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26 ACUTE POLIOMYELITIS may be in the midst of a densely infiltrated zone (Wickman, Forssner and Sjovall, Harbitz and Scheel, Strauss).

An exact knowledge of these relations of the interstitial to the nerve cell changes is of great importance as regards pathogenesis.

D. Brain. — Redlich was the first to demonstrate that exudate around the vessel walls and small foci of round cells — evidences of inflammatory reaction — occur also in the brain. In all hitherto examined cases these brain lesions have been minute. Harbitz and Scheel made the most thorough investigations of the nature and site of the cerebral lesions in acute poliomyelitis. They found the basal part of the brain, particularly the neighborhood of the Sylvian fossa and the central ganglia, as a rule, to be more intensely inflamed than the surface convolutions; and that the central was the gyrus most often implicated. They further noted the involvement of the cerebral meninges, especially in the region of the Sylvian fossa. I shall later refer to two cases of encephalitis reported by them.

In five cases, in which Wickman had an opportunity thoroughly to' examine the brain, much milder changes were present, especially in the pia. But even in these cases, scattered small foci, mostly in the central ganglia and in the central gyri, were observed (Plate II, Fig. 8).

The cerebellum presents analogous pathologic changes.

E. Other Organs. — The internal organs have seldom been investigated. Parenchymatous degeneration of the heart, liver and kidney — occasionally even a distinct nephritis — has been reported. Inflammatory changes outside the nervous system have been observed in the pericardium only, and in but a single case (Wickman).

Reparative and Cicatrising Stage. — I shall describe this stage very briefly as from every point of view it is of much less interest than the acute stage. I have already related how, even at the height of the small celled infiltration granule-containing cells, which develop from the polyblasts, appear between the small round cells. Absorption thus commences. After a while, if the process were diffuse and severe, one can no longer speak of the tissue proper ; the tissue is dissolved, melted and ingested by these granular cells (Plate II, Fig. 9), which lie partly scattered, but mainly concentrated around the lymphatics of the vessels. The support-