As the arachnoid and pia mater are closely connected and form virtually one membrane, they are always associated in their inflammations. The term Meningitis is commonly used without qualification to express inflammation of the soft membranes, which is much more frequent than that of the dura mater. The nutrient vessels of the brain pass in from the vessels running in the subarachnoid space, and their sheaths are direct continuations from the loose tissue of the pia mater. Hence in. all forms of inflammation there is liable to be an extension inwards for a certain distance along the vessels. This is important, as serious irritation of the cortical substance of the brain is apt to result.

(A) Simple Acute Leptomeningitis

This name is applied to nonspecific inflammations of the pia-arachnoid. Some irritant obtains access to the membranes, and as it is carried by the circulating cerebrospinal fluid the inflammation is usually of a spreading character. The cause is usually traumatic and the irritant is Septic in nature; but it may arise from disease of the bones, especially in ear disease, and in that case the meningitis may be associated with abscess of the brain. There are cases in which the meningitis has an embolic origin, septic matter being transported in pyaemia or ulcerative endocarditis. It has also been found associated with acute specific fevers, typhoid, scarlet fever, small-pox, and with other acute diseases, especially pneumonia. In a large proportion of cases, the infective agent is the capsule-coccus of Fraenkel (see p. 346), and this is the case in traumatic as well as in other cases.

In the milder cases there may be simply a serous exudation in the subarachnoid space, constituting an inflammatory oedema. But in most instances the case goes on till pus, or pus with fibrine, forms. The first appearances are visible in the neighbourhood of the veins, in the sulci between the convolutions. Very often there is a white or yellow band visible on either side of the vein, and this consists of accumulated leucocytes, it may be with fibrine. As the exudation increases the veins become buried in it, and the whole subarachnoid space becomes filled. The spaces being filled out the pia-arachnoid forms a bulky solid layer which may be separated from the surface of the brain, and form a mould of the convolutions on its under surface. Under the microscope the membranes on the surface of the brain and in the sulci are seen to be packed with leucocytes (see Fig. 345), and the inflammation extends, as already noted, in some measure along the perivascular spaces into the superficial part of the brain substance.

Acute leptomeningitis is most frequently a disease of the convexity of the brain. It may have its origin as in traumatic cases in a particular locality, and it may here be more intense in its manifestations. Sometimes the arachnoid and dura mater are united by fibrinous exudation in such localities. But the infection generally spreads over the surface of the brain, so that on exposure of the hemispheres it is manifestly visible. The affection does not always confine itself to the convexity, but may extend to the base of the brain. There is not commonly any dilatation of the ventricles (hydrocephalus) in this disease, but the ventricles may contain a yellow semi-purulent fluid. It is to be noted that, in accordance with what is stated in the Introduction to this section, the exudation is-in the subarachnoid space, not on the free surface of the arachnoid.

From a case of cerebro spinal meningitis, a, a, The soft membranes infiltrated with pus, also passing into the sulci, b, b.

Fig. 345. - From a case of cerebro-spinal meningitis, a, a, The soft membranes infiltrated with pus, also passing into the sulci, b, b. Dilated veins are seen in the midst. Vessels also prominent in the brain substance.

Simple chronic meningitis of the brain and spinal cord is usually secondary. There are thickenings and adhesions of these membranes in different forms of insanity, 1n diseases of the bones, in the various scleroses of the cord, especially posterior sclerosis. It is to be remembered also that a chronic inflammation of the membranes of the cord may be propagated from the peripheral nerves along their sheaths.

(B) Epidemic Cerebro-Spinal Meningitis

This disease, which occurs usually in the form of local epidemics but also in isolated cases, is to be grouped along with the acute specific fevers, depending like them on a specific morbid poison. The infective agent has been found in many instances of this disease to be the capsule-coccus of Fraenkel, which seems to have as one of its seats of election the membranes of the brain (see above). It is to be noted that pneumonia is not infrequently associated with the form of disease under consideration.

The soft membranes present evidences microscopically of acute inflammation, at first in the form of serous exudation with few leucocytes and red corpuscles. This condition is only seen in cases which have died very early after the onset of the disease, for the exudation soon takes on a purulent character as in simple meningitis. The exudation is here also in the subarachnoid space, and penetrates, along the sheath of the nutrient arteries, into the nervous tissue beneath (see Fig. 345). There is very seldom any exudation on the surface of the arachnoid, and the dura mater hardly ever takes part in the disease. The exudation is most marked in the sulci between the convolutions of the cerebral convexity, in the fissure of Sylvius, on the surface of the pons, the upper surface of the cerebellum. It sometimes extends into the lateral ventricles. In the spinal cord it is most abundant in the lumbar region, and is almost confined to the posterior surface, where it often surrounds the posterior roots.

The disease is fatal in over 50 per cent, of the cases, but when recovery occurs it is usually complete. Sometimes, however, there is permanent damage to the nervous structures, as evidenced by deafness, etc.

Apart from the condition of the meninges and the occasional occurrence of pneumonia the organs of the body show appearances similar to those in other acute fevers. The spleen is enlarged but not very greatly so. The follicles of the intestine are swollen. The liver and kidneys are enlarged and show parenchymatous infiltration. There are sometimes abscesses in the muscles and pus in the joints. The skin has a peculiar tendency to show petechia? during life, and becomes deeply stained by the blood colouring-matter after death.