LECT. XVII. VELOCITY OF THE CIRCULATION. 321 



traction of the left ventricle, a hundred and ninety-two pul- 

 sations would be necessary to make the entire mass circu- 

 late ; that is, about two minutes and a half. 



It is, however, but right to observe, that we cannot 

 assume that all the blood of the ventricle would be expelled 

 at each pulsation ; consequently, the numbers given must 

 always be below the true ones. 



It remains now for us to speak of the velocity of the 

 blood in the different vessels of the circulatory system. 

 Assuming the section of the orifice of the left ventricle to 

 be equal to that of the aorta, and that the sum of the sec- 

 tions of the different branches into which it divides, is also 

 the same, it follows that if the same quantity of blood pass 

 every where in the same interval of time, the velocity will 

 be the same in every vessel. But the sections of the 

 arterial and venous trunks are not really equal to those of 

 their ramifications. The most simple observation proves, 

 on the contrary, that the sum of the sections of the small 

 vessels is more considerable than that of the trunks. Look 

 at the heart of an ox, in which we have divided the arte- 

 rial and venous trunks, the orifice of the aorta is about 28 

 millimetres, while that of one of its trunks is about 20, and 

 the other 16 : the venae cavse have a total diameter of 76 

 millimetres. The well-known law of Castelli ought to be 

 applied, in order to obtain the velocity of the blood at dif- 

 ferent parts of its course : this velocity will always be in 

 the inverse ratio of the sections. If we could accurately 

 estimate the ratios which exist between the sections of the 

 various vessels in which the blood circulates, it would be 

 easy to determine what would be the velocity in all the 

 vessels ; the quantity of blood thrown out of the left ven- 

 tricle being known, as well as the time employed in expell- 

 ing it. 



I shall content myself with showing you by one illustra- 

 21 



