PROBLEMS OF THE DOUBLE CIRCULATION IN VERTEBRATES

Abstract

SUMMARY In a mammal such as a sheep there is a foetal double circulation and an adult double circulation. Both double circulations have as common factors: (a) separate return of oxygenated and deoxygenated blood to the heart, (b) anatomical separation (partial or complete) of the two streams of blood during their passage through the heart, (c) separate pathways by which the two streams leave the heart. Amongst the fishes, only in Dipnoi does the pulmonary vein enter a separate portion of the atrial division of the heart. This pulmonary vein develops in a very similar way to that in which the corresponding vein of a mammal develops. In the Tetrapoda a completely divided heart is found only in Mammalia, Aves and Crocodilia; partial division is met with in other reptiles and in Amphibia. No living amphibian has an interventricular septum. The heart of the lung‐fish Lepidosiren, in the possession of partial inter‐auricular and interventricular septa, shows an amount of division greater than that of any living amphibian. The hearts of Dipnoi differ from those of other vertebrates in the nature of the ‘valve’ in the auriculo‐ventricular canal. The interpretation of the reptilian heart has given rise to much controversy. The ventricle is primarily divided into ventral and dorsal cavities. These cavities probably correspond to the right and left cavities in a lung‐fish. The dorsal cavity becomes subdivided by a secondary septum; it is this secondary septum which in crocodiles and birds becomes the definitive interventricular septum. The similarity of the four‐chambered hearts of birds and mammals is misleading. In mammals the interventricular septum is the primary septum; in birds it is formed from the secondary septum which is met with in some reptiles. The foregoing conclusion, reached by Goodrich as the result of comparative studies, is supported by the modes of development of the septa. Mammalian and avian hearts also show differences in the mode of development of the atrial septum and the foramina in it. The alinement of the chambers during the development of the heart of Lepidosiren results in close approximation of the interventricular septum to both the interatrial septum and the septum of the conus. In higher vertebrates the alinement is not so perfect, in that the base of the bulbus (conus) is far to the right of the interventricular septum. The final development of these septa is different in mammals and birds. The conus, which in Teleostei, some Elasmobranchii and Dipnoi tends to lose its contractile properties, becomes progressively reduced in the higher vertebrates. Eventually it is partially incorporated into the right ventricle and also forms part of the roots of the aorta and pulmonary artery. The splitting of the conus into two paths in the Theropsida and three in the Sauropsida is of great significance. The association of the carotids with the right systemic arch in the sauropsids and with the left in theropsids is equally significant. The persistence of the left systemic arch in reptiles may be explained with reference to the relative volumes of the systemic and pulmonary circulations and may also be related to the embryonic circulation. The blood supply to the heart muscle itself is also of great importance, and a comparison of the coronary systems of fishes with higher vertebrates shows that the evolution of this system has proceeded hand‐in‐hand with the division of the heart. The reduced coronary system of amphibians, taken together with the absence of a ventricular septum and viewed in the light of current ideas concerning the evolutionary position of Recent Amphibia, suggests that the heart of these animals is to be regarded as highly specialized. As has repeatedly been made clear, the Dipnoi are not to be considered as ancestral to the tetrapods, yet they are the nearest group of fish to the early land vertebrates of which the soft parts are available for study. The hearts of dipnoans possess several features which demonstrate their specialized nature; nevertheless they also show other features which cannot all be accounted for by convergence. In this framework an attempt has been made to assess the evolutionary importance of the dipnoan ventricular septum, and it seems to be more in agreement with the facts to assume that a ventricular septum was present in the heart of the first tetrapods than to imagine that the condition seen in living amphibians represents an ancestral one. If the first tetrapods possessed an interventricular septum homologous with that of Dipnoi, then this septum has been lost in Recent Amphibia. It was replaced by a secondary septum in the sauropsid reptiles which gave rise to the birds, and so this secondary septum forms the interventricular septum of birds; but the original septum was apparently retained by the theropsid reptiles and so became the interventricular septum of the mammals.