1. The presence of clasmatocytes in the adventitial sheath of the blood vessels of the normal brain has been shown. Thus far, these are the only mesodermal phagocytes, the presence of which in the central nervous system has proven to be demonstrable by the supravital technique.

2. The clasmatocytes are vacuolated even normally.

3. The vacuolation increases in the clasmatocytes of the whole brain as result of a localized injury, and it increases also throughout the central nervous system in the early stages of an herpetic encephalitis, even before gross damage to nerve cells or myelin can be demonstrated.

4. The endothelial cells of blood vessels have not exhibited any signs of multiplication or of vacuolation. They have not apparently given rise to any of these phagocytes, nor taken any part in the phagocytic reaction themselves.

5. Along capillaries, the phagocytic cells are seen only rarely, and it is possible that they have wandered along these vessels from the nearby venules and arterioles.

6. Trypan blue, injected without pressure into the subarachnoid space, finds its way rapidly down the perivascular sheath and enters the clasmatocytes of the sheath, deep within the substance of the brain. From this fact may be deduced the functional patency of the perivascular channels and their continuity with the subarachnoid space.

7. Lymphocytes are seen normally in the perivascular sheath, and the presence of the large young forms along with the intermediate and the small old cells suggests that they develop in this situation and give rise to the lymphocytes of the cerebrospinal fluid by wandering out into the subarachnoid space.

8. In response to an herpetic encephalitis, as well as other injury, a huge increase in the number of lymphocytes and of clasmatocytes occurs in the perivascular tissues.

9. The multiplication of both cell types seems to begin simultaneously, but never at the same spot, any one point along a vessel exhibiting a marked preponderance of either one form or the other.

10. The resting clasmatocytes cannot readily be differentiated from any other fusiform cell in sections of fixed tissues; while the lymphocyte may appear, in such sections, as an apparently isolated small round mononuclear cell, with no visible relation to the perivascular apparatus.

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