THE MECHANISM OF THE HEART PUMP 897 



ampulla may be recorded on a moving smoked surface. The large size of these sounds 

 makes it difficult to use them on any animal smaller than the ass or horse. In smaller 

 animals, such as the dog, the question has been investigated by the use of a manometer 

 such as that of Hiirthle. In this instrument (Fig. 396) the changes of pressure are 



FIG. 396. Diagram to show construction of Hiirthle's membrane manometer. 



recorded by the oscillations of a thick rubber membrane which covers a very small 

 tambour. The tambour is filled with magnesium sulphate solution, which is also used 

 to fill the tube connecting with the heart. This tube can be inserted in the same way 

 as Marey's cardiac sound. 



Even Hiirthle's instrument is inadequate to give a correct representation of the very 

 rapid changes of pressure occurring in the contracting ventricle. A study of the theory 

 of recording instruments by Otto Frank has enabled him to lay down certain funda- 

 mental requirements of such a recording instrument. In order that an instrument 

 may reproduce correctly rapid changes of pressure, the mass moved must be as small as 

 possible in order to reduce the momentum, and therefore the tendency to overthrow of the 

 instrument, to the greatest possible extent. Moreover, the movement of fluid into and 

 out of the instrument, which accompanies each change of pressure, must occur with 

 the smallest possible friction. This is accomplished, as in Hiirthle's instrument, by 

 using a very small tambour, covered with a strong tightly stretched membrane connected 

 by as short and wide a tube as is feasible, with the heart or blood-vessel where it is 

 desired to register changes of pressure. A lever is entirely got rid of, the minute 

 oscillations of the membrane being recorded by means of a beam of light which impinges 



EF 



O 



FIG. 397. Diagram of Piper's manometer. 



on a mirror attached to the rubber membrane and reflected on to a moving photographic 

 surface. In Fig. 397 is represented the construction of Piper's manometer, built on 

 the principles laid down by Frank. 



It consists of a tube armed with a stilette, A, which fits it accurately. At o is a tap 

 which, when opened, will permit the passage of the stilette, and can close the tube en- 

 tirely when the stilette is withdrawn. About 2 cms. above the lower extremity of the 

 tube is a small drum-like enlargement, closed on one side by a thick membrane, E. 

 On the edge of this membrane is fixed by means of shellac a minute mirror, F, 1 mm. in 

 diameter. With the stilette protruding, the manometer is thrust directly into the 

 cavity of the heart, and fixed in position by a purse-string suture through the super- 

 ficial part of the heart muscle, tied tightly round the end of the manometer. The stilette 

 is then withdrawn and the tap turned off, but alterations in pressure in the cavity of the 

 heart causes minute oscillations of the membrane, which can be recorded and magnified to 

 any desired extent by means of a beam of light reflected from the mirror on to a moving 

 photographic plate or paper. The advantage of this optical method of registration is 

 that the magnification can be increased to any extent without altering the mass moved. 

 The ' figure of merit ' of this manometer, i.e. its own period of vibration, when filled with 

 fluid, is about 250 per second with a thick membrane, so that it can record with perfect 

 accuracy all such rapid changes of pressure as may occur even in the left ventricle. 



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