XVIII.—The Conducting System of the Marsupial Heart

Abstract

Elsewhere two of the present authors (Davies and Francis, 1941) have expressed theopinion that a specialised cardiac conducting system, consisting of nodal and Purkinje fibres, is a newly evolved one in birds and mammals. The general morphology and topography of this system in the heart of the bird (Davies, 1930 a, 1930 b) is similar to that in the heart of the eutherian mammal, and we have postulated (Davies and Francis, 1941) that the system has undergone parallel evolution in these two classes of homoiothermal vertebrates in response to functional requirements, and that in particular its presence can be correlated with the rapidity of the heart-rate in proportion to its size. On the other hand, Keith and Flack (1907), Keith and Mackenzie (1910), and Mackenzie (1913) maintain that the sinu-atrial node, atrio-ventricular node, and atrio-ventricular bundle of the mammalian heart are remnants of more extensive tissues of similar structure in the hearts of lower vertebrates. They trace the evolution of the conducting system of the mammalian heart from a simpler and more definite form which they described in the fish, and state that as one ascends the animal scale the concentration and reduction of nodal tissue becomes more marked.

The Conducting System of the Vertebrate Heart

“…Blair & Davies (1935)~ however, observed that while the order of the fibre size in the OX is the same as that given by T. Lewis for the dog, in the human heart (aged 14 years) the order is different, the thickness increasing in the order: nodal, atrial, A.-V. bundle, ventricular, Purkinje. As stated above (p. 181)~ Davies & Francis (1942) have shown that the proportional sizes of the various types of muscle fibres change with age. Lewis ( I~z s ) , in the case of the dog, has expressed as 'The Law of Cardiac Muscle' a relation between the size (breadth) of a cardiac muscle fibre, its rate of conduction, rhythmicity, length of systole and glycogen content, the fibre size, glycogen content and rate of conduction increasing in the following order: nodal, ventricular, atrial, Purkinje; the duration of systole and rhythmicity diminishing in the same order.…”

“…T. In this connexion Blair & Davies (1935) described differences in the histological structure of the two nodes, the fibres of the A,-V. node being thicker, more cylindrical and more branched than those of the s.-A. node is rendered inactive by cooling, the remainder is capable of initiating the normal heart rhythm.…”

“…Subsequent to this discovery many investigators more closely localized the origin of the contractile wave to the region of this structure, e.g. Blair & Davies (1935) noted that the node is a much more extensive structure than the description by Keith & Flack implied, and observed that this emphasizes the difficulty of experimental removal of the entire node and throws open to question the validity of such experimental work as claimed, without histological verification, that after removal of the node the right atrium continued to beat normally, and that this indicated the presence of other (ectopic) centres in the atrium with the same rhythmic powers as the node. T.…”

“…As stated above (pp. For example, in the marsupial heart (Davies & Francis, 1942), whereas the ordinary atrial and ventricular muscle fibres increase markedly in size from the foetal to the adult stage, the nodal ,fibres are fully developed in the foetal stage. Consistent with this view are the following observations .…”

Histological Findings of the Atrioventricular Conductive System in Street Heroin Addicts, Victims of Sudden Unexpected Death

“…More detailed descriptions of the anatomy and histology of the normal AV system and its perfusing artery have been reported elsewhere .…”