on an interruption of the motions of the nervous power, which interruption, he conceived, may be occasioned either, first, by morbid conditions producing a very general compression of the brain as the origin of the nerves; or, second, by causes destroying directly, and independently of compression, the mobility of the nervous power.

The most frequent causes of cerebral compression he considered to be vascular distension and effusion, the latter of which, he supposed, may consist either of blood or of serum. The states both of over-distension and of effusion may be produced, he farther conceived, by whatever increases the afflux and impetus of the blood in the arteries of the head; but they are more frequently produced by causes which operate by preventing the free return of the venous blood from the vessels of the head to the right ventricle of the heart.

In employing the doctrine of cerebral compression to explain the phenomena of apoplexy, Dr Cullen seems to have been aware that this doctrine is not unattended by physical difficulties. The cranium," he remarked in his lectures (Works, vol. ii. p. 339), "is very accurately fitted to its contents; and any turgescence of the cerebral vessels must certainly be limited by it. If the bloodvessels are, therefore, upon any occasion, preternaturally distended, they must occupy the place allotted to them and to the medullary substance. I would, therefore, not consider the state as a compression on the origin of the nerves, but rather as a general dilatation of the whole cortical substance and of the vessels interposed in it, operating by a compression on the whole medullary substance." It may be doubted, however, whether, by any restriction of the nature here suggested, the doctrine of cerebral

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compression could be reconciled with the actual physical conditions of the encephalon, as suggested by Dr Monro secundus, and more fully established by the recent experiments of Drs Sanders and Seeds and of Dr Kellie, and by the reasonings of Drs Carson, Abercrombie, Serres, and others. From these it would арpear, that, as the substance of the brain is nearly incompressible, as the parietes of the cranium are unyielding, and as the atmospheric pressure can, so long as the cranium is entire, act on the encephalic bloodvessels only in one direction, the absolute amount of the contents of this cavity cannot undergo any change in health or in disease, whatever variations may occur in the relative proportions of the several substances, solid and fluid, of which these contents consist. No portion of these contents, therefore, can be withdrawn, without its place being simultaneously occupied by some equivalent; nor can any thing exuberant be intruded, without an equivalent displacement. It is these physical difficulties in the doctrine of cerebral compression,-which do not seem to be, as yet, everywhere fully appreciated,-that has rendered it necessary to seek for some other explanation of the changes which occur, in apoplectic affections, in the exercise of the animal and intellectual functions. How far such an explanation may be found in the varying rapidity with which the blood may circulate through the cavity of the cranium, or in the variations that may take place in the relative proportion of this fluid contained in the several sets of encephalic vessels, the arteries, the capillaries, the veins, and the sinuses, or in the several portions of each of them, are questions still open for consideration. M. Magendie

has satisfied himself that there naturally exists, within the cranium and spinal canal, a certain quantity of serous fluid capable of passing into the spinal canal, in the event of an accumulation of blood occurring, from any cause, within the cranium, and of passing back again into the cranium on the cessation of such vascular turgescence. This ingenious speculation would, if duly corroborated, afford a simple explanation, consistent with the physical principle above adverted to, of those cases of general vascular turgescence of the encephalon, -occurring, as we have reason to believe, in a sudden manner, and without any possibility of a proportional wasting of the solid substance of the brain,-which from time to time present themselves to our observation.

In conceiving, from the results of post-mortem examinations, that there is a foundation for the common distinction of apoplexy from compression into the Sanguineous and the Serous, Dr Cullen was satisfied that this distinction cannot be very usefully applied to practice, as both kinds may often depend on the same cause, —that is, a venous plethora, and therefore require very nearly the same method of cure. In more recent times, Dr Abercrombie has called in question the existence of any modification of apoplexy that can properly be called Serous; contending, in the first place, conformably with Dr Cullen's view, that there is no foundation for the distinction between sanguineous and serous apoplexy, in the persons attacked, in the symptoms exhibited, or in the mode of treatment required; second, Dr Abercrombie considers it as in the highest degree improbable, on pathological grounds, that serous effusion should occur in the brain as a primary

disease, and accumulate with such rapidity as to produce the symptoms of an apoplectic attack; and third, he contends, that where serous fluid is found effused, its quantity bears no proportion to the degree of the apoplectic symptoms, and its presence, consequently, cannot be considered as the cause of these symptoms. Dr Abercrombie insists very strenuously and ably on the existence of a form of apoplexy which occurs, independently of either sanguineous or serous effusion, and in which, when death occurs, we cannot, on inspection, discover in the brain any satisfactory deviation from the healthy structure. This affection he proposes to call simple apoplexy; and he conceives that many of those cases of apoplexy terminating fatally, in which serous effusion has been met with to a greater or less extent, were really simple in their origin, and that this effusion is to be regarded as having supervened in the course of the disease, and not as having been its primary cause.

Dr Cullen, as we have seen (p. 193), recognised the possibility of apoplexy occurring independently of any of those morbid conditions which he conceived capable of occasioning compression of the brain, supposing, as he observes, that the mobility of the nervous power may, by the agency of certain morbific causes, be directly destroyed; or, in the language of Dr Abercrombie, he admitted a form of simple apoplexy. He was well aware, also, of the existence of several sources of fallacy, in judging from the appearances that present themselves on the dissection of persons dying of apoplexy, as to the actual cause of the disease. In particular, he notices, in his lectures, that, in some cases in

which morbid substances, solid or fluid, are found within the cranium, these are of too inconsiderable bulk to have produced general compression of the brain; and he was consequently led to suspect that a partial compression is sufficient to produce a pretty general collapse over the whole medullary substance. In other cases, as he has pointed out in his First Lines, though the foreign matters found within the cranium are of considerable amount, so that the appearances upon dissection might lead us to think that the apoplexy had depended entirely upon compression, there is reason to believe that these foreign matters have been developed in the progress of the apoplexy rather than operated as its cause. "Whatever," says he, "takes off or diminishes the mobility of the nervous power, may very much retard the motion of the blood in the vessels of the brain, and that, perhaps, to the degree of increasing exhalation or even of occasioning rupture and effusion; so that, in such cases, the marks of compression may appear on dissection, though the disease had truly depended on causes destroying the mobility of the nervous power."

In his account of the Chorea or Dance of St Vitus, Dr Cullen remarks (§ 1353) that, "as in this disease there seem to be propensities to motion, so various fits of leaping and running occur in the persons affected; and there have been instances of this disease, consisting of such convulsive motions, appearing as an epidemic in a certain corner of this country." I find, among Dr Cullen's papers, a letter addressed to him by Dr Farquharson, then of Dundee and afterwards of Edinburgh, which he had probably in view in this