Although the general structure of the lacertilian heart as revealed by gross dissection has been long known (Owen, 1866; Huxley, 1871; Wiedersheim, 1886; and more recently Meinertz, 1952) the study of its detailed anatomy as disclosed by histological methods and serial sectioning is of comparatively recent date, and, furthermore, investigations of this kind have been carried out on relatively few species and genera, such as Lacerta viridis and muralis (Külbs and Lange, 1911), Lacerta agilis (Krause, 1922), Tiliqua scincoides (Rau, 1924), Uromastix hardwickii (Bhatia, 1929), Hemidactylus flaviviridis (Mahendra, 1942) and Varanus monitor (Mathur, 1944). Much of the interest of these investigations and the discussion concerning them centres around the subdivision of the ventricular cavity and the precise positions of the various orifices, especially the arterial ones. Here Mahendra's work (1942) is of particular interest because his subdivision of the ventricle provides a convenient basis for its description in the Sauria, and thus more readily facilitates comparison between the various species.1
It is well-known that in most reptiles (except crocodiles, which have a complete interventricular septum) the ventricular cavity is incompletely divided by a muscular ridge projecting in from the ventral wall. This projection (called “Muskelleiste” by German workers, and “interventricular septum” or “septum ventriculorum” by most English workers) is probably not equivalent to the entire interventricular septum of higher forms, and therefore Mahendra (1942) has preferred to call it the “muscular ridge”. This name, however, has been rather an unfortunate choice, for Rau (1924) in one of his figures, had previously labelled as “muscular ridge” an entirely different projection from the apical wall of the ventricle in some reptiles, at the place of separation of arterial and venous blood.
[Footnote] 1Only when this paper had been prepared for publication did I have access to the paper by Prakash on the heart of the Indian spiny-tailed lizard, Uromastix hardwickii (Proc. Rajasthan Acad. Sc., 4, 1–12, 1953). In this paper, Prakash has also followed Mahendra's method of description and his conclusions in regard to the gross structure are in substantial agreement with mine. He has not, however, considered the coronary circulation nor the conducting system.
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).
Fig. 15.—A sagittal section (X 80) of the A-V junction, just to the right of the intriatrial septum. The section is viewed from the left. The atrial musculature is invaginated into the base of the ventricle as the A-V funnel. The epicardial tissue (Ep) accompanies this invagination for some distance, completely separating the atrial from the ventricular musculature. More caudally, however, these are seen to become directly continuous. This section also shows the right septal cusp (R. S. C.) of the bell-shaped valve with its concavity directed towards the ventricle and its caudal margin attached to the ventricular wall both ventrally and dorsally. Fig. 16.—Ventral view of the hear of Letolopisma grande in situ. Note the sac-like diverticulars (S. D.) of the right atrium projecting between the diverging carotid arteries. Fig. 17.—A wax-plate reconstruction of the sinu-atrial junctional region, from the ventral aspect. The ventral wall of the sinus venosus (S. V.) and the dorsal wall of the right atrium (R. A.) can be seen. The rostral (R. S. V.) and caudal (C. S. V.) cusps of the S. A. valve project into the right atrium. At its left-hand end the rostral cusp continues on to the dorsal part of the interatrial septum (I. S.). A white arrow passes through the opening of the pulmonary vein (P. V.) into the left atrium.
Figs. 18 21 are thick hand-cut transverse sections of the ventricle seen from above. The sections proceed in a caudal direction. × 16. Fig. 18.—Shows the muscular ridge separating the large cavum dorsale from the small cavum pulmonale (into which a hair has been inserted). Fig. 19.—This section includes a portion of both the muscular and apical ridges. The muscular ridge again separates the cavum pulmonale ventrally and to the right (with a hair inserted in it) from the cavum dorsale dorsally. The apical ridge can be seen running dorso-ventrally across the cavum dorsale, dividing it into the cavum venosum on the right, and cavum arteriosum on the left. Fig. 20.—The muscular ridge is not present at this level, but the apical ridge is prominent. Fig. 21.—Apex of the ventricle. Here muscular trabeculae divide the ventricular cavity into a number of regular spaces into which hairs have been inserted. Fig. 22.—Photomicrograph of ventricular muscle to show the characteristic appearance of the bulged portions of the fibres when these are cut in transverse section Azan × 600. Fig. 23.—Photomicrograph of a nest of epitheloid cells situated between the right and left systemic arteries in the adventitia of the truncus. The actual paraganglion cells may be distinguished by their distinct nucleoli (as indicated by arrow). Three fat cells are seen in the cell-nest in this section H. P. F. × 615. Fig. 24.—Subepicardial vein communicating with intertrabecula space H. P. F. × 290.
Fig. 25.—Photomicrograph of ventricular muscle showing the bulgings of the fibres around certain nuclei. It can be seen that these bulgings are quite localised and are not present along the entire length of the fibre. Azan. × 600. Fig. 26.—Photomicrograph of atrial muscle also showing occasional bulging of the fibres. Azan. × 650. Fig. 27.—Photomicrograph of ventricular muscle showing small masses of glycogen (arrows) within the individual fibres. Haematoxylin and Best's carmine × 720.
In an attempt to clarify this issue, I have made a detailed study, both gross and microscopic, of the heart of Leiolopisma grande, in which I have also studied the distribution of the coronary arteries, and the cardiac paraganglion cells which Trinci (1912) and Palme (1934) have described in other reptiles in the adventitia of the arteries of the truncus.