THE CAPILLARIES. 131 



found, by experiments on different animals, to be supported by 

 the force of the blood in them, is equivalent to the height of 

 the column which the force of the blood in the human aorta 

 would support, he calculated that about 4 Ibs. 4 oz. avoirdu- 

 pois would indicate the static force with which the blood is im- 

 pelled into the human aorta. By the same calculation, he 

 estimated the force of the circulation in the aorta of the mare 

 to be about 11 Ibs. 9 oz. avoirdupois: and that in the radial 

 artery at the human wrist only 4 drs. We have already seen 

 that the muscular force of the right ventricle is equal to only 

 one-half that of the left, consequently, if Poiseuille's estimate 

 of the latter be correct, the force with which the blood is pro- 

 pelled into the lungs will only be equal to 2 Ibs. 2 oz. avoir- 

 dupois. 



The amounts above stated indicate the pressure exerted by 

 the blood at the several parts of the arterial system at the time 

 of the ventricular contraction. During the dilatation, this 

 pressure is somewhat diminished. Hales observed, that the 

 column of blood in the tube inserted into an artery, falls an 

 inch, or rather more, after each pulse ; Ludwig has observed 

 the same, and recorded it more minutely. The pressure is also 

 influenced by the various circumstances which affect the action 

 of the heart ; the diminution or increase of the pressure being 

 proportioned to the weaker or stronger action of this organ. 

 Valentin observed that, on increasing the amount of blood by 

 the injection of a fresh quantity into it, the pressure in the 

 vessels was also increased, while a contrary effect ensued on 

 diminishing the quantity of blood. 



Velocity of the Blood in the Arteries. 



The velocity of the stream of blood is greater in the arteries 

 than in any other part of the circulatory system, and in them 

 it is greatest in the neighborhood of the heart, and during the 

 ventricular systole; the rate of movement diminishing during 

 the diastole of the ventricles, and in the parts of the arterial 

 system most distant from the heart. From Volkmann's ex- 

 periments with thehsemodromometer, it may be concluded that 

 the blood moves in the large arteries near the heart at the rate 

 of about ten or twelve inches per second. Vierordt calculated 

 the rapidity of the stream at about the same rate in the arteries 

 near the heart, and at two and a quarter inches per second in 

 the arteries of the foot. 



THE CAPILLARIES. 



In all organic textures, except some parts of the corpora 

 cavernosa of the penis, and of the uterine placenta, and of the 



