effusion, which fills the sulci, and raises the membrane from the surface of the brain. Instead of being transparent, and allowing the vessels of the pia mater to shine through, the surface looks milky to a greater or less extent, and more particularly on the surface of the hemispheres. Owing to the obliteration of the sulci by the serum, the affected portion of the cerebrum often looks as if the convolutions were compressed, but on removal of the serum the cause of this appearance at once becomes evident. The subarachnoid effusion may be independent of any affection of the arachnoid, and be solely due to congestion of the pia mater; we should then find no adhesion between the tissues, the arachnoid retains its transparency, and the fluid is more prone to follow the laws of gravitation, and form pouches at the dependent portions of the organ.

It may fairly be questioned whether the subarachnoid fluid is, in any way, due to the action of the arachnoid membrane. Whenever we find serum between the layers of the arachnoid, and in the ventricles, to adhere to the received terminology, it is right to refer it to that membrane; but the vicinity of a congeries of vessels, and the known laws of transudation, certainly favor the view that subarachnoid effusion is attributable to the pia mater only. Congestion of the vascular network contained in this membrane is extremely frequent, though we are more frequently left to infer it, during life, than that we find it after death. It is the very nature of congestion to disappear in articulo mortis. Still, there is a sufficient number of affections in which the vessels of the pia mater have been shown to be gorged with blood, without any further accompanying morbid conditions of the adjoining structures; thus, we meet with it in cases of pertussis, of fever, of capillary bronchitis, or disease of the heart. The congestion may, in the latter case more particularly, attain an extreme degree, so that, the blood not having room in the veins, we find dark venous-colored blood even in the larger arteries.

There is no doubt that, occasionally, the subarachnoid fluid is attri

Enlarged Pacchionian bodies.

butable to cadaveric changes; it is, therefore, necessary to be circumspect in at once attributing its presence to antecedent morbid action. The amount and position, and, more particularly, the concomitant appearances of the pia mater and arachoid, must assist us in determining the question in the individual case.

It is not yet decided whether the small nodules, which are found in almost every brain, but become more numerous with advancing age, and have received the name glandulæ Pacchioni,' are pathological products, or normal constituents. A superficial examination suffices to show that they are not what their first discoverer assumed them to be, conglobate lymphatic glands. They consist of an irregular fibrous network, containing some albuminous granular matter, and generally occupy the vicinity of the mesial line of the surface. They often cause perforations of the dura mater, and may thus appear to belong to this membrane, when the brain is taken out in all its envelops. Their development, at times, is so considerable as even to induce absorption and thinning of the skullcap. The term arachnoid granulations, applied to them by Louis, is probably as correct a designation as any that has been given them, and we should be inclined to attribute to them no higher importance than that belonging to warty indurations on the surface of the body. Luschka, who has especially investigated the Pacchionian bodies, confirms this view, and describes them as cactus-like projections from the arachnoid, of a fibrous organization, vascular, and covered by a scanty epithelium. He regards them as normal constituents of the membrane, but states that the hypertrophy to which they are liable may be the cause of death by the pressure they occasion.

The arachnoid is occasionally found to present an unctuous sensation to the finger, without any marked morbid changes being discoverable in its vicinity; though there is generally considerable disease in some part of the brain, which would account for a change of nutrition and secretion in a membrane like the arachnoid. This is the case in four out of five cases in which Dr. Bright applies the term to the arachnoid.

HEMORRHAGE INTO THE ARACHNOID.

We come now to the consideration of hemorrhage into the cavity of the arachnoid, a subject which presents a peculiar interest, from being the form of cerebral sanguineous effusion most common in childhood; the reverse condition obtains in adult life, when hemorrhage within the cerebral tissue is the prevailing form. In neither case do we generally succeed in tracing the mouths of the vessels from which the discharge has proceeded; though it is easier to do so than in most other hemorrhages. In fact, Dr. Watson' lays it down as a rule, that, while the hemorrhages occurring in the lungs, and other organs of the body, are due to exhalation, those occurring within the cranium are attributable to the actual rupture of a larger vessel. We have already alluded to

Ant. Pacchioni Dissert. Epistolaris de Glandulis Conglobatis Duræ Meningis. Romæ, 1705. 2 Lectures on the Practice of Physic, &c. vol. i. p. 494.

one variety of hemorrhage occurring in the tissues of the head in the new-born infant, which we stated to be the result of the mechanical pressure exerted during parturition; the same cause may give rise to arachnoidal effusion, which then commonly and speedily proves fatal. In this case, we find a larger or smaller quantity of fluid blood investing the upper surface of the brain; but, if the child survives the immediate shock of the apoplectic seizure, the effused blood will undergo that series of changes, which indicate the tendency of the natural processes to restore the parts to their normal condition. The first step is the formation of a coagulum; the fluid portions are first absorbed, then, by degrees, the coloring matter passes through various changes till it also disappears, and we then find, in the place of the original coagulum, a membranous formation, which is more or less organized, and may be shown to contain a capillary system of its own. It is a singular act in the pathology of the disease, to which MM. Rilliet and Barthez were the first to draw attention, that, unlike cerebral hemorrhage in the adult, it rarely gives rise to paralysis in the child; a circumstance explained by Dr. West, upon the principle of the blood in the latter instance being almost invariably effused into the cavity of the arachnoid, in consequence of which, the effects of the pressure act more uniformly upon all the contents of the cranium. The changes in the effused blood may, however, assume another form than the one above described, and it is important to be aware of the circumstance, as it may stimulate hydrocephalus. Instead of the serum being absorbed,

This drawing represents a portion of dura mater divided from above, and showing a sac which was filled with coagulated blood. The coats of the sac presented nearly the same thickness as the dura mater.-St. Bartholomew's Museum. Series vi. No. 25.

it may become inclosed in a false membrane, and remain as a persistent sac, exerting an amount of constant pressure upon the subjacent brain, sufficient to cause the flattening of the surface, and to induce, as they generally do, a considerable impairment of the intellect. When these

1 Lectures on the Diseases of Infancy and Childhood. By C. West, M. D., London, 1848, p. 40.

cysts are once formed they have a great power of passive resistance, and rarely diminish in size.

Three interesting and instructive cases of superficial hemorrhage occurring in the adult, are given by Dr. Abercrombie;1 and we also find a few instances recorded by Dr. Bright. In the former, nothing was found either in the brain or the other viscera to account for the effusion; in one of the cases contained in the Medical Reports there was a hypertrophy of the heart; in another, the hemorrhage was due to the rupture of a small aneurism; in a third, no lesions were found; and in two others the hemorrhage was attributable to a fall, and the viscera do not appear to have presented any disorganization.

ARACHNITIS.

Arachnitis is a term used synonymously with meningitis, to designate inflammation of the arachnoid and the pia mater. The nosologist may at the desk draw numerous fine distinctions, and classify symptoms so as to produce a uniform system of morbid processes; but nature does not bind herself to laws of this description. This remark applies forcibly to the attempts made to dissever inflammation of the visceral plate of the arachnoid from inflammation of the subjacent pia mater. Whether in the course of pathological research we shall be justified in establishing more systematic divisions than we are now able to do, is not the question; but we deem it especially our duty, throughout this work, to place before the student of pathology facts which he may recognize in the dead-house, from their having been previously observed by trustworthy inquirers, rather than to show him a maze of systems which have long been the opprobrium of scientific medicine. The subject immediately before us is a stumbling-block to the practitioner, as much or more than it has been to the pathological anatomist. The former has allowed early and traditional impressions of the necessity of employing powerful antiphlogistic treatment in all cases where heat of surface, restlessness, a tendency to, or actual convulsions, the drawn-in thumb, and other well-known symptoms, seemed to indicate inflammatory action; and many a child has been sacrificed at the altar of school-science, where the avoidance of injurious influences, or a strengthening regimen, would have succeeded in restoring the patient. Similar observations apply to much of our treatment in cerebral affections of adults, in whom cephalic symptoms are, even at the present day, too uniformly looked upon as the legitimate excuse for the application of our entire antiphlogistic apparatus. These remarks have suggested themselves by the great difference in the causes and course of several diseases, which are each due to, or connected with, inflammatory conditions of the pia mater and arachnoid; and there is also one disease which must be classed with the former, which with symptoms resembling those of meningitis leaves no

1 Pathol. and Pract. Researches, &c. p. 242.

2 Medical Reports, &c., p. 266. See, also, Mr. Prescott Hewett's paper in the MedicoChir. Trans., vol. xxviii.

trace of inflammatory or morbid action. Under the first head we class simple meningitis and tubercular meningitis; under the second, we allude to hydrencephaloid disease, to which Marshall Hall' first drew attention, and the analogue of which is presented to us in the adult, in that form of cephalic disease, termed somewhat indefinitely, serous apoplexy. In a case of simple acute meningitis, three points especially deserve attention: the vascularity of the membranes, the adventitious membrane formed between the arachnoid and the pia mater, and the effusion of serum or pus in the same position. We may, according to the stage and duration of the disease, meet with either of these appearances, or they may be combined in the same individual. If the patient has died in the early stage of the disease, we find, on removing the dura mater, that the subjacent membrane shows a great increase of vascularity, which may be so intense as almost to resemble the effusion of blood; the eye and the touch will, however, speedily detect the real nature of the discoloration. The congestion spreads more or less over the surface, or appears in circumscribed patches; on removing a portion of the arachnoid, we shall find the congested vessels dipping down with the pia mater between the convolutions. If effusion of lymph has taken place, a membranous expansion will be found here and there to intervene between the two meninges, causing a sort of marbled appearance, or bands stretching from one convolution to another. The effused lymph attains the thickness of a wafer and more, and most commonly occupies the upper portion of the hemispheres. The lymph itself dips down into the convolutions, and presents the same variations of density and consistency that this product of inflammation offers elsewhere in proportion to the date of its effusion. Some serous effusion beneath the arachnoid of the

Portion of upper cerebral hemisphere of a young woman, aged 27, with purulent effusion under the arachnoid: there were two yellow symmetrical patches, one on each parietal surface, concealing the subjacent convolutions.

base, especially about the optic chiasma, which causes the part to resemble the appearance of jelly, and a small amount of similar fluid (from two drachms to an ounce) in the ventricles, are often found in this form