526 Royal Society. 



In this paper additional evidence is given by the author in corrobo- 

 ration of the principles which he had announced in a former commu- 

 nication, 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 stales 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 de- 

 velopment, a regular gradation, when followed throughout the dif- 

 ferent orders of Mammalia and Birds; and that it e.xtends even to 

 some Reptiles. 



The force with which the circulating blood is impelled by the ge- 

 neral venous trunks into the heart, and which is dependent on the 

 action of the arterial system, and the degreeofcompression arising from 

 muscular action, combined with the resistance of the valves of the 

 veins, and which 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 sud- 

 den variations. Any increase taking place in this force tends to pro- 

 duce distension of the right ventricle of the heart, followed by dis- 

 turbance in the valvular action of the tricuspid membrane, owing to 

 the displacement of its parts, which thus allows of a considerable re- 

 flux 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 man- 

 ner, especially in the adult period j for at birth, when the right ventricle 

 is unyielding, itscarcely exists; and in various states of disease ihe tri- 

 cuspid valve acts with too much or with too little efficacy. The Pachy- 

 dermata 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 a))pear, 

 indeed, to possess a peculiar additional provision for effectually se- 

 curing the permanent performance of this office, which the author 

 compares to that of a safety-valve. A similar function, subject to si- 

 milar gradations, is likewise traced in different orders of Birds. It is 

 but slight in the Gallinaceae ; 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 Passerinse and Scansores. Crocodiles 



* An abstract of this paper will be found in Lond. and Edinb. Phil. Mag., 

 vol. vii. p. 207. 



