182 STUDIES IN ADVANCED PHYSIOLOGY. 



would be the fact that the artery seems to be distended, 

 and the blood in it under considerable pressure. This 

 pressure is, of course, produced by the resistance offered to 

 the flow of blood through the small arteries and capillaries 

 on the one end and the continued pouring in of blood by 

 the heart at the other end. The condition of things differs 

 in no way from that of a city water- works system. In that 

 case the central pump usually forces the water into the 

 mains at such a rate that often a heavy pressure results. 

 Especially so in cases of fire when the central pumps are 

 made to run faster than usual. If there were no resistance 

 at the farther ends of these water mains, no pressure would 

 arise, but the water would flow out at one end as rapidly as 

 it entered at the other. But the pressure in the mains re- 

 sults from the internal resistance due to friction given to 

 the stream as it tries to traverse the smaller pipes at the 

 distal extremity. In such a system with the pump going at 

 a regular rate the pressure in the mains would rise and rise 

 until finally a pressure would result which would be able to 

 force out at its peripheral end as much water in a given 

 time as the pump poured into it at the other end. Thus, 

 in such a system, if in a case of fire the central pump should 

 increase its speed the pressure throughout all the mains 

 would at once rise, and continue to do so until it would be 

 able to force through all the finer ramifications as much 

 water as entered. It is easy to determine what the amount 

 of this pressure is in the case of the blood-vessels. From 

 general considerations which govern the supply of all streams 

 flowing in closed tubes, the pressure must be greatest near- 

 est the heart and become gradually less towards the peri- 

 phery. Just as in water mains the pressure is greater near 

 the station and gradually decreases toward more distant 

 places. 



Arterial Pressure. 



The blood pressure in the human arteries has been 

 measured in cases of amputation. Experiments of this kind 



