The nature of the increased vascular resistance in chronic hypertension has been the subject of much investigation and speculation. The evidence strongly suggests that this resistance is mainly at arteriolar level; hence the arterioles are often referred to as the resistance vessels. When hypertension was first recognized, it was thought that structural changes in the arterioles and capillaries might account for the increased resistance (Gull and Sutton, 1872), but these changes could not subsequently be confirmed. More recently, Folkow (1956) has suggested that medial hypertrophy might have the effect of narrowing the lumen; but so far as the author is aware, the existence of medial hypertrophy in the arterioles has never been established by actual measurements of the cross-sectional area of the wall, as Barrett (1963) did so elegantly in the case of the larger arteries. It has been clearly shown that the arteriolar wall/lumen ratio is increased in hypertension (Kernohan, Anderson, and Keith, 1929;Morlock, 1939), but this does not necessarily indicate an increase in the substance of the wall; it could be due to a decrease in diameter.Satisfactory measurements cannot be made on arterioles in ordinary histological preparations because the vessels are collapsed and contracted. Since they are collapsed, they appear flattened in cross-section instead of being circular, and this makes it impossible to obtain a valid measure of diameter. Because they are contracted-and contracted to varying degrees (MacWilliam and Mackie, 1908) as shown by differing degrees of undulation of the internal elastic lamina-the wall/lumen ratio is of limited significance. Both these difficulties can be overcome by distending the arterioles to such a degree that the internal elastic lamina Received June 2, 1965. becomes completely smooth. This makes all the arterioles cylindrical, so that the diameter can be measured accurately. It also abolishes the varying degree of contraction and makes it possible to measure all the arterioles in the same state. This method has the further great advantage that all arterioles in a section can be measured, not only those cut at right angles; for, however obliquely a distended vessel may be cut, the shortest diameter is the true diameter. Distending the arterioles should not be regarded as an unnatural procedure. On the contrary, by reproducing the systolic pressure existing during life, it is likely that the calibre of these vessels is restored to a more natural state than exists in uninjected post-mortem material (Van Citters, Wagner, and Rushmer, 1962).The small intestine was chosen for this study because the evidence suggests that this is a representative portion of the peripheral vascular bed in hypertension. It is also usually readily available at necropsy. In our first report on this work (Short and Thomson, 1959) we compared the crosssectional area of the wall and the wall/lumen ratio ofarteries in the mesentery (the vasa recta) in hypertensive and control cases, and also described the arteriogra...
