3I 6 TEXT-BOOK OF PHYSIOLOGY. 



The Velocity in the Capillaries. The rate of flow in the capil- 

 lary vessels can not be experimentally determined. It has been esti- 

 mated by Vierordt at 0.5 mm. per second in his own retinal capillaries ; 

 bv Weber at 0.8 mm. In frogs the velocity can be fairly well deter- 

 mined by observing the time required for a corpuscle to pass over one 

 or more divisions of an ocular micrometer. Weber calculated in this 

 way that the velocity is 0.5 mm. per second. 



As the velocity varies inversely with the sectional area, it becomes 

 possible to approximately determine the relation of the sectional area 

 of the capillary system to that of the aorta from the above-mentioned 

 velocities. If it be assumed that the velocity in the aorta averages 

 300 mm. and in the capillaries 0.5 mm. per second, then the sectional 

 area of the capillaries is to that of the aorta as 600 to i. 



The Velocity in the Veins. In the venous system the velocity 

 increases in proportion as the sectional area decreases. In the jugu- 



Arteries. Capillaries. Veins. 



FIG. 150. , Blood-pressure. , Velocity. o o o o, Sectional area. 



lar vein Volkmann found the velocity 225 mm. per second, which 

 was about one-half that in the aorta of the same animal. The reason 

 for the slow rate of movement in the jugular vein is to be found in the 

 fact that the sectional area of the combined venae cavae is about twice 

 that of the aorta; hence the relation of the sectional area of the cap- 

 illary system to the sectional area of the venae cavae is about 300 to i . 

 The blood-pressure, the velocity of the blood, the sectional area 

 of the vascular apparatus, and their relation one to the other are 

 shown in Fig. 150. 



THE PULSE. 



It has been stated (see Blood-pressure) that with each contraction 

 of the left ventricle a definite volume of blood is discharged into the 

 aorta which at the moment is already full of blood ; that this incoming 



