VI.—The Primitive Conducting Mechanisms of the Vertebrate Heart. An Introduction to the Study of their Appearance and Development in Lepidosiren paradoxa

Abstract

The arrangement of the structures subserving the phenomena of the heart-beat has long been the study of anatomists and morphologists. Current descriptions are incomplete in essential particulars, and much confusion and uncertainty has arisen from the well-known fact that apparently the structures which conduct the impulses initiating muscular contraction in other parts of the body are duplicated or supported, in the heart, by a specialised mechanism. This mechanism is said to consist of atypical muscular fibres, accompanied by nerves, and arranged in anatomically distinct portions, nodal and bundle. In the nodal portions of the mechanism the fibres are smaller than those of the myocardium, and form a close entanglement, probably syncytial; while in the bundle of His the fibres vary in size in different animals. Ventricular extensions, again of widely divergent types, are well known under the name of Purkinje fibres. While two nodal portions are described, one, the sino-atrial node, at the commencement of the venous portion of the heart, the other, the atrio-ventricular node, at the end of it, only one distinctive pathway establishing continuity throughout the special mechanism has been demonstrated, namely, the atrioventricular bundle of His. Subsidiary structures of a like kind have been demonstrated, for example Kent's accessory atrio-ventricular bundle, but “no distinctive pathway between the sino-atrial and atrio-ventricular nodes has yet been demonstrated,” although the results of experiment have indicated the probable existence of such a communication (9).