The term meningitis denotes an inflammation of the cerebrospinal meninges. The common form is leptomeningitis, an acute inflammation of the pia-arachnoid; a much rarer type is pachymeningitis, an inflammation of the dura mater, which is a chronic condition. The former is represented by the purulent, tuberculous or cerebrospinal form, while pachymeningitis is usually the result of a constitutional disorder, syphilis and occasionally tuberculosis.
It is customary to include with the group of meningitis changes in the cerebrospinal membranes that are secondary to an invasion by spirochetes, Trichina, Torula and Cysticercus, and by carcinoma, sarcoma and other malignant growths. Whatever the etiologic factors, the meninges exhibit a manifest reaction. It pertains to the mesodermal components of the pia-arachnoid and dura, differing mainly in intensity and extensity.
In the pia, mainly of the spinal cord, Key and Retzius1 differentiated two main layers: an external, inconstant one, consisting of parallel
HASSIN GB. HISTOLOGIC STUDIES IN MENINGITIS. Arch NeurPsych. 1932;28(4):789–809. doi:https://doi.org/10.1001/archneurpsyc.1932.02240040034003
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