262 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxi. 



of the capillaries) is about 700 times as great as that of the 

 truncated apex representing the aorta. Thus the velocity of 

 blood in the capillaries is not more than ^-^ of that in the aorta. 

 The veins are larger than the corresponding arteries, and so the 

 rate there is proportionally slower. 



In the Arteries. The velocity of the stream of blood is greater 

 in the arteries than in any other part of the circulatory system, 

 and in them it is greatest in the neighbourhood of the heart, and 

 during the ventricular systole. The rate of movement diminishes 

 during the diastole of the ventricles, and in the parts of the 

 arterial system most distant from the heart. 



A few of the results obtained by different observers may be 

 here given. 



In the carotid of the dog, the velocity is 205 350 mm. per second. 

 horse 306 



metatarsal 56 



In very round numbers we may state the average speed in the 

 large arteries as a foot per second. 



Estimation of the Velocity. Various instru- 

 ments have been devised for measuring the velocity 

 of the blood-stream in the arteries. Ludwig's 

 Stromuhr (fig. 258), which is the best instrument 

 to use, consists of a U-shaped glass tube dilated 

 at a and a', the ends of which, h and i, are of 

 known calibre. The bulbs can be filled by a 

 common opening at k. The instrument is so con- 

 trived that at b and V, the glass part is firmly fixed 

 into metal cylinders, attached to a circular hori- 

 zontal table c c', capable of horizontal movement on 

 a similar table d d' about the vertical axis marked 

 in figure by a dotted line. The openings in c c', 

 when the instrument is in position, as in fig. 258. 

 corresponds exactly with those in d d' ; but if c c' 

 be turned at right angles to its present position, 

 there is no communication between h and a and i 

 and a', but h communicates directly with i ; and if 

 turned through two right angles c' communicates 

 with d. and c with d', and there is no direct commu- 

 nication between h and i. The experiment is per- 

 formed in the following way : The artery to be 

 experimented upon is divided and connected with 

 two cannulfe and tubes which fit it accurately with 

 h and i ; h is the central end, and i the peripheral : 

 the bulb a is filled with olive oil up to a point 

 rather lower than k, and a' and the remainder of a 

 is filled with defibrinated blood ; the tube on k is 

 then carefully clamped : the tubes d and d' are also 

 filled with defibrinated blood. When everything is 



ready, the blood is allowed to flow into a through h, and it pushes before 

 it the oil, and that the defibrinated blood into the artery through i ; a' is 

 then full of oil ; when the blood reaches the former level of the oil in a. 



Fig. 258. Ludwig's 

 Stromuhr. 



