Observations, as determined by injections of India ink through the coronary arteries of isolated and perfused beating hearts, were presented. Tissue blocks were removed from the right and left ventricular walls and the interventricular septum, sectioned and cleared for microscopic examination and photomicrography. The term microcirculatory bed was defined as those vessels 100 p and smaller in diameter. The descriptions were based on 76 normal adult hearts of both sexes of six domestic species.The arterioles divided by dichotomous branching until the final division resulted in two daughter capillaries. At no time was a capillary observed leaving an arteriole larger than 15 p in diameter. The arterioles had a modified type of end artery pattern. Arterial anastomoses were uncommon in the areas and species examined. A specific area of a cardiac muscle fascicle was supplied by several arterioles whose capillaries intermeshed. The implications of this concept in relation to micro-embolism were explored.Capillary anastomoses were frequent but the connecting vessel was only about 20 p in length. Three types of capillary anastomoses were identified. Capillaries did not cross the muscle fascicles of the myocardium.Collecting venules were often distinctive in their morphological pattern. "Turnip root" patterns were observed in all species and represented the local venous collection of a muscle fascicle. A venule's capillary components were usually from several arteriolar sources.Supported in part by research grant HE 04217-05 from the National Heart Institute, U. S . Public Health Service. This paper will present some observations on the morphological pattern of the microcirculation in the ventricular myocardium of adult domestic mammals. This pattern was determined by injections of India ink through the coronary arteries of isolated and perfused beating hearts.The distribution and general pattern of the major arteries and veins has been adequately established for the human heart and those of domestic mammals. In the past, the dissector has examined only the larger vessels because injection materials failed to penetrate the smaller channels in the myocardium. The microscopist has concentrated his attention on the morphological features of the vessel walls and has neglected the definitive pattern of the microcirculation.The microcirculatory bed in different tissues is assembled in an almost endless number of variations and other than in the heart has been studied by numerous investigators. A satisfactory method of visualizing the capillary bed was devel-AM. J. ANAT., 116: 355-374.oped by Wearn ('28a) and used primarily for capillary/heart fiber ratio studies but the morphological pattern in the ventricular myocardium was not fully explored. Functional studies and pathological findings indicate that the larger coronary vessels are true end arteries. Extensive research now being conducted on circulatory diseases makes it imperative that the structural pattern of the microcirculation be investigated.The descriptions tha...
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