PHYSICAL FORCES OF THE CIRCULATION. 287 



When a small artery is cut across and the orifice directed up- 

 wards, the blood is thrown two or three feet in jerks. When a 

 vein is cut, the blood only trickles gently from its orifice, the 

 force depending much upon the position of the part ; and it is 

 well known that bleeding from a vein in the leg can easily be 

 stopped by placing the limb in a position more elevated than the 

 rest of the body, so as to remove the force of gravity from acting 

 on the blood. 



By means of a special form of gauge (the mercurial mano- 

 meter) which will presently be described the exact difference 

 in the pressure exerted by the blood against the vessel walls in 

 the different parts of the circulation can be accurately estimated, 

 and it has been found by direct experiment that the blood pres- 

 sure varies just as one would be led to expect from a considera- 

 tion of its physical relationships, namely, the direction and rate 

 of the current and the varying width of the bed in which it 

 flows. 



The rate of the fall in pressure observed in the vessels passing 

 from the left ventricle to the right auricle is not even, but in the 

 arterioles it falls suddenly, and a great difference therefore always 

 exists between the arterial and venous pressure (v. p. 294). Since 

 there is a permanent high pressure in the arteries as compared 

 with that in the capillaries and veins, there can be no difficulty 

 in explaining the permanent flow through the capillaries from 

 arteries to veins. 



The fundamental problem that must be clearly understood in 

 studying the dynamics of the circulation is, how the high pressure 

 in the arteries is kept up, or, in other words, how the arteries can 

 exert so much pressure on the blood when the capillary out-flow 

 is so wide and free. 



From the description already given of the action of the heart, 

 it appears that each beat of the ventricle pumps some six ounces 

 of blood into the aorta, which blood, though coming to the left 

 ventricle from the pulmonary circulation, may, on account of 

 the exact co-operation of the two sides of the heart, be said to 

 be pumped out of the systemic veins, and thus, as far as the 

 physical forces are concerned, the pulmonary circulation may be 



