The capillaries constitute a most essential unit of the circulatory system,1 the part about which least is known. Technical improvements have greatly facilitated capillary studies. The apparatus used for our studies was recently described in detail.2
NORMAL CAPILLARIES OF THE SKIN
The first impression one gains in a study of the capillaries is that even when normal they are subject to a wide range of variation. not only in different persons, but within the same person. This is true of all anatomic parts. One must therefore guard against drawing conclusions from small differences in structure or in reaction.
—It is known that the capillaries are the finest terminal connecting loops between the arterioles and the venules. Each capillary is divided into a narrow, constricted arterial limb and a wider, more dilated venous limb. The diameter of the arterial side may be less than the diameter of a