20G HANDBOOK OF PHYSIOLOGY. 



which the main artery has been ultimately broken up. The sectional 

 area of the capillaries is several hundred times that of the aorta, and 

 the friction generated by the passage of the blood through these minute 

 channels opposes a considerable hindrance or resistance in its course. 

 The resistance thus set up is called peripheral resistance. The fric- 

 tion is greater in the arterioles where the current is comparatively rapid 

 than in the capillaries where it is slow. 



That the blood exerts considerable pressure 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 expe- 

 rimented with. If a large artery be punctured, the blood may be pro- 

 jected upward for many feet, whereas if a small artery be similarly dealt 

 with the jet does not rise to such a height. Another marked feature 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 jerky character of the outflow. 

 If the artery be cut across, the jet issues with force, chiefly from the 

 central end, unless there is considerable anastomosis of vessels in the 

 neighborhood, when the jet from the peripheral end may be as forcible 

 and as intermittent as that from the other end. The intermittent flow 

 in the arteries which is due to the intermittent 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 systolic 

 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 persently return, but we may say here, 

 that in a normal condition the pulse is a characteristic of the arterial, 

 and is absent from the venous flow. At the same time it must be recol- 

 lected that in the veins the blood exercises a pressure on its containing 

 vessel, but as we shall see presently this is small when compared with 

 the arterial blood-pressure. As might be expected, therefore, the blood 

 is riot expelled with so much force if a vein be punctured or cut, and 

 further, 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. 



The result produced by the experiment of cutting or puncturing a 

 blood vessel may be modified by introducing into the vessel a glass 

 tube of a calibre corresponding to that of the vessel, and allowing the 

 blood to rise in it. If the vessel be an artery, the blood will rise 

 several feet, according to the distance of the vessel from the heart, and 

 when it has reached its highest point will be seen to oscillate with 

 the heart's beats. This experiment shows that the pressure which the 



