358 THE CIRCULATION OF THE BLOOD. [CHAP. XXVin. 



force and a velocity exactly adapted to overcome such natural 

 obstacles as the action of the vascular system must naturally create. 



To estimate the force of the heart, we must ascertain the pressure 

 which the blood exercises on the walls of the blood-vessels during 

 life, and we must measure the rate at which it flows through them. 



A fluid flowing through a tube exerts a double force, one in the 

 direction of the long axis of the tube, the force of the stream, of 

 which the velocity gives a measure, and another, which is the 

 pressure of the fluid against the wall of the containing tube or the 

 lateral pressure. This latter force is always proportionate to the 

 resistances which the fluid has to encounter to its flow. The longer 

 the tube, through which the fluid passes, the rougher its walls, the 

 narrower the opening through which it escapes, and the more 

 glutinous the fluid, the greater the lateral pressure.* 



A tube fixed into the walls of the tube through which the fluid 

 flows, and at right angles to it, affords a simple means of measuring 

 the lateral pressure, by the height to which the fluid will rise in it. 

 By equally simple means we may measure both forces, if the 

 measuring tube be prolonged into the other tube with its orifice 

 opposite to the stream. The height which the column of fluid will 

 attain in a tube so arranged, will indicate the altitude from which 

 it must have fallen, to acquire the velocity and force with which it 

 streams through the main tube. 



Pitot, a distinguished French engineer, who lived about the 

 middle of the last century, employed a tube of this kind for mea- 

 suring the velocity of the stream in rivers. The tube was bent 

 at a right angle, into two unequal branches, and the smaller or 

 horizontal branch was immersed in the stream with its mouth 

 opposed to it. The height of the column sustained in the tube 

 afforded a measure of the force and velocity of the stream, that 

 height being such as the water must have fallen from, in order to 

 have acquired the same velocity. 



Hales adopted a similar method to measure the pressure of 

 the blood in the arteries. He inserted into the left crural artery 

 of a mare, a brass pipe, whose bore was one-sixth of an inch 

 in diameter; and to that, by means of another brass pipe, he 

 fixed a glass tube of nearly the same diameter, which was nine 

 feet in length. When the blood was allowed to flow into this tube 

 it rose in it to a height eight feet three inches above the level of 

 the left ventricle of the heart. After considerable loss of blood, 

 however, the power of maintaining a column of this height ceased, 



* See Volkmann, die Hsernadynamik nach Versuchen. Cap. i. Leipzig, 1850. 



