MEASUREMENT OF THE BLOOD PRESSURE. 291 



impaired. For example, the heart's beat may be stopped by the 

 stimulation of the inhibitory nerve fibres of the vagus, in which 

 case the blood pressure rapidly falls, as shown by the curve taken 

 by the graphic method. Or weakness of the heart-beat may arise 

 from disease (fatty degeneration) of the muscle, when signs of low 

 arterial tension can be recognized in the human subject. 



Any insufficiency of the aortic valves, whose duty it is to close 

 the proximal end of the arteries, that permits the blood to flow 

 backward into the ventricle, allows the pressure in the arteries to 

 fall between each ventricular systole, so that the characteristic 

 "pulse of unfilled arteries" is recognized by the physician. 



The resiliency of the arterial coats may also be destroyed to a 

 certain extent by degeneration of the tissue, in which case the 

 large arteries become greatly distended, and unable to exert their 

 normal steady pressure on the blood. 



Injuries of the nerve centres are often associated with paralysis 

 of the muscular arterioles, and fall of blood pressure ; but the 

 effect upon the blood pressure of dilatation of the small arteries 

 can be best seen by experimenting on the nerves that control their 

 contraction in the lower animals. If paralysis or inhibition of 

 the vasomotor mechanisms be experimentally produced, the result 

 on the arterial pressure is the same, namely, a sudden fall, which 

 may reach zero : all opposition to the outflow of blood from the 

 arteries being stopped, they cease to be tense, even though the 

 ventricle continue to beat and pump the blood into them. 



MEASUREMENT OF THE BLOOD PRESSURE. 



The first attempt at direct measurement of blood pressure 

 was made by the Rev. Stephen Hales about the middle of last 

 century, who, wishing to compare the motion of fluids in animals 

 with that of plants, connected a tube in an artery of a living 

 animal, and found that the blood was ejected with considerable 

 force, and that when the artery of a horse was brought into union 

 with a long upright tube, the blood reached a height of about 

 three yards. 



The blood itself is not now used as a measure, because so much 



