83 input to the ions is balanced by a cooling to atomic oxygen, the major neutral constituent, and measurements of the difference between ion and neutral tem perature permit the determination of atomic oxygen concentrations. Using this approach, ALCA YDE et al. 6 have shown from data taken above Saint Santin, France, that the atomic oxygen concentration at 200 km is slightly larger in winter than summer. () The molecular concentrations at heights near 200 km can also be derived from a determination of the ratio of the molecular-ion concentration to elec tron concentration, p (Cox and EVANS 7). It can be shown from the steady state form of the continuity equation for 0+ ions that (20.2) where Q(O+) represents the photoionization coefficient of oxygen atoms, and k and k19 represent the rate coefficients of reactions between 0+ ions and 1S molecular nitrogen Illld oxygen (reactions (12.18) and (12.19)). Then the ratio of atomic oxygen to weighted molecular sum is given by: (l-p)Ne [OJ (20.3) Q(O+) , [kMJ where Ne represents the electron concentration.

The present report, compares two murine models of virus induced chronic relapsing demyelination. MHV-induced demyelination in the BALB/c mouse results from the direct virus mediated cytolysis of oligodendrocytes. Extensive remyelination by oligodendrocytes is noted. Recurrent demyel- ination occurs in small areas. Infectious virus persists and 34 Fig. 2: Demyelination in SJL/J mice infected with TMEV. A) Multifocal areas of perivascular demyelination in the spinal cord (110 days post infection). Para- phenylene diamine stain. X 250. B) Perivascular inflammatory infiltration within the white matter of the spinal cord (22 days post infec- tion). Paraphenylene diamine stain. X600. C) Localization of TMEV associated antigen in the cytoplasm of oligodendrocytes (45 days post infec- tion). Vibratome section stained with the peroxidase-anti peroxidase technique. X 400. D) Immunoperoxidase staining of viral antigen within inner and outer loops of an oligodendrocyte (45 days post infectin) X 60,000. E) Longitudinal section showing viral antigen within Schmidt-Lanterman incisures (80 days post infection). X 49,000. viral antigens are localized within oligodendrocytes and their processes. TMEV-induced demyelination in SJL/J mice is asso- ciated with perivascular inflammatory infilrates and is dimin- ished by immunosuppressive measures. Remyelination by oligo- dendrocytes is delayed and incomplete. Chronic demyelination is widespread and associated with perivascular inflammatory infiltrates. The virus persists and viral antigen is local- ized within oligodendrocytes.