The Purkinje fibers of the heart* ramify into the mass of ventricular myocardium, carrying with them the signal to contract. From their termini the signal is spread through the ID system. The Purkinje fibers are not nerves. They're actually modified strands of cardiac muscle. Recall that all muscle tissue is "excitable" and this property can be used to carry information. That's what's happened here. What is a secondary function in contractile myocytes has been transmuted into the main role of these not-quite-muscle cells.
The cells of the Purkinje fibers are noncontractile. They contain a few scanty, scattered, and more or less nonfunctional sarcomeres; they also contain a good deal of glycogen and a PAS stain will really make them stand out. This image shows the bundle of fibers that runs alongside the interventricular septum. There's such a bundle along both sides of the septum, one distributing to the right ventricle and one to the left ventricle. The Purkinje fibers are larger and less well stained than the ordinary contractile myocardium nearby.
Dog heart; H&E stain, paraffin section, 200x
*Yes, it's the same Purkinje who described the large integrator neurons of the brain. The term "Purkinje cell" can refer to either location, so "Purkinje fiber" is perhaps a better term in the context of the myocardial conduction fibers.
