380 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



measures the height of the mauometric column of mercury the downward pres- 

 sure of which in one limb of the tube is just equal to the downward pressure 

 of the fluid in the other. In order to adapt this " U-tube " to the study of the 

 blood-pressure, that limb of the tube which is to communicate with the artery 

 or vein is capped with a cock which can be closed. Into this same limb, a little 

 way below the cock, opens at right angles a short straight glass tube, which is 

 to communicate with the blood-vessel through a long flexible tube of lead, sup- 

 ported by the iron frame, and a short glass cannula tied into the blood-vessel 

 itself. Two short pieces of india-rubber tube join the lead tube to the manometer 

 and the cannula. Before the blood-vessel is connected with the manometer, the 

 latter is filled with fluid between the surface of the mercury next the blood- 

 vessel and the outer end of the lead tube, which fluid is such that when mixed 

 with blood it prevents or greatly retards coagulation. With this same fluid 

 the glass cannula in the blood-vessel is also filled, and then this cannula and 

 the lead tube are connected. The cock at the upper end of the " proximal 

 limb " of the manometer is to facilitate this filling, being connected by a rub- 

 ber tube with a " pressure bottle," and is closed when the filling has been 

 accomplished. The fluid introduced by Poiseuille and still generally used is 

 a strong watery solution of sodium carbonate. A solution of magnesium sul- 

 phate is also good. If, in injecting this fluid, the column of mercury in the 

 "distal limb" is brought to about the height which is expected to indicate the 

 blood-pressure, but little blood will escape from the blood-vessel when the 

 clamp is taken from it, and coagulation may not set in for a long time. 



The Recording Mercurial Manometer and the Graphic Method. 

 When the arterial pressure is under observation, the combined respiratory 

 and cardiac fluctuations of the mercurial column are so complex and fre- 

 quent that it is very hard to read off their course accurately even with the 

 help of a millimeter-scale placed beside the tube. In 1847 this difficulty led 

 the German physiologist Carl Ludwig to convert the mercurial manometer 

 into a self-registering instrument. This invention marked an epoch not 

 merely in the investigation of the circulation, but in the whole science of 

 physiology, by beginning the present " graphic method " of physiological 

 work, which has led to an immense advance of knowledge in many depart- 

 ments. Ludwig devised the " recording manometer " by placing upon the 

 mercury in the distal air-containing limb of Poiseuille's instrument an ivory 

 float, bearing a light, stiff, vertical rod (see Fig. 100). Any fluctuation of the 

 mercurial column caused float and rod to rise and fall like a piston. The rod 

 projected well above the manometer, at the mouth of which a delicate bear- 

 ing was provided to keep the motion of the rod vertical. A very delicate 

 pen placed horizontally was fastened at right angles to the upper end of the 

 rod. If a firm vertical surface, covered with paper, were now placed lightly 

 in contact with the pen, a rise of the mercury would cause a corresponding 

 vertical line to be marked upon the paper, and a succeeding fall would 

 cause the descending pen to inscribe a second line covering the first. If 

 now the vertical surface were made to move past the pen at a uniform rate, 



