THE BLOODVESSELS 69 



It has been seen that the amount of side pressure exercised 

 by fluid in a tube is ascertained by means of a manometer ; 

 precisely the same principle is employed to ascertain the side 

 pressure in a bloodvessel, here called blood-pressure. It is 

 interesting to observe that the first blood-pressure experiment 

 ever performed was carried out on a horse, a vertical tube being 

 placed in the femoral artery, in which the blood rose to a height 

 of 8 feet 3 inches. It is, of course, for several reasons incon- 

 venient to work with a long tube, and in consequence most 

 manometers are U-shaped tubes containing mercury which is 

 made to balance the pressure to be measured. The greater 

 specific gravity of this metal enables a tube nearly fourteen times 

 shorter to be employed. The inertia of mercury renders it 

 useless for recording delicate or sudden alterations in pressure ; 

 these are obtained by means of spring manometers. 



In Fig. 28 is shown the apparatus employed in determining 

 blood-pressure with the ordinary mercury manometer. 



The Mean Arterial Pressure is increased at each systole of the 

 ventricles, and falls at each diastole ; the maximum pressure is 

 therefore known as systolic, the minimum as diastolic. In the 

 aorta of the dog the systolic pressure may be 168 mm. (6| inches), 

 the diastolic only 100 mm. (4 inches). The difference between 

 these two pressures is sometimes spoken of as the pulse 

 pressure or mean pressure. The further the vessel is situated 

 from the heart, the less the difference between systolic and 

 diastolic pressures, until finally in the capillaries there is no 

 difference, and consequently no pulse. 



In the aorta of the horse the blood-pressure may vary from 

 321 mm. to 150 mm. (12J inches to 6 inches) ; dog, 168 mm. 

 to 100 mm. (6| inches to 4J inches) ; sheep, 206 mm. to 156 mm. 

 (8 inches to 6 J inches). 



In the carotid of the horse the pressure is from 325 to 215 mm. 

 (12 J inches to 8 inches), equal to a column of blood 9 J feet to 

 13! feet in height. 



Neither size of body nor pulse-rate bear any relation to the 

 amount of blood-pressure present in the arteries of an animal. 

 The pressure in the carotid of the goose is nearly the same as in 

 the carotid of the horse. 



Influence of Muscular Work on blood-pressure is very marked ; 

 the pressure is raised because muscular contraction causes 

 mechanical compression of the vessels of the muscles. Every 

 movement of the body, every movement of a limb or part, 

 affects the blood-pressure in it ; in the large arteries the pressure 

 from this cause will be found to be constantly varying, unless the 

 animal be absolutely immobilised. There is also another important 

 cause — viz., the increased force and frequency of the heart-beat. 



