Arterio‐venous Anastomoses in the External Ear

In studies based on both injected preparations and histological material, arterio‐venous anastomoses were found to be present in large numbers in the external ears of eight species of animal. These highly specialized blood vessels, which are much larger than capillaries and form direct connexions between an artery and a vein, are present on both surfaces of the ear and are situated mainly at two levels: in the deepest layer of the skin, and on and in the perichondrium. 2. In the different species the arterio‐venous anastomoses show certain variations in their size and shape, and in the thickness and cellular content of their walls, but the basic features of these vessels are the same in all. The arterio‐venous anastomoses run a more or less tortuous course; they have a thick wall, which is composed of variable numbers of epithelioid and modified smooth muscle cells, irregularly disposed; there is no internal elastic lamina. Studies made in the sheep's ear showed that the arterio‐venous anastomoses have a very rich innervation. 3. It is suggested that one important function of the arterio‐venous anastomoses in the ears is to permit a greatly increased flow of blood through these exposed extremities and thus keep them warm in a cold environment.