390 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



unloading the veins in the immediate neighborhood of the heart, and so remove 

 some part of the resistance to be overcome by the contractions of the cardiac 

 muscle. When we come to the detailed study of the heart it will appear also 

 that a slight force of suction is generated by the heart itself, which force adds 

 its effects upon the flow of venous blood to those of the elasticity of the lungs 

 and of the contraction of the muscles of inspiration. 



It must here be repeated, however, that the heart is quite competent to 

 maintain the circulation unaided. This is proven as follows : If in an anes- 

 thetized mammal a cannula be placed in the windpipe, the chest be widely 

 opened, and artificial respiration be established, the circulation, though modi- 

 fied, continues to be effective. By the opening of the chest its aspiration has 

 been ended, and can, no longer assist in the venous return. If, further, the 

 animal be drugged in such a manner as completely to paralyze the skeletal 

 muscles throughout the body, their contractions can exert no influence upon 

 the venous return ; yet the circulation is still kept up by the heart, unaided 

 either by the elasticity of the lungs, by the contractions of the muscles which 

 produce inspiration, or by those of any other skeletal muscles. 



E. THE SPEED OF THE BLOOD IN THE ARTERIES, CAPILLARIES, 



AND VEINS. 



If we keep as our text, in discussing the circulation, the character of the 

 capillary flow, it will be seen that we have now accounted for the facts that 

 the capillary flow is toward the veins ; that it shows much friction ; that it is 

 continuous, pulseless, and under low pressure. We have not yet accounted 

 for the fact that it is slow. We must now do so, but must first state and 

 account for the speed of the blood in the arteries and veins. 



The Measurement of the Blood-speed in Large Vessels ; the " Strom- 

 uhr." The speed of the blood in the larger veins and arteries must be meas- 

 ured indirectly. We can picture to ourselves the volume of blood which moves 

 past a given point in a given blood-vessel in one second, as a cylinder of 

 blood having the same diameter as the interior of the blood-vessel. The 

 length of this cylinder will then be expressed by the same number which will 

 express the velocity with which a particle of the blood would pass the given 

 point in one second, provided that this velocity be uniform and be the same 

 for all the particles. In order, then, to learn the average speed of the blood 

 at a given point of an artery or vein during a certain number of seconds, we 

 have only to measure the calibre of the blood-vessel and the quantity of 

 blood which passes the selected point during the period of observation. 

 From these two measurements the speed can be obtained by calculation. But 

 these two measurements are not quite easy. The physical properties of the 

 blood-vessels, especially of the veins, make their calibres variable and hard 

 to estimate justly as affected by the conditions present during an experiment. 

 The means adopted for measuring the quantity of blood passing a point in a 

 given time necessarily alters the resistance encountered by the flow, and so of 

 itself affects both the rate of flow and the blood-pressure ; and, with the 



