The endocardium of 12 human hearts was studied following methylene blue perfusion and immersion within eight hours after death. Photomicrographs presented show two major types of endings: complex unencapsulated endings, and end‐nets. Complex unencapsulated endings are discrete, much branched, highly variable “bush‐like” endings, which originate from myelinated fibers. These structures were found only in the atria, and particularly at atrio‐venous junctions. The close resemblance of complex unencapsulated endings to stretch receptors seen in other tissues suggests that they serve as “baroreceptors” in the heart. End‐nets are composed of repeatedly branching and anastomosing fine beaded fibers. The fine, beaded fibers originate from myelinated fibers, but the number and origin of the parent fibers is not known. In this study, end‐nets were found in the atria, on the atrioventricular valves, and in the ventricular endocardium. The function of the end‐net is unknown, and while most evidence suggests that it is sensory, motor function cannot be ruled out. Copyright © 1968 Wiley‐Liss, Inc.
