THE CIRCULATION OF THE BLOOD. 313 



If the systemic vascular apparatus be conceived of as a system 

 of tubes which have symmetrically divided and subdivided, and have 

 again united and reunited in a corresponding manner, it is clear that 

 the total sectional area will steadily increase from the beginning to 

 the middle of the system, and then as steadily decrease from the middle 

 to the end of the system. In such a system the same volume of blood 

 must pass through any given section in a unit of time if the balance 

 of the circulation is to be maintained. As the velocity of a fluid is 

 inversely as the sectional area of the tubes through which it flows, it 

 follows that the initial mean velocity of the blood in the aorta will 

 steadily decrease as it flows into the steadily enlarging blood-path 

 until it reaches a minimal value in the middle of the capillary system ; 

 and that it will again steadily increase as it flows into the narrowing 

 blood-path until it reaches the heart. The initial mean velocity of 

 the blood in the aorta will not be attained in the venae cavae, for the 

 reason that the total sectional area of the latter is somewhat greater 

 than that of the former. The same facts hold true for the pulmonic 

 vascular system. 



The Velocity in the Aorta. From the well-known fact that the 

 velocity with which a fluid is flowing through a tube may be deter- 

 mined by dividing its sectional area into the quantity discharged 

 in a unit of time, attempts have been made to determine the mean 

 velocity of the blood at the beginning of the aorta. If it be assumed 

 that the volume discharged at each contraction is 180 c.c., as stated 

 by Vierordt, and the number of heart-beats per minute at 72, the 

 total volume discharged per minute would be 12,960 c.c., or 215 c.c. 

 per second. The sectional area of the aorta is 6.15 sq. cm. On the 

 principle above stated, these two factors would show a velocity of 

 350 mm. per second. An objection to this estimate is that the 

 amount of blood discharged i. e., the contraction volume is much 

 larger than recent investigations warrant. Different observers have 

 estimated that in man the contraction volume is considerably less, 

 probably not more than 80 c.c. If this is true, the mean velocity in 

 the aorta would be less than in the carotids. 



The Velocity in the Arteries. The mean velocity of the blood 

 in the larger and more superficially lying arteries has been determined 

 by Volkmann with the hemodromometer, by Ludwig and Dogiel 

 with the stromuhr, and by other investigators with different forms of 

 apparatus. 



Since neither the blood nor any particle placed in it can be seen 

 through the walls of the artery, it occurred to Volkmann to inter- 

 calate along the course of a vessel a U-shaped glass tube about one 

 meter in length with a lumen the diameter of that of the selected 

 vessel, into and through which the blood could be made to flow. 

 The mechanic construction of the apparatus is such (Fig. 147) that 



