It is well known that when blood flows through a small artery, the cells occupy the axial stream, leaving a comparatively cell-poor peripheral zone; Bayliss (1959), however, has suggested that this effect may not be as marked as had been previously believed. It might be expected that a branch leaving such an artery approximately at right angles would sample mainly the peripheral zone, and this effect has, in fact, been observed by Krogh (1929) who first used the term 'plasma skimming' to describe the phenomenon. The only previous quantitative investigation is that of Pappenheimer (1958) who studied plasma skimming in glass capillary tubes and found that the blood emerging from the main tube contained 47-5 % of cells, while that from a side branch contained only 39 00. The uterine artery of the rat appears to be a vessel in which plasma skimming would occur, since it gives to the uterus a series of branches which leave the main trunk at right angles. However, it has been shown (Moffat, 1959) that the orifice of each of these branches is provided with a pair of intraarterial cushions. These are streamlined longitudinally and pass on either side of the orifice of the collateral vessel, projecting into the lumen of the main trunk (Fig. 1). Similar cushions have been described in the arteries of many organs of a variety of species, including man. Since these cushions project into the region of the axial stream, it occurred to us that their function might be to abolish plasma skimming, or even to produce the opposite effect by increasing the cell concentration of the blood passing into the branch.The object of the present study was to make a quantitative investigation of plasma skimming in the intestinal arteries of the rat, where there are no intra-arterial cushions, and to investigate the effect of the presence of such cushions in the uterine artery. The latter vessel was studied in both adult and immature rats, since in young animals the cushions are extremely small in relation to the size of the lumen (Moffat, 1959).

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