298 PHYSIOLOGY. 



come and the vis a tergo that is impelling the blood-current. Thus 

 the arterial pressure depends upon the relation existing between the 

 blood thrown out by the ventricles and the quantity that can pass 

 through the capillaries in the same time. 



Science has possessed for a long time the means of knowing what 

 is, in inelastic tubes, which are the seat of the flow, the force of 

 afflux for each point of their length, and what, also, is the quantity 

 of that force which has been consumed by the resistances. 



In order that the student may gain some knowledge of the causes 

 that produce variations in the pressure as well as the means of 

 measuring and recording it, attention will be turned briefly to the 

 physical world to note the simplest possible apparatus that can 

 convey even a vague idea of this property of the blood's circulation. 



Suppose a reservoir full of liquid to a certain level, and from 

 the bottom of which runs a pipe of uniform caliber. The tubes 

 which branch from this main pipe are of equal caliber and are placed 

 at equal distances from one another. The upright tubes have 

 received the name of manometers. If, now, there be a flow of the 

 liquid it will be because of a difference of pressure at the reservoir 

 and outlet. During this flow the liquid in the various manometers will 

 contain columns of the liquid whose tops would be in contact with a 

 straight line drawn from the superior surface of the contents of the 

 reservoir to the point of egress. This slanting line is known as the 

 pressure-slope. The manometer nearest the reservoir contains the 

 highest column of liquid, the next one a column of less height, etc., the 

 lowest being attained in the upright tube farthest from the heart or 

 reservoir. 



The height to which the liquid rises in a manometer sensibly 

 indicates the intensity of the force of afflux at that point. And, as 

 it decreases from the orifice of entry to that of exit, it must be con- 

 cluded therefrom that the force of the flow of the liquid decreases 

 of itself. It has been demonstrated in physics that the resistances 

 which liquids meet with in ducts of a uniform caliber are proportional 

 to the length of the latter. It follows, therefore, that, when the 

 flow is established in the tube the more distant from the ingress a 

 point of that tube is, the more the liquid which passes through it 

 will have lost its initial force in consequence of resistances. 



The more narrow the caliber of the tube, the greater is the 

 resistance to the liquid. Up to the time of Rev. Stephen Hales, an 

 English vicar, the methods of noting blood-pressure were crude in 

 the extreme. It was known that the blood exerted considerable 



