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In this paper additional evidence is given by the author in corrobo­ration of the principles which he had announced in a former commu­nication, which was read to the Royal Society in May 1835, on the influence of the tricuspid valve of the heart on the circulation of the blood. His object is to demonstrate that the tricuspid valve in man occasionally serves the purpose of a safety-valve, being constructed so as to allow of the reflux of the blood from the ventricle into the auricle, during the varying states of distension to which the right cavities of the heart are at times subjected; that a similar function is maintained in the greater number of animals possessing a double circulation, and also that in the different orders of these animals the structure of this valve is expressly adapted to the production of an effect of this kind, in various degrees, corresponding with the respective characters and habits of each tribe. He is thus led to conclude that the function which the tricuspid valve exercises exhibits, in the extent of its developement, a regular gradation, when followed throughout the dif­ferent orders of Mammalia and Birds; and that it extends even to some Reptiles. The force with which the circulating blood is impelled by the ge­neral venous trunks into the heart, and which is dependent on the action of the arterial system, the degree of compression arising from muscular action, combined with the resistance of the valves of the veins, and is also influenced by occasional accumulations of blood from rapid absorption, from impeded respiration, and from cold applied to the surface of the body, is shown to be subject to great and sud­den variations. Any increase taking place in this force tends to pro­duce distension of the right ventricle of the heart, followed by dis­turbance in the valvular action of the tricuspid membrane, owing to the displacement of its parts, which thus allows of a considerable re­flux of blood into the auricle. Among the Mammalia, the lowest degree of this action, corresponding to that of a safety-valve, is found in the rodent, the marsupial, and the canine tribes. The next in degree is that which occurs in the order of Edentata and the feline genus. The Quadrumana occupy the next place in the scale of gradation. The human conformation exhibits this function in a very conspicuous man­ner, especially in the adult period; for at birth, when the right ventricle is unyielding, it scarcely exists; and in various states of disease the tri­cuspid valve acts with too much or with too little efficacy. The Pachydermata and Ruminantia come next in succession. The Seal exhibits this peculiarity in a still higher degree; but in no order of Mammalia does it exist to so great an extent as in the Cetacea, which appear, indeed, to possess a peculiar additional provision for effectually securing the permanent performance of this office, which the author compares to that of a safety-valve. A similar function, subject to si­milar gradations, is likewise traced in different orders of Birds. It is but slight in the Gallinaceæ; and rather greater in the predaceous tribes. In some of the Waders it exists to a considerable extent; but is greatest of all in the orders of Passerinæ and Scansores. Crocodiles and the Ornithorhynchus present some traces of this peculiar provi­sion relatively to the circulation.

On the safety-valve of the right ventricle of the heart in man; and on the gradations of the same apparatus in Mammalia and birds