THE BLOOD FLOW 407 



value at the arteriocapillary junction. On the venous side, such 

 high values are not encountered under ordinary conditions. Thus, if 

 the accompanying determinations of Burton-Opitz^ are used as a 

 guide, it must be concluded that the speed of the venous blood is only 

 about one-fourth as great as that of the arterial, viz. : 



Ext. jugular vein 80. mm. in a second 



Renal vein 63 . mm. in a second 



Mesenteric vein 83 . 6 mm. in a second 



Femoral vein 61.6 mm. in a second 



It is slowest in the vicinity of the capillaries and fastest in the central 

 veins; moreover, when the blood reaches the neighborhood of the 

 heart, it is brought under the influence of the right auricle and shows 

 alterations in flow similar to those encountered in the central arterial 

 trunks. Thus, it has been proved by Burton-Opitz^ that the influx 

 into the right auricle is not constant, but is diminished during the 

 periods of high intra-auricular pressure, z.e., during the systole 

 of the auricles and again during the systole of the ventricles. It 

 may be surmised that the heart influences the current in the pulmonary 

 veins in a very similar manner. 



The capillaries, of course, are not accessible to any one of the 

 instruments described previously. In the frog, however, fairly 

 accurate results may be obtained by placing a translucent capillary 

 area, such as the web or mesentery, under the microscope in such a 

 way that a rather straight capillary comes to lie directly across the 

 divisions of an ocular micrometer. The time is then determined 

 when a certain erythrocyte enters and leaves this capillary. The 

 length of this tubule is ascertained later on by determining the mag- 

 nification, which requires a comparison of the ocular micrometer with 

 the stage micrometer. By this procedure Weber^ and Volkmann* 

 have found the velocity of the capillary blood stream to be 0.5 to 0.8 

 mm. in a second. 



Vierordt^ has also described a method which is applicable to man 

 and depends upon the following entoptic observation. As the red 

 cells traverse the retinal blood-vessels they cast their shadows upon 

 the underlying rods and cones. The visual sensations set up by the 

 latter may be rendered clearly perceptible in an indirect manner by 

 fixedly gazing at a white surface placed at a distance of 11-16 cm. 

 in front of the eyes. Having first determined the speed of the pro- 

 jected shadows upon the screen, the speed of the red cells in the retinal 

 vessels may be ascertained in accordance with the proportion : 



7 be 



a :b = c:x:x = — 



1 Pfluger's Archiv, cxxiv, 1908, 469. 



2 Am. Jour, of Physiol., vii, 1902, 435. 



3 Archiv fiir Anat. und Physiol., 1838, 450. 

 * Haemodynamik, 1850. 



^ Archiv fiir physiol. Heilkunde, xv, 1856. 



