almost as soon as it appears, but which is sufficient to prevent the drying of the membrane." " To render this phenomenon of vital physics still more manifest," he adds, " it is necessary to inject a certain quantity of water, at 30° R., into the veins of the animal which is subjected to the experiment; immediately the liquid exhalation of the pia mater takes place in a more rapid manner, and consequently becomes more ap parent." We ought to be content with M. Majendie's statement respecting this experi ment : the point in question is by no means of sufficient consequence to warrant the repetition of so cruel an experiment.
Majendie's experiments have demonstrated further that this fluid can be as quickly rege nerated as the aqueous humour of the eye. He found that on puncturing the theca of the spinal cord, and perforating both layers of arachnoid membrane, the fluid quickly escapes at first as a fine continuous jet, and afterwards per sallurn in correspondence with the efforts of expiration. If the orifice be closed up and the animal left to go at large for twenty-four hours, the fluid is reproduced in as conside rable -quantity as before the first experiment.