While the essential structure of the lacertilian heart is now fairly well established, and further study can only hope to reveal differences in detail as between different lizards, the position in regard to the conducting system of the heart is rather different. Although a conducting system of specialised muscle fibres is certainly present in mammalian and avian hearts (in spite of the contention that it is fictitious in certain species—Glomset and Glomset, 1940a, 1940b), there is considerable uncertainty about lower vertebrates (fish, amphibia, reptiles). Some believe that the muscular fibres connecting the various chambers are specialised, that is to say, that they may be clearly distinguished histologically from the rest of the myocardium; and that, in the evolution of the mammalian heart, the nodal tissue at the atrio-ventricular junction, like that at the sinu-atrial junction, has undergone reduction and concentration (Keith and Flack, 1907; Keith and Mackenzie, 1910; Mackenzie, 1913). Others, while agreeing that there is muscular continuity between the chambers of the heart, deny that this muscle shows any structural specialisation, so that “the S-A node, A-V node and A-V bundle of the hearts of mammals cannot be considered as remnants of more extensive tissues of similar structure in a lowly, generalised vertebrate heart like that of the spotted salamander” (Davies and Francis, 1941a, p. 125). According to this view, the conducting systems of birds and mammals are parallel neomorphic developments to be correlated with the more rapid rate of contraction of the hearts in these homoiothermal vertebrates than is the case in poikilothermal vertebrates (fish, amphibia, reptiles).

Because birds and mammals have evolved from the great Sauropsidan and Theropsidan branches of reptiles (Goodrich, 1916), the reptiles obviously occupy a vital position in this controversy; for, if the conducting systems of birds and mammals are neomorphic developments with no counterpart in lower forms, they must then have arisen independently in those phyla; but if, on the other hand, they are remnants of a more extensive system in lower forms, we should surely expect to find these specialised tissues particularly well-developed in the reptilian heart.

Since Davies and Francis (1946) have fully reviewed the extensive literature on this subject, I need refer here only to that which is relevant to the reptilian heart.

Gaskell (1882, 1883) first claimed that in the frog and tortoise, the muscle of the S-A and A-V connexions, in contrast to ordinary cardiac muscle, is of an embryonic character, with a less distinct transverse striation, more sarcoplasm, and a slower rate of conduction to explain the interval between the contractions of the various chambers. This view was later taken up and elaborated by Keith and Mackenzie (1910) and Mackenzie (1913) after the discovery of the mammalian S-A node (Keith and Flack, 1907). They described, in the lizard and tortoise, an almost complete ring of “nodal tissue” around the junction of the sinus and the atrium: a ring of similar tissue was said to occur at the A-V junction. Since then, however, others have had great difficulty confirming these observations—many have found no obvious differences in the histological structure of the muscle at the S-A junction (Külbs and Lange, 1911; Külbs, 1912; Laurens, 1915; Swett, 1923); and at the A-V junction too, although generally workers have been more rectant to deny the presence of specialised muscle here, most of them regard it as much less distinctive than Keith and Mackenzie imply (Külbs and Lange, 1911; Külbs, 1912; Laurens, 1913, 1915). Swett (1923) goes even further and says categorically that in the alligator the A-V connexion is of ordinary cardiac musculature. More recently, Davies et al. (1952) have also been quite unequivocal on this matter, and have stated that there is no specialised nodal tissue or Purkinje fibres in any part of the heart of the alligator or crocodile—the conducting system, they say, appears as a structural entity only in birds and mammals. Nevertheless, Robb (1953) has recently questioned this by claiming that, in the turtle, “Purkinje-like tissue extends in scattered strands from the sinus venosus throughout the atria, forms a considerable portion of the A-V funnel and is distributed widely (deeply as well as subendocardially) within the ventricle” (p. 13).