WHEN A TISSUE is deprived of blood flow for a limited timewhich may vary from minutes to hours-it is generally assumed that its capacity to survive will depend on the functional properties of the parenchymal cells. Less attention has beengivento the possibility that ischemia may modify the state of the blood or injure the blood vessels and thus impair the return of the blood flow, with irreversible damage to paren-chyma occurring as a secondary nge. Thereis evidence, both from the work of others1'2 and from our ownwork, 3 that early changes in the cere-bral vasculature may play a role in the pathogenesis of permanent ischemic injury to the brain. In the first paper of this series3 it was shown that if the rabbit brain was submitted to periods of ischemia exceeding 5 mm, and the blood flowwas then allowedtoreturn, localizedareas of thebrain failed to perfuse. It was evident that obstructions to blood flow had de-veloped, buttheir nature was not demonstrated. The present study was undertaken to explore this problem in more detail. The distribution and extent of the vascular lesions were determined as a function of the dura-tion of the ischemic insult, and the cause of theobstruction was investi-gated by experimental alteration of the conditions under which the ischemia was performed.

Material and Methods The experiments were carried out on albino rabbits weighng 1.4-3.6 kg. Nine were kept as controls; 40 were submitted to cerebral ischemia alone, and 22 to cerebral ischemia preceded and/or foDowed by administration of one or more drugs. The procedure for producing cerebral ischemia has been descrbed in detail else-where. 3 Briefly, it involved 2 steps:(1) sectioning the basilar artery, and (2) lamp-ing the common carotid arteries and constricting the neck with a pneumatic cuffin-flated to 350 mm. Hg, while artificial respiration was maintained by a tracheostomy