Attempts have been made to classify the various forms of meningitis on a bacterial basis. When, however, it is realized that almost any known form of organism may be present in the exudate, and that the symptoms dependent on these various infections closely resemble one another, merely differing in their acuteness and intensity, it would appear that some other form of classification is preferable.
The following method of classification will suffice:--
=1.= =Meningitis of traumatic origin.= The organisms either obtain direct entrance to the meningeal space by means of a compound fracture of the vault or base of the skull, or reach their destination along the course of the numerous emissary and diploic veins that bring the extra- and intra-cranial channels into communication.
=2.= =Meningitis secondary to disease of neighbouring regions.=
(1) The accessory sinuses of the nose and ear.
(3) The soft parts--scalp, orbit, nose, and face.
=3.= =Meningitis secondary to disease of more distant regions.= Through the medium of the blood-stream bacteria may be carried to the meningeal region in many acute infections--more especially in diseases of the lung.
=Some anatomical considerations of meningitis.= The _pia_ is closely applied to the surface of the brain, dipping down into all the fissures, both major and minor. The _arachnoid_ is only reflected into the greater fissures--the mesial longitudinal fissure, the fissure of Sylvius, Rolando, &c. The pia and arachnoid are therefore separated from one another by a potential space over the convolutions themselves, and by an actual space in the region of many of the cortical fissures.
On the under aspect of the brain the two membranes are so disposed and so separated from one another as to form three great basins or cisternæ, three great spaces acting as a water-bed for the brain. These are the _cisterna magna_, the arachnoid bridging over the wide interval between the back part of the under surface of the cerebellum and the medulla, the _cisterna pontis_, the angular space between the pons, medulla, and cerebellum, and the _cisterna basalis_, the arachnoid there covering in the structure situated in the interpeduncular space, including the Circle of Willis.