The venous occlusion plethysmograph has been widely used for the measurement of forearm blood flow. It was pointed out by Grant & Pearson (1938) that the rate of increase of forearm volume occurring when the collecting pressure was applied to the arm might be affected by the venous return from the hand entering the forearm. In order to remove this source of error they applied a blood-pressure cuff to the wrist. This was inflated to a high pressure (about 200 mm. Hg) and thus the hand was almost completely cut off from the general circulation. The application of this pressure squeezes some blood into the forearm, and it was observed that after a sudden initial increase the forearm volume diminished, becoming constant after about 10-15 sec. It was assumed that by this time the forearm blood flow, if affected by this procedure, would have settled down to a steady rate. It was the practice of these workers to record three inflow curves in quick succession, starting the first 15 sec. after the application of the wrist cuff. The average of these three was taken as the forearm blood flow at the time concerned. It was observed that the flows measured in this way were the same as those obtained by inflating the wrist cuff and leaving it inflated for half an hour or so, taking groups of three readings meanwhile at the same time intervals as before. For this reason the inflation of the wrist cuff just prior to each series of inflow curves was regarded as justifiable. The relation between the three consecutive readings following the application was not described.It was felt that a fuller examination of the possible changes in blood flow resulting from the inflation of the wrist cuff would be desirable. The results of such an investigation are described in this paper. METHODSSince records of forearm blood flow were required at frequent and regular intervals, the automatic blood-flow recorder described by Kerslake (1949) was used for these experiments. The same system of volume recording was used since it presents no disadvantages when used at ground level,

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4. The jugular veins in the baby's neck are occluded by finger pressure and there follows an increase in skull volume, which is rapid at first, but which decreases exponentially as venous drainage diverts to non-occluded channels such as the vertebral venous plexus. At the instant of jugular occlusion the rate of skull volume increase is representative of the rate of flow in the jugular vessels prior to occlusion, and so provides an index of the relative magnitude of the intracranial blood flow. The method thus allows changes in intracranial blood flow to be followed. When occlusion is released cranial volume decreases, initially rapidly, but slowing exponentially as resting volume is regained.5. A theoretical model of the events occurring during the inflow and outflow phases has been developed, and a formula derived which allows an estimation to be made of the flow of blood through uncompressed channels. The measured value of jugular blood flow can then be augmented to an estimate of total intracranial flow.6. The mean cerebral blood flow of sixteen normal babies was estimated to be 40 ml. 100 g-1. min-' (S.D. = + 11t63).

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