PRECAPILLARY arteriovenous anastomoses are a normal part of the microcirculatory bed of skin, muscle, lung, heart, intestine, liver, spleen, kidney, ear, and eye.1,2 These have been labeled "preferential channels"3 or "throughfare channels" by Chambers and Zweifach.4 Whether similar anastomoses occur in the brain parenchyma has been a subject of debate. Rowbotham and Little5-9 observed precapillary arteriovenous anastomoses in the pial circulatory system but not in the brain parenchyma of man, while Pfeifer10,11 and Solnitsky12 saw them in both.Benda and Brownell13 and Swank and Hain14 injected particles up to 60\g=m\ in diameter into the carotid arteries of dogs and observed that they passed to the cerebral veins. Because the average capillary diameter has been found to be 5.8\g=m\ by Ueda,15 the passage of large emboli from artery to vein means that there is a shunt bypassing the capillaries or that the capillaries have a tremendous capacity for dilatation. Despite this it has been the conclusion of the majority of studies of the angioarchitecture of the brain in both human beings and experimental animals that, with the exception of the leptomeningeal circulation, there are normally no arteriovenous anastomoses in the cerebral vascular system. Because of this disagreement, we decided to reinvestigate the subject.
Material and MethodsTwelve normal human brains with the pa¬ tients ranging in age at death from 6 months to 96 years and five normal dog brains were selected. After fixation for four weeks in 15% neutral formaldehyde (formalin) solution, fro¬ zen sections 200µ to 500µ, in thickness were made from cerebrum (frontal, parietal, tempo¬ ral, and occipital lobes) and cerebellum, and the vascular system was stained selectively by a metallic impregnation method developed by T. Hasewaga and J. R. Ravens.16 One dog was injected slowly with approxi¬ mately 10 ml/kg of 14% trypan blue human plasma solution into the right femoral vein.The dog was killed, and the brain was fixed for seven days in 15% neutral formalin solution, after which one hemisphere was cut coronally and the other sagittally into blocks 2 to 3 mm thick. From these blocks frozen sections 200µ to , µ in thickness were made, employing dehydration, clearing, and mounting.In addition, frozen sections µ, to 150µ thick were made and stained with eosin picric acid solution (100 ml of alcohol eosin solution and four drops of picric acid) for 10 to 15 sec¬ onds. Dehydration, clearing, and mounting were employed immediately after staining. Arterioles were stained violet-blue and venules, light violet or pink.Another dog was infused with 14% trypan blue human plasma solution; and after it was killed, 60 ml of a mixture of india ink in 4% gelatin solution was injected into both common carotid arteries. The brain was removed and fixed in 95% alcohol for seven days, and one hemisphere was cut coronally, the other sagit¬ tally into 2 to 3 mm thicknesses. These were placed in 95% alcohol for seven days, in ab¬ solute alcohol for three days, and immersed in a so...