CIRCULATION. 101 



An instrument based on the principles jus! described is valuable for the 

 study of rapid changes of velocity. 1 In an artery, its needle oscillates rhyth- 

 mically, showing that there the speed of the 1)1 1 varies during each beat of 



the heart, being greatly accelerated by the systole of the ventricle, and retarded 



by the cessation of the systole. It will be remembered that the microscope 

 directly shows faint rhythmic accelerations in the minute arteries of the frog. 

 In the veins rhythmic changes of speed do not occur except near the heart 

 from respiratory causes. 



The Speed of the Blood in the Arteries. — The stromuhr shows that the 

 speed of the blood is liable to great variations. This fact, and the range of 

 speed in the arteries, are fairly exhibited by the results obtained by Dogiel 

 from the common carotid artery of a dog, the experiment upon which lasted 

 127 seconds. During this time six observations were made which varied in 

 length from 14 to 30 seconds each. For one of these periods the average 

 speed was 243 millimeters in one second ; for another period, 520 millimeters. 

 These were the extremes of speed noted in this case. 2 The speed in the 

 arteries diminishes toward the capillaries. 



The Speed of the Blood in the Veins. — The speed in a vein tends to be 

 slower than that in an artery of about the same importance, but is not neces- 

 sarily so. 3 It increases from the capillaries toward the heart. 



The Speed of the Blood in the Capillaries. — The rate of the capillary 

 flow may be measured directly under the microscope. Certain physiologists 

 have also observed the movement of the blood in the retinal capillaries of 

 their own eyes, and have measured its rate there.' Both methods show that 

 in the capillaries the speed is very much less than in the large arteries or large 

 veins. In the capillaries of the web of the frog's foot it is only about 0.5 

 millimeter in one second. In those of the mesentery of a young dog it has 

 been found to be 0.8 millimeter; in those of the human retina, from 0.6 to 

 0.9 millimeter. 



Speed and Pressure of the Blood Compared. — If now we compare the 

 speed with the pressure of the blood in the arteries, in the capillaries, and in 

 the veins, we shall be struck by both similarities ami differences. In the 

 arteries both pressure and speed rhythmically rise and fall together \ and both 

 the mean pressure and the mean speed decline from the heart to the capillaries. 

 In the capillaries both pressure and speed are pulseless and low, — very low 

 compared with the great arteries. In the veins, however, the pressure is 

 everywhere lower than in the capillaries and falls Prom the capillaries to the 

 heart; the speed is everywhere higher than in the capillaries and rises from 



1 M. L. Lortet: Recherches sur /" vitesse du coura </" tang danslea art&res du cheval au m a 

 d'un nouvel hemodromiifiriijili:, l'.nis. lsr.7. 



2 J. Dogiel : loc. eil. 



3 E. Cyon und P. Steinmann : " Die Geechwindigkeil iK-s Blutstroma in den Venen," Bulletin 

 de I'Academie Imperial* den Sciences ,1c St. Peiersbmirfj, 1871 ; also in E. I !yon : QesamnuUe physio- 

 logisehe Arbeitm, 1888. S. 110. 



1 K. Vierordt : Die Erscheinungen und Oeselze der StromgeachwindigkeUen da Blutee, etc., 1862, 

 8.41,111. 



