THE MECHANISM OF THE HEART PUMP 1011 



inserted in the same way as Marey's cardiac sound. The object of 

 making the tambour small and the rubber membrane thick is to limit 

 the excursions as far as possible and thereby diminish the total amount 

 of fluid moved -with any given rise of pressure. 



A much more perfect form of manometer has been devised by 

 Otto Frank on the basis of a very complete investigation of the 

 theoretical requirements of such an instrument. It is evident that 

 the total mass moved for the purpose of making the record must 

 be as small as possible, in order to diminish the momentum of the 

 moving parts and therefore the tendency of the instrument to 

 enter into vibrations of its own, which will add to and deform the 

 changes of pressure which it is sought to record. On the other hand, if 

 the vibrations of the instrument be damped by introducing artificial 



FIG. 393. 



friction, the instrument will be impaired in its power of responding 

 to quick changes of pressure. The vibration frequency of the instru- 

 ment itself should therefore be so rapid that there is no fear of 

 confounding any such vibrations with the alterations of pressure in 

 the vascular system. The construction of Frank's instrument is shown 

 in Fig. 393. It consists of a short vertical tube which is prolonged 

 at its lower end into a cone for connection with the arterial cannula. 

 The tube 6 is closed by a tap c, by which any possible air bubbles can 

 be let out. It is important to avoid even the smallest air bubble in such 

 an apparatus, as well as any rubber connections, as these may undergo 

 compression or distension with changes of pressure and therefore slow 

 the vibration frequency of the instrument and distort the curves. The 

 apparatus is filled with liquid from a reservoir by the tube/. On the 

 narrowed end of e the manometer capsule is cemented. This is covered 

 with a rubber membrane, care being taken to exclude air bubbles 

 when the membrane is tied on. A small stand is fixed on e which 

 carries a metal fork G. The two limbs of the fork carry, one a wedge- 



