144 



HANDBOOK OF PHYSIOLOGY. / 



ing to drive it on in the direction of least resistance. The only direction 

 in which it can be driven is onwards towards the capillaries, and so the 



blood-pressure in the arteries is one of the 

 great agents in maintaining the circulation. 

 The relations which exist between the 

 arteries and their contained blood are thus 

 so obviously 's& importance to the carrying 

 on of the qjjeulation, that it becomes neces- 

 sary to be able to gauge the alterations in 

 blood- pressure very accurately. This may be 

 done by means of a mercurial manometer in 

 the following way: The short horizontal 

 limb of this (Fig. 131) is connected, by 

 means of an elastic tube and canula, with 

 the interior of an artery; a solution of sodium 

 or potassium carbonate being previously in- 

 troduced into this part of the apparatus to 

 prevent coagulation of the blood. The 

 blood-pressure is thus communicated to the 

 upper part of the mercurial column; and the 

 depth to which the latter sinks, added to the 

 height to which it rises in the other, will 

 give the height of the mercurial column 

 which the blood-pressure balances; the 

 weight of the soda solution being subtracted. 

 For the estimation of the arterial tension at any given moment, no 

 further apparatus than this, which is called Poiseuilles's Jmmadynamo- 

 meter, is necessary; but for noting the variations of pressure in the ar- 

 terial system, as well as its absolute amount, the instrument is usually 

 combined with a registering apparatus, and in this form is called a kymo- 

 graph. 



The kymograph, invented by Ludwig, is composed of a haemadyna- 

 mometer, the open mercurial column of which supports a floating piston 

 and vertical rod, with short horizontal pen (Fig. 132). The pen is 

 adjusted in contact with a sheet of paper, which is caused to move at a 

 uniform rate by clockwork; and thus the up-and-down movements of the 

 mercurial column, which are communicated to the rod and pen, are 

 marked or registered on the moving paper, as in the registering appa- 

 ratus of the sphygmograph, and minute variations are graphically re- 

 corded (Fig. 134). 



For some purposes the spring kymograph of Fick (Fig. 135) is pref- 

 erable to the mercurial kymograph. It consists of a hollow C-shaped 

 spring, filled with fluid, the interior of which is brought into connection 

 with the interior of an artery, by means of a flexible metallic tube and 



FIG. 131. Diagram of mercu 

 rial manometer. 



