Circulatory cycles in the vertebrates.

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 nutritive function. Explanation for the complete absence of a ductus venosus in some mammals is lacking. (c) A hepatic portal system is a characteristic of all vertebrates, so far as the conveying of blood to the liver from the absorptive part of the gut is concerned. It is pointed out, however, that in certain groups large amounts of blood from the body wall and caudal half of the body also pass through the liver. The best explanation for this may be that it is an accident of development, for there does not seem to be any obvious function served by this condition. The importance of the drainage of non-absorptive portions of the alimentary tract and other organs through the liver for specific physiological purposes is considered. (d) Despite the universal occurrence of a renal portal system in either the adults or developmental stages of all vertebrates, its physiologic importance is still an unsettled question. Phylogenetically it is certainly one of the fundamental characteristics of vertebrates. (e) The reason for the position of the gills in the arterial circuit from the heart to the rest of the body is obvious. The retention of the “cstigial branchial apparatus, including the vessels, in higher vertebrates is one of the best examples of persistence of ancestral structures, not for their primitive physiological function during a transitory period, but because they are the building blocks from which greatly modified specialized parts of the higher forms are constructed. (f) The position of tetrapod lungs in the circulatory sequence, receiving as they do non-aerated blood, is contrasted to that of the swim-bladder and lung-fish type of lung which receive at least partially aerated blood which has already passed through the gills. This is in line with the ‘storage tank’ function of the latter type of lung and the swim-bladder, as compared to the aerating function of the true lung.