THE CONDUCTING SYSTEM OF THE VERTEBRATE HEART

Abstract

In the warmblooded heart, the. conduction system is a neomorphic development, associated with the more rapid rate of the heart (in proportion to size). Ontogenetic development of these structures in the mammal supports this view. In the lower vertebrate hearts (i.e. salamander) no specialized conduction tissue is present. The sequence of the cardiac cycle, however, is similar to that of the higher vertebrate, though each chamber has its own intrinsic rhythm. The glycogen content of the frog heart, which is also devoid of specialized tissue, is inversely proportional to the in-trinsie rhythmic rates of the several chambers. In mammals and birds, parallel evolution of the specialized conducting system has taken place; small differences in the topography of the specialized conducting fibres in closely allied spp,, or in different animals of the same spp., may be attributed to developmental variation.