Circulatory Cycles in the Vertebrates*

Abstract

Summary 1. The following significant facts have been pointed out: (a) The yolk sac of Osteichthyes (bony fishes) and Amphibia is supplied by blood from body veins, while that of Selachii (cartilaginous fishes) and Amniota receives blood by way of direct branches from the dorsal aorta. (b) The allantois is always supplied directly from the aorta by the umbilical arteries. Its venous drainage is at first by way of the umbilical veins directly to the heart, but later part or all of it passes through the liver sinuses. (c) A hepatic portal system exists in all Vertebrata, but in developing Amniota a by‐pass around the liver sinuses is provided in Aves and Reptilia by the meatus venosus, and is often provided in Mammalia also by the ductus venosus. These structures allow the allantoic vein blood and some of the vitelline and intestinal vein blood to reach the heart directly. Anterior abdominal wall and urinary bladder blood and some from the caudal region and posterior extremity drain into the hepatic portal and liver sinuses in Amphibia and Reptilia, and to a slight extent in birds and monotremes through the anterior abdominal vein or its homologue. (d) A renal portal circulation is present in all Vertebrata during embryonic life. It is present in the adults of all except Mammalia. It is apparently modified by the presence of venous anastomoses between afferent and efferent renal veins in many of these. Its anatomical and physiological relation to the renal tubules has been denied by at least one recent investigator. (e) The gills always receive blood directly from the heart by way of the ventral aorta and are drained by efferent arteries leading directly to the roots of the dorsal aorta. Head arteries arise from some of the efferent branchials, and in certain fishes pulmonary arteries also arise from them. Typical tetrapod lungs receive their functional supply by arteries directly from the heart or ventral aorta, and drain by veins directly into the heart. On the other hand, the lungs of lung‐fishes and the swim‐bladder of other fishes receive blood either from the dorsal aorta or from branches of the efferent branchial arteries and drain into both the systemic venous system (sometimes in part via the renal portal) and the hepatic portal in most fishes, and more or less directly to the heart in Amia, Cladista and Dipneusti. 2. The following possibilities and problems have been discussed: (a) The blood supply of the yolk sac of Osteichthyes and Amphibia is fundamentally different from that of Selachii and other vertebrates, indicating a difference in the physiology of the yolk sac and in its evolutionary development in these two groups. In this respect, then, selachian ontogeny is much closer to that of the amniotes than to that of other anamniotes. (b) The logic in the by‐passing of the liver by allantoic blood in those forms where the allantois is primarily respiratory is pointed out, as well as the possible carbohydrate regulating function of the liver in those foetuses (mammals) where the allantois has a nutritiv...

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