The membranes or envelopes of the brain and cord were formerly reckoned as three in number: the dura mater, lying next to the inner surface of the bones of the cranium and spinal column, forming an internal periosteum; the pia mater, which is in immediate contact with the brain and cord; and the arachnoid, which was described as a distinct membrane loosely attached to the pia mater, but more firmly united to the dura, and constituting a closed sac or serous cavity called the cavity of the arachnoid. This view is now abandoned by most anatomists, who regard the external arachnoid as simply a pavement epithelium covering the internal surface of the dura, and the visceral arachnoid as constituting the external layer of the pia. The term cavity of the arachnoid has, however, been generally retained for the sake of convenience; and it gives rise to no obscurity if we remember that by it we simply mean the space between the dura and the pia.
The diseases of the membranes which I shall consider are—congestion and inflammation of the dura, including hæmatoma; congestion and simple and tubercular inflammation of the pia; and meningeal apoplexy.
The membranes of the brain and those of the cord may be separately diseased, or both may be affected at the same time. Simultaneous inflammation of the cerebral and spinal meninges is apt to occur epidemically, and is supposed to be zymotic in its origin. This is described in another article. (See EPIDEMIC CEREBRO-SPINAL MENINGITIS, Vol. I. p. 795.)
Inflammation of the Dura Mater of the Brain.
SYNONYM.—Pachymeningitis. Either surface of the dura may be inflamed separately, and in many cases the symptoms are sufficiently definite to render it possible to diagnosticate the situation of the disease in this respect during life, especially when taken in connection with the causes. For this reason it is usual to speak of external and internal pachymeningitis. A peculiar form of the latter is accompanied by the effusion of blood upon the arachnoidal surface, which sometimes forms a tumor called hæmatoma. Congestion is also sometimes included in the diseases of the dura mater, but is rarely alluded to by writers on pathology or clinical medicine.
Congestion of the Dura Mater.
Except as the first stage of inflammation, congestion of the dura can hardly exist unless in connection with the same condition of the pia or of the brain. Its causes are either local, such as thrombosis of the sinuses, syphilitic or other growths in the membranes; or remote, the principal being obstructions to the general circulation, including valvular disease of the heart, emphysema and other affections of the lungs, renal disease, compression of the superior cava or jugulars by aneurism and other tumors, delayed or suppressed menstruation, etc. The symptoms cannot be distinguished from those of congestion of the pia or of the brain—namely, headache, a sensation of throbbing, pressure, or weight in the head, somnolence, etc. The absence of fever would serve to distinguish the affection from an inflammatory condition of the membranes or brain.