THE CIRCULATION THROUGH THE BLOOD VESSELS 2IQ 



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

 be projected 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 almost as forcible and as intermittent as that from 

 the central end. The intermittent flow in the arteries 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 the containing vessels, 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 

 cannula and connecting the cannula with a tall vertical glass tube. When the 

 blood in the vessel is released to the cannula, a column of blood will rise in the 

 tube at once to the height that can be supported by thfe 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 

 certain height. In the case of the rabbit's carotid it is equal to 90 to 120 cm. 

 of blood, or rather more than the same height of water. In the case of the 

 vein, if a similar experiment be performed, blood will rise in the tube only 

 for 8 or 10 cm. or less. 



The usual method of estimating the amount of blood-pressure differs 

 somewhat from the foregoing simple experiment. Instead of a simple 

 straight tube or glass manometer for measuring the pressure, a U-shaped tube 

 containing mercury, the mercury manometer, is employed. The artery is 

 connected with the manometer by means of the cannula inserted into the 

 vessel as before, an arrangement being made whereby the cannula, tubes, etc., 



