264 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXI. 



of blood-pressure is thus very different from one of velocity; the 

 velocity like the pressure falls from the arteries to the capillaries, 

 but unlike it, rises again in the veins. 



We must now study the methods by which blood-pressure is 

 measured and recorded, and the main causes that produce variations 

 in its amount. 



In order that we may understand the methods that are used for 

 this purpose, it will be first necessary for us to consider some of the 

 general laws of fluid pressure, and then to study the methods that 

 are employed in an artificial schema of the circulation. 



Fluid pressure is a different thing from the pressure of a solid, 

 and is exercised equally in all directions. If a cylindrical vessel, 

 placed vertically, is filled with a cylinder of ice, the pressure of the 

 ice will be exercised on the bottom of the cylinder, but not on its 

 sides. When the ice melts, the water presses on the sides also, and 

 if a hole is made in the cylinder below the level of the upper surface 

 of the water, the water will flow out of the hole, and the force with 

 which it escapes will be proportional to the depth of the hole beneath 

 the surface. If we take a square centimetre as the unit of area, the 

 actual pressure excited on it is h x d x g> where h is the height of the 

 free surface above the level where we are measuring the pressure, d 

 its density, and g the acceleration of gravity (981). Suppose a 

 gramme of water to flow out, we may consider that this gramme has 

 fallen through a height or head Ji in centimetres from the free surface 

 to the opening ; it comes practically from the top, because it is there 

 that the liquid disappears from inside the vessel. In falling the 

 height h, it gives out kg ergs of work. 



The unit of force is called a dyne ; a moving body is said to possess 

 momentum ; this is measured by the product of its mass and its velocity ; thus the 

 effective quantity of motion of a body may be large on account of its having a large 

 mass (for instance, a heavy waggon rolling down a hill), or large velocity (for instance, 

 a bullet speeding through the air). A force continuously applied to a moving mass 

 produces a continuous increase in its rate of movement ; this is termed acceleration, 

 and force may be defined as the rate of change of momentum ; it can be measured, 

 therefore, by observing the amount of momentum it generates in a measured time, 

 and dividing by that time. If a gramme is taken as the unit of mass, a centimetre 

 as the unit of length, and a second as the unit of time, the unit of force 



= momentum = gramme-centimetre per second 



Time. Time in seconds. 



= gramme-centimetre per second, per second = 1 dyne. 



The unit which corresponds to the dyne in the measurement of work is called an 

 erg, that is, the work done in lifting a gramme weight through the height of one 

 centimetre ; the weight of a gramme is 981 dynes, and the work done in lifting it 

 one centimetre is 981 ergs. 



The kinetic energy of a body moving with velocity v is X mass 

 x^ 2 , or for one gramme \ I G L \ hence if all the work that liquid can do 

 is spent in giving kinetic energy to it, the velocity with which it will 



