CHAP, iv.] THE VASCULAR MECHANISM. 179 



(iKni-polarisable) electrodes are placed one on each side of some part of 

 a blood vessel, artery or vein, say the right jugular or femoral vein 

 (previously laid bare and insulated), and are connected with a Wheat- 

 stone bridge and galvanometer, as in the usual way of observing 

 changes in electrical resistance. If a solution of salt be now injected 

 into some other vessel, say the left jugular, the blood laden with the 

 extra quantity of salt, when it reaches the seat of the electrodes will 

 give rise to a change in the electrical resistance through the blood 

 vessel with its contained blood between the electrodes, and this will be 

 indicated by a movement of the galvanometer. If the times of the 

 injection, and of the movement of the galvanometer be noted, the 

 interval between the two will give the time it takes the blood con- 

 taining the salt to pass from the seat of injection to the seat of the 

 electrodes. 



In the horse this time has been experimentally determined at 

 about 30 sees, and in the dog at about 15 sees. In man it 

 is probably from 20 to 25 sees. 



We may arrive at a similar result indirectly by means of a 

 calculation. Taking the quantity of blood as -Jg- of the body 

 weight, the blood of a man weighing 75 kilos would be about 

 5,760 grm. If 180 grms. left the ventricle at each beat, a 

 quantity equivalent to the whole blood would pass through the 

 heart in 32 beats, i.e. in less than half a minute. 



Taking the rate of flow through the capillaries at about 1 mm. 

 a sec., it would take a corpuscle as long a time to get through 

 about 20 mm. of capillaries as to perform the whole circuit. 

 Hence, if any corpuscle had in its circuit to pass through 10 mm. 

 of capillaries, half the whole time of its journey would be spent in 

 the narrow channels of the capillaries. Inasmuch as the purposes 

 served by the blood are chiefly carried out in the capillaries, it is 

 obviously of advantage that its stay in them should be prolonged. 

 Since, however, the average length of a capillary is about '5 mm., 

 about half a second is spent in the capillaries of the tissues and 

 another half second in the capillaries of the lungs. 



107. We may now briefly summarise the broad features of 

 the circulation, which we have seen may be explained on purely 

 physical principles, it being assumed" that the ventricle delivers 

 a certain quantity of blood with a certain force into the aorta 

 at regular intervals, and that the physical properties of the blood 

 vessels remain the same. 



We have seen that, owing to the peripheral resistance offered 

 by the capillaries and small vessels, the direct effect of the 

 ventricular stroke is to establish in the arteries a mean arterial 

 pressure, which is greatest at the root of the aorta and diminishes 

 towards the small arteries ; some of it being used up to drive the 

 Mood from the aorta to the small arteries, but which ret tins at 

 the region of the small arteries sufficient power to drive through the 

 small arteries, capillaries and veins just as much blood as is being 



