H^MADYNAMOMETER. 663 



Secondly, the force of the heart, and therefore, the rate of motion of 

 the blood, is wasted by the slight loss from friction between the par- 

 ticles of the elastic coat of the vessels, occurring in their distension 

 and recoil, and likewise by the disturbance of the artery and the sur- 

 rounding tissues. Lastly, an efficient cause of retardation in the 

 arterial blood-current, is the obvious increase in the total capacity of 

 the branches of the arteries, as compared with that of the trunks from 

 which they arise; for not only do the united diameters of two or more 

 branches exceed the diameter of the parent trunk, but, though of 

 course in a much less degree, the combined areas of two or more 

 branches, are usually larger than the area of the parent trunk. The 

 combined areas of the two iliac arteries, into which the abdominal 

 aorta divides, are, however, larger than that of the aorta itself. Open- 

 ing an artery, which not only causes hemorrhage, but also diminishes 

 the resistance in the arteries, increases the velocity of the blood in the 

 opened vessel : this result may be exhibited by experiments with arti- 

 ficial tubes injected with water, and then opened. 



The/0n?e of the heart, or the blood pressure, in the arteries, has 

 been frequently investigated, both by the earlier and later physiol- 

 ogists. Stephen Hales found that, on fitting a long tube containing 

 water, into the crural artery of horses, the force of the blood-current 

 was sufficient to elevate the water in the tube, to heights varying, in 

 different cases, from 8 feet 3 inches to 9 feet 8 inches. From these 

 and other experiments, he inferred that the pressure of the blood in 

 the large arteries of the human body, would support a column of blood 

 90 inches, or 7 feet 6 inches high, or a weight of 3 pounds 7 ounces 

 per square inch. More recently, Poiseuille invented the hcemadyna- 

 mometer, a much more convenient instrument, in which a short column 

 of mercury is substituted for the longer column of water in Hales's 

 apparatus. This instrument, as now improved, and named a ma- 

 nometer^ consists of a U-shaped glass tube, having one of its stems or 

 legs longer than the other; the shorter leg is bent horizontally, and 

 provided at its end with a stopcock, and with a piece of elastic tube, 

 so that it can readily be adapted to the cut end of a divided artery in 

 a living animal. The lower curved part, and 3 or 4 inches of both 

 legs of the U-shaped tube, are filled with mercury, and the space in 

 the short leg, between the surface of the mercury and the stopcock 

 against which the artery is fixed, is occupied with a solution of com- 

 mon salt, sulphate of soda or Glauber's salt, or carbonate of soda, so 

 as to prevent the coagulation of the blood when it enters the appa- 

 ratus. At the back of the tube is fixed a graduated scale, the zero 

 of which corresponds with the level of the mercury when at rest in 

 both legs. When the horizontal part of the short feg of this instru- 

 ment is connected with an artery, and the stopcock is opened, the ap- 

 paratus being maintained in a vertical position, the force of the blood- 

 current depresses the mercury in the shorter, and raises it in the 

 longer leg. The difference between the level of the mercury in the 

 two legs, gives the height of the mercurial column supported by the 

 blood pressure. But the level of the mercurial column in the longer 

 leg is very inconstant ; for it is raised at each ventricular systole, and 



