208 THE CIRCULATION OF THE BLOOD 



where it is desirable that the conditions of blood flow should be most con- 

 stant. In fact, we shall find that it is through the interaction of this same 

 group of factors, together with the possibility of variations through the regu- 

 lation of their nerve- motor mechanisms, that we have the great variations 

 and adjustments of blood pressure, speed of flow, volume of flow, and the 

 regulation of volume in particular parts of the body. 



Arterial Blood Pressure. That the blood exerts considerable pres- 

 sure upon the arterial walls in keeping them in a stretched or distended 

 condition may be readily shown by puncturing any artery; the blood is 

 instantly projected with great force through the opening, and the jet rises to 

 a considerable height, the exact level of which varies with the size of the artery 

 experimented upon. If a large artery be punctured the blood may be pro- 

 jected upward for several feet, whereas if it is a small artery the jet does not 

 rise so high. Another characteristic of the jet of blood from a cut artery, 

 particularly well marked if the vessel be a large one and near the heart, is the 

 intermittent character of the outflow. If the artery be cut across, the jet 

 issues with force, chiefly from the central end. If there is considerable 

 anastomosis of vessels in the neighborhood the jet from the peripheral end 

 may be as forcible and as intermittent as that from the central end. The 

 intermittent flow in the arteries which is due to the action of the heart, and 

 which represents the systolic and diastolic alterations of blood pressure, 

 may be felt if the finger be placed upon a sufficiently superficial artery. The 

 finger is apparently raised and lowered by the intermittent distention of the 

 vessel occurring at each heart-beat. This intermittent distention of the 

 artery is what is known as the Pulse, to the further consideration of which 

 we shall presently return, but we may say here that in the normal condition 

 the pulse is a characteristic of the arterial, and is absent from the venous, flow. 



At the same time it must be recollected that in the veins also the blood 

 exercises a pressure on its containing vessel, though it is small when compared 

 with the arterial pressure. As might be expected, therefore, the blood is 

 not expelled with so much force if a vein be punctured or cut. The flow 

 from the cut vein is continuous and not intermittent, and the greater amount 

 of blood comes from the peripheral and not from the central end, as is the 

 case when an artery is severed. 



Methods of Measuring Arterial Blood Pressure. The pressure in 

 an artery may be measured by cutting the vessel and introducing into it a 

 glass tube which has a tall vertical limb. A column of blood will rise in the 

 tube at once to the height that can be supported by the pressure in that par- 

 ticular vessel. If the vessel be an artery, the blood will rise several feet, 

 according to the distance of the vessel from the heart, and when the pressure 

 has reached its highest point it will be seen to oscillate with the heart-beats. 

 This experiment shows that the pressure which the blood exerts upon the 

 walls of the containing artery equals the pressure of a column of blood of a 



