1. The effects of viremia on the CNS of cynomolgus monkeys were studied by comprehensive histological examination following inoculations of approximately equal amounts of the same strain of poliomyelitis virus into the femoral vein, carotid artery, and vertebral artery, respectively, of four animals in each case.

2. The intravenous injections failed to produce lesions in the CNS, indicating that an effective mechanism exists for the removal of virus from the blood stream. While not absolute, the degree of protection of the CNS afforded by this mechanism appears to be of major importance.

3. Inoculations into the carotid artery failed to produce significant lesions in the CNS in two animals; only a few minor lesions in one; and bulbar paralysis in one. The neuronal areas supplied by the carotid artery are in general those of low susceptibility to poliomyelitis virus.

4. Inoculations into the vertebral artery, which supplies neuronal areas of high susceptibility, produced in all four animals severe symmetrical and widely distributed lesions in the brainstem, chiefly the motor centers of the pons, medulla, and cord, and maximal in the cord. Involvement of all of the various affected areas appeared to be simultaneous.

5. Viremic invasion of the CNS appears to occur at many points by direct passage of virus from capillary to neuron, and not at a single area of specialized vascular permeability.

6. Comparison of the two routes of arterial inoculation indicates that: (a) the localizations of CNS lesions from viremia depend largely upon the susceptibilities of exposed nerve cells in a given region; (b) in areas of high neuron susceptibility the blood-neuron "barrier" does not present an obstacle of importance to the passage of virus.

7. Invasion of the CNS from the blood results in a notable difference in the initial localization of lesions from that produced by invasion by way of the peripheral nerves, the latter tending to occur in isolated nuclear groups, usually in the lower brainstem, related to the regional supply.

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