ARTERIAL PRESSURE. THE PULSE. 249 



becomes more capacious from the aorta to the capillaries the 

 rate of flow in it becomes proportionately slower, and as the 

 total area of the channels diminishes again from the capilla- 

 ries to the venae cavae, so does the rate of flow quicken, just 

 as a river current slackens when it spreads out, and flows 

 faster where it is confined to a narrower channel ; a fact taken 

 advantage of in the construction of Eads' jetties at the mouth 

 of the Mississippi, the object of which is to make the water 

 flow in a narrower channel and so with a more rapid current 

 in that part of the river. Actual measurements as to the rate 

 of flow in the arteries cannot be made on man, but from ex- 

 periments on lower animals it is calculated that in the human 

 carotid the blood flows about 400 millimetres (16 inches) in a 

 second. In the capillaries the current travels only from 0.5 

 to 0.75 mm. (-fa to -fa inch) in a second. The total time 

 taken by a portion of blood in making a complete circulation 

 has been measured by injecting some easily detected sub- 

 stance into an artery on one side of the body and noting the 

 time which elapses before it can be found in a corresponding 

 vein on the oppos : e side. In dogs this time is 15 seconds, 

 and it is calculated for man at about 23 seconds. Of this 

 total time about half a second is spent in the systemic and 

 another half second in the pulmonary capillaries, as each por- 

 tion of blood on its course from the last artery to the first 

 vein passes through a length of capillary which on the aver- 

 age is 0.5 mm. (^ inch). The rate of flow in the great veins 

 is about 100 mm. (4 inche,s) in a second, but is subject to con- 

 siderable variations dependent on the respiratory and other 

 movements of the Body; in the small veins it is much slower. 

 Secondary Causes of the Circulation. x Wh^le the heart's 

 beat is the great driving force of the circulation, certain other 

 things help more or less viz., gravity, compression of the 

 veins, and aspiration of the thorax. All of them are, how- 

 ever, quite subsidiary; experiment on the dead Body shows 

 that the injection of whipped blood into the aorta under a 

 less force than that exerted by the left ventricle during life 

 is more than sufficient to drive it round and back by the venae 

 cavae. Not infrequently the statement is made in books that, 

 probably, the systemic capillaries have an attractive force for 

 arterial blood and the pulmonary capillaries for venous blood, 

 but there is not the slightest evidence of the correctness of 

 such a supposition, nor any necessity for making it. 



