ACTION OF THE VALVES. 43 



tions, but the above is the principle made use of at the present day in esti- 

 mating the pressure of the blood in different parts of the circulatory system. 



Hales estimated, from experiments upon living animals, the height to 

 which the blood would rise in a tube connected with the aorta of the human 

 subject, at 7 feet 6 inches (228'6 centimetres), and gave the area of the left 

 ventricle as 15 square inches (96'67 square centimetres). From this he cal- 

 culated the force of the left ventricle as equal to 51*5 pounds (about 23 kilos.). 

 Although this estimate is merely an approximation, it seems to be based on 

 more reasonable data than any other. 



The apparatus of Marey for registering the contractions of the different 

 cavities of the heart enabled him to ascertain the comparative force of the 

 two ventricles and the right auricle ; the situation of the left auricle pre- 

 cluding the possibility of introducing a sound into its cavity. By first sub- 

 jecting the bags to known degrees of pressure, the line of elevation of a 

 lever may be graduated so as to represent the degrees of the cardiometer. In 

 analyzing traces made by the left ventricle, the right ventricle and right 

 auricle, in the horse, Marey found that as a general rule, the comparative 

 force of the right and left ventricles is as one to three. The force of the 

 right auricle is comparatively insignificant, being in one case, as compared 

 with the right ventricle, only as one to ten. 



Action of the Valves. In man and the warm-blooded animals, there are 

 no valves at the orifices by which the veins open into the auricles. As has 

 already been seen, compared with the ventricles, the force of the auricles is 

 insignificant ; and it has f arthermore been shown that the ventricles may be 

 filled with blood and the circulation continue when the auricles are entirely 

 passive. Although the orifices are not provided with valves, the circular 

 arrangement of the fibres about the veins is such, that during the contraction 

 of the auricles the openings are considerably narrowed and regurgitation can 

 not take place to any great extent. The force of the blood flowing into the 

 auricles likewise offers an obstacle to its return. . There is really no valvu- 

 lar apparatus which operates to prevent regurgitation from the heart into 

 the veins ; for the valvular folds, which are so abundant in the general venous 

 system and particularly in the veins of the extremities, do not exist in the 

 vense cavse. The continuous flow of blood from the veins into the auricles, 

 the feeble character of the auricular contractions, the arrangement of the 

 fibres around the orifices of the vessels, and the great size of the auriculq- ven- 

 tricular openings, are conditions which provide sufficiently for the flow of 

 blood into the ventricles. 



Action of the Auriculo - Ventricular Valves. After the ventricles have 

 become completely distended by the auricular systole, they take on their con- 

 traction, which is very many times more powerful than the contraction of 

 the auricles. They force open the valves which close the orifices of the pul- 

 monary artery and aorta and empty their contents into these vessels. To 

 accomplish this, at the moment of the ventricular systole, there is a complete 

 closure of the auriculo- ventricular valves, leaving only the auriculo- ventricu- 

 lar opening through which the blood can pass. That these valves close at 



