THE CIRCULATION OF THE BLOOD AND LYMPH 83 



curves of the great arteries and great veins. To obtain satisfactory 

 tracings of the swiftly-changing endocardiac pressure is a task of the 

 highest technical difficulty, and it is only in very recent years that 

 it has been accomplished with any approach to accuracy by the 

 use of elastic manometers, in which the blood-pressure is counter- 

 balanced, not by the weight of a column of liquid, as in the mercurial 

 manometer, but by the tension of an elastic disc or of a spring. 

 One of the earliest of these was the now perhaps somewhat obsolete 

 C-spring manometer of Fick (an adaptation of Bourdon's pressure- 

 gauge), of which a diagram is given in Fig. 21. Probably the most 

 perfect elastic manometers of the modern type are the improved 

 instrument of Fick (Fig. 22), with the various modifications it has 

 undergone in the hands of v. Frey and others, and especially the 

 manometers of Hiirthle. 



Hiirthle's spring manometer consists of a small drum covered 

 with an indiarubber membrane, loosely arranged so as not to vibrate 

 with a period of its own. The drum is connected with the heart or 

 with a vessel, and the blood-pressure is transmitted to a steel spring 



FIG. 22. PICK'S ELASTIC MANOMETER. 



a a is a metal piece tunnelled by a narrow canal of about i mm. in diameter, which 

 enlarges below to a shallow saucer-shaped space b. The wide opening of b is covered 

 by a thin piece of indiarubber c, to the centre of which an ivory button d is attached. 

 The button presses on a strong steel spring/ which is attached at one end to the brass 

 frame ee, and at the other, by means of an intermediate piece g, to the lever A ; b is 

 filled with a few drops of water, but the canal a contains only air. When a is con- 

 nected with the interior of the heart or of an artery, the changes of pressure are trans- 

 mitted to the spring, and recorded by the writing-point of the lever. 



by means of a light metal disc fastened on the membrane. The spring 

 acts on a writing lever. The instrument is so constructed that for a 

 given change of pressure the quantity of liquid displaced is as small 

 as possible, and it is on this that its capacity to follow sudden 

 variations of pressure chiefly depends. The manometer is connected 

 with the cavity of the heart by an appropriately-curved cannula of 

 metal or glass, which, after being filled with some liquid that 

 prevents coagulation (Practical Exercises, p. 185), is pushed through 

 the jugular vein into the right auricle or ventricle, or through the 

 carotid artery and aorta into the left ventricle. Some observers fill 

 only the cannula with liquid, and leave the capsule of the elastic 

 manometer and as much of the connections as possible full of air. 

 Others fill the whole system with liquid. And around the question 

 of the relative merits of ' transmission ' by liquid and by air has raged 



