ANATOMICAL AND PHYSICAL DATA 85 



tube between the first two vertical tubes. In general the energy of 

 position represented by the lateral pressure at any point is equal to 

 the energy used up in overcoming the resistance of the portion of the 

 path beyond this point. 



Velocity of Outflow. It has been found by experiment that v, the. 

 mean velocity of outflow, when the tube is not of very small calibre, 

 varies directly as the diameter, and therefore the volume of outflow 

 as the cube of the diameter. In fine capillary tubes the mean velocity 

 is proportional to the square, and the volume of outflow to the fourth 

 power of the diameter (Poiseuille). If, for example, the linear velocity 

 of the blood in a capillary of 10 p, in diameter is \ mm. per sec., it will 

 be four times as great (or 2 mm. per sec.) in a capillary of 20 p diameter, 

 and one-fourth as great (or mm. per sec.) in a capillary of 5 p. diameter, 

 the pressure being supposed equal in all. The volume of outflow per 

 second is obtained by multiplying the cross-section by the linear 

 velocity. The cross-section of a circular capillary, 10 /* in diameter, 

 is TT (5 x TfiW) 2 = sav > TaioiF sc l- mm - The outflow will be ^ gsffo x i 

 = 05^0 cub. mm. per sec. The outflow from the capillary of ,.o /* 

 diameter would be sixteen times as much, from the 5 /* capillary only 

 one-sixteenth as much. Some idea of the extremely minute scale 

 on which the blood-flow through a single capillary takes place may 

 be obtained if we consider that for the capillary of 10 /* diameter a 

 flow of -2500TJ cub. mm. per sec. would scarcely amount to I cub. mm 

 in six hours, or to i c.c. in 250 days. 



When the initial energy is obtained in any other way than by means 

 of a ' head ' of water in a reservoir say, by the descent of a piston 

 which keeps up a constant pressure in a cylinder filled with liquid 

 the results are exactly the same. Even when the horizontal tube is 

 distensible and elastic, there is no difference when once the tube has 

 taken up its position of equilibrium for any given pressure, and that 

 pressure does not vary. 



Flow with Intermittent Pressure. When this acts on a rigid tube, 

 everything is the same as before. When the pressure alters, the 

 flow at once comes to correspond with the new pressure. Water 

 thrown by a force-pump into a system of rigid tubes escapes at every 

 stroke of the pump in exactly the quantity in which it enters, for 

 water is practically incompressible, and the total quantity present 

 at one time in the system cannot be sensibly altered. In the intervals 

 between the strokes the flow ceases; in other words, it is intermittent. 

 It is very different with a system of distensible and elastic tubes. 

 During each stroke the tubes expand, and make room for a portion 

 of the extra liquid thrown into them, so that a smaller quantity flows 

 out than passes in. In the intervals between the strokes the distended 

 tubes, in virtue of their elasticity, tend to regain their original calibre. 

 Pressure is thus exerted upon the liquid, and it continues to be forced 

 out, so that when the strokes of the pump succeed each other with 

 sufficient rapidity, the outflow becomes continuous. This is the state 

 of affairs in the vascular system. The intermittent action of the 

 heart is toned down in the elastic vessels to a continuous steady flow. 



SECTION II. THE BEAT OF THE HEART IN ITS PHYSICAL OR 

 MECHANICAL RELATIONS. 



Events in the Cardiac Cycle. In the frog's heart the contraction 

 can be seen to begin about the mouths of the great veins which open 

 into the sinus venosus. Thence it spreads in succession over the 



