Inflammation of the pia mater (true meningitis) is the most important of the inflammatory affections of the membranes of the brain. In its essence it is inflammation of a loose areolar tissue. It is impossible to depict its general features without distinguishing two totally different forms of the disease.

A. The First Form

a. Its product is, in general, a yellow or yellowish-green, fibrinous, or purulent exudation into the tissue of the pia mater. This product is effused according to the circumstances of the case, sometimes at separate spots, as between some-of the convolutions, or along the course of the larger venous trunks of the pia mater, while the membrane itself is elsewhere infiltrated with an opaque grayish serum: at other times the exudation is made up of flakes of fibrin, and is diluted with a considerable quantity of serum; it is of a grayish-yellow, or variegated greenish color, and pervades the pia mater uniformly; or it may contain but little serum, and be pure fibrin, or pus, diffused in large quantity through the pia mater and its prolongations between the convolutions. In the last case the membrane is manifestly increased in thickness, and may be easily separated from the brain, sometimes without injury, but at other times not without scaling off the surface of the brain with it. It may also be easily torn.

B. In this form the disease usually extends over the convexity of the hemispheres and as it approaches the base of the brain, diminishes in intensity. It rarely occurs at the base of the brain at all.

r. The individuals who present this form of disease, are in the youthful period, the bloom of life; they are usually strong, at any rate they show no trace of the tubercular dyscrasia.

§. The disease is usually unaccompanied with (acute) hydrocephalus; at least the exudations found in the ventricles are mostly slight: so also softening of the stomach does not ordinarily result from it.

e. Except at its periphery, the brain is unaltered by the disease.

This form of meningitis is very frequently a primary and idiopathic disease; at times it is met with frequently, and according to Forget and others, is even an epidemic: and further, it is the usual result of concussion of the brain. Sometimes it is a secondary affection, and as such, it succeeds other inflammations, extensive exudations on serous membranes, for instance, pneumonia, etc, or it is induced by the contiguity of the membrane to circumscribed inflammation going on in the dura mater and cranial bones, by phlebitis of the venous trunks of the pia mater, or of the sinuses of the dura mater by the advance of inflammation or abscess of the brain, and so on; or, lastly, it may be excited by the irritation which adventitious growths of various kinds produce within the cranium.

It usually, as I have already pointed out, spreads over a large artificial area, and is sometimes accompanied with inflammation of the membranes of the spinal cord: even when the inflammation has advanced from some other tissue merely to circumscribed spots of the pia mater, it usually spreads out very rapidly into an extensive (general) meningitis. To this, however, there are occasional exceptions; for inflammation and even suppuration sometimes continue to be limited to their original site; as, for instance, is sometimes the case with the inflammation and suppuration dependent on caries of the skull.

The inflammation is always acute, and more acute in proportion to its intensity, and to the abundance of the exudation. What is called chronic meningitis is really nothing but the various terminations, and the metamorphoses of the products, of an acute inflammation.

Much as it might have been expected, yet are arachnitis on the one hand, and inflammation of the periphery of the brain on the other, far from being constant accompaniments of inflammation of the pia mater. Exudation on the free surface of the arachnoid very rarely occurs, and it is found only when meningitis reaches its most intense degree: the surface of the brain, on the contrary, is certainly more frequently involved. In some rare cases it may be concluded from the terminations and consequences of the inflammation, that it has extended from the pia mater to the surface of the brain in one direction, and in the other to the arachnoid and dura mater, and even to the bones of the skull.

This form of meningitis is frequently fatal; but it often terminates in resolution. When its products change into a cellular or fibroid tissue, the inflammation leaves the pia mater a thickened, whitish, tough, dense membrane; the arachnoid shares in the thickening, the pia mater acquires unnatural connections with the surface of the brain, the cerebral arachnoid adheres to the dura mater, and at length even the dura mater and the skull unite. In some rare cases of this kind, the surface of the brain and skull are bound together by a series of successive normal and false membranes. Such terminations of meningitis are frequently found in mental disease, especially in cases of secondary imbecility.

Meningitis terminates in suppuration only when the inflammation is local, and passes to the pia mater from other contiguous structures.

B. The Second Form

a. The products of the second form of inflammation of the pia mater, though commonly mixed with yellow or yellowish-green spots of fibrin, consist in greatest part of an opaline, flocculent, albumino-serous, gelatinous, sero-purulent, and usually very abundant exudation, the color of which is grayish, sometimes shot with yellow or faint green. The opacity of the infiltrated tissue of the pia mater, and of the arachnoid, is palpable in consequence of the transparency of the product. The first-mentioned fibrinous exudation very often assumes the form of granular, plastic nodules, which become tuberculous.

B. It occurs almost exclusively at the base of the brain: and the peculiar product, which was mentioned second, especially accumulates between the hemispheres of the cerebrum on each side, from the optic commissure in front, to the pons, and even over the medulla oblongata behind. In this situation it is deposited amongst the numerous vessels and bands of cellular tissue which pass across the spaces between the pia mater and the bridge-like arachnoid. From thence it may be traced into the fissures of Sylvius, and the longitudinal fissure of the cerebrum, and so on to the convex surface of the hemispheres; for the fibrinous product (that which tends to become tubercle) accumulates along the vascular trunks which run in the fissures, viz. - the arteries and veins of the fissures of Sylvius and corpus callosum, and the latter often appear completely enveloped in the exudation.

y. From these points the inflammation always extends also to the choroid plexuses and the lining membrane of the ventricles, particularly the lateral ventricles, and there gives rise to the exudation of a similar product, from which a distinct purulent sediment is often deposited: thus it is combined with acute hydrocephalus; and very often it is associated also with softening of the stomach.

d. The brain is always in a state of (acute) oedema or serous infiltration, and of (hydrocephalic) swelling. (Compare Hydrocephalus, treated of below.) At those parts where the process is most intense, and particularly in the fissures of Sylvius, the cerebral convolutions, especially at their superficial parts, become the seat of red or yellow softening.

e. The subjects of this form of the disease are mostly children, although it is frequent also at later periods of life. I shall have again to notice that the individuals who are attacked with it are mostly persons of bad constitution, especially of a tubercular dyscrasia, or those in whom tubercle is actually deposited.

This form of meningitis is sometimes primary, sometimes secondary; when attended with tubercular exudation, it is generally secondary, and combined with tuberculosis of other organs. Frequently also, more frequently than the first form, it comes on secondarily as an attendant on various diseases of the brain, such as adventitious growths, particularly tubercle, inflammation, abscess, softening of the brain, and so on, in broken-down constitutions.

The peculiarity of the product of this meningitis appears therefore to arise from exhaustion of the fibrin of the blood by previous exudations of tubercle, or from hoematosis, resulting from the influence of some of the above-named diseases, pre-existing in the brain. Both in its quality, and in the condition on which that quality depends, the product is strikingly analogous to the gelatinous pasty products which are furnished by pneumonia under the like circumstances of a defibrinated condition of the mass of the blood, and are attended with considerable hepatization and extensive tuberculosis.

Though convinced that in distinguishing two forms of inflammation of the pia mater, and in my portraiture of them, I have been perfectly true to nature; yet I must say expressly that the disease presents other exceptional characters.

a. Instances occur, on the one hand, of meningitis at the base of the brain, in which the exudation is fibrinous (plastic), and even purulent;

B. And on the other hand, it is by no means rare for meningitis, on the convex surface of the hemispheres, to furnish a tubercular exudation. Isolated spots of inflammation are then found scattered through the pia mater, especially through the part of it which dips between the convolutions, and it appears infiltrated with a yellow granular exudation. The spots of inflammation are very commonly, too, the seat of hemorrhage, and the membrane adjoining the tubercular mass appears quite filled with coagulated blood. The adjacent surface of the brain is generally in a state of red (inflammatory) softening. These processes are, almost without exception, combined with tuberculosis of other organs, especially with tubercle in the brain; and they appear to be analogous to the lobular pneumoniae which are attended with tubercular exudation - tubercular infiltration of the lungs.

The meningitis which occupies the base of the brain, very often extends into the neurilemma of the cerebral nerves, as they pass off through the inflamed part.

4. Adventitious Growths

Exclusively of growths, which originate in the brain, and implant themselves secondarily, and in part only, in the pia mater, this class of disease is restricted in the pia mater to tubercle.