VELOCITY AND PRESSURE OF BLOOD-FLOW. 485 



velocity during a given time, but also the variations in velocity 

 coincident with the heart beat or other changes that may occur 

 during the period of observation. By means of instruments of 

 this kind it is possible to determine not only the velocity in any 

 large artery or vein at a given moment, but also the total volume 

 of flow into or out of an organ during a given period of time. 

 Data of the latter kind give us an idea of the relative quantity of 

 blood supplied to each organ, and the differences in this respect 

 between the several organs. Burton-Opitz* on the basis of experi- 

 ments made by himself and others gives the following figures, which 

 express the blood-supply per minute for each 100 grams of organ: 



% 



Posterior extremity 5 c.c. Spleen 58 c.c. 



Skeletal muscle 12 c.c. Liver (venous) 59 c.c. 



Head 20 c.c. Liver (total) 84 c.c. 



Stomach 21 c.c. Brain 136 c.c. 



Liver (arterial) 25 c.c. Kidney 150 c.c. 



Intestines 31 c.c. Thyroid Gland 560 c.c. 



Mean Velocity of the Blood-flow in the Arteries, Veins, and 

 Capillaries. Actual determinations of the average velocity in the 

 large arteries and veins give such results as the following: Carotid 

 of horse (Volkmann), 300 mms. per second; (Chauveau) 297 nuns. 

 Carotid of the dog (Vierordt), 260 mms. 



The flow in the carotid, as in the other large arteries, is not, 

 however, uniform; there is a marked acceleration or pulse at each 

 systole of the heart during which the velocity is greatly augmented. 

 Thus, in the carotid of the horse it has been shown by the hemo- 

 dromograph that during the systole the velocity may reach 520 

 mms. and may fall to 150 mms. during the diastole. It is found, also, 

 that this difference between the systolic velocity and the diastolic 

 velocity tends to disappear as the arteries become smaller, and, as 

 was said above, disappears altogether in the capillaries, in which 

 the pulse caused by the heart beat is lacking. The smaller the artery, 

 therefore, the more uniform is the movement of the blood. 



The flow in the large veins is uniform or approximately 

 uniform and increases as one approaches the heart, although the 

 velocity in the large veins near the heart is somewhat slower 

 than in the large arteries of the same region, owing to the fact 

 that the total area of the venous bed is larger than that of the 

 arterial bed. Burton-Opitzf gives the following average figures 

 obtained from experiments upon anesthetized dogs. Jugular, 

 147 mms.; femoral, 61.6 mms.; renal, 63 mms.; mesenteric vein, 

 84.9 mms. In the capillaries, however, the velocity is relatively 

 very small. From direct observations made by means of the 

 microscope and from indirect observations in the case of man, 



* Burton-Opitz, "Quarterly Jour, of Experimental Physiology," 4, 117, 1911. 

 t Burton-Opitz, "Am. Journal of Physiology," vols. 7 and 9, and "Pfliiger's 

 Archiv," vols. 123 and 124, 1908. 



