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The continuity of the transverse tubules in muscle with the extracellular space is of great physiological significance. Such a continuity is difficult to demonstrate morphologically, if, as in the case in skeletal muscle of the frog and other animals, the transverse tubules are very small or, as in the case of frog slow muscle (1), the tubules are not only very small, but are few in number. Nevertheless, the continuity of transverse tubules with the extracellular space has now clearly been demonstrated by using ferritin (1, 2) and the peroxidase reaction (3). In recent communications (4, 5), we have pointed out that in the ventricles of mammalian hearts there are two populations of muscle fibers: those that have transverse tubules (ventricular fibers, V fibers), and those that do not. The latter we have named P fibers, since one of the places in which they are invariably found is the so called Purkinje network on the endocardial surface of the ventricles. Because the transverse tubules of cardiac muscle are large in diameter, their continuity with the extracellular space can be easily demonstrated on morphological grounds alone. Nevertheless, to confirm the absence of transverse tubules in P fibers, we performed tracer experi-

PURKINJE FIBERS OF THE HEART EXAMINED WITH THE PEROXIDASE REACTION