CIRCULA TION. 



127 



have been opened, the cannula may also be passed into the heart through a small 

 wound in an auricle or even through the Malls of the ventricle itself. The end 

 of the cannula which remains without the animal's body is connected, air-tight, 

 with a rigid tube of small, carefully chosen calibre, and as short as the condi- 

 tions of the experiment permit. The other end of this tube is not, as in the 

 mercurial manometer, left as an open mouth, but is connected, air-tight, with 

 a very small metallic chamber, which constitutes, practically, a dilated blind 

 extremity of the system formed by the tube and the cannula together. The 

 roof of this small metallic chamber is a highly elastic disk either of thin metal 

 or of india-rubber. Except for this small disk, all parts of the chamber, tube, 

 and cannula are rigid. In the instruments of some observers, the entire cavity 

 of the system formed by the chamber, tube, and cannula is filled with liquid, 

 viz. the solution which checks coagulation. Other observers introduce this 



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SSL 



Fig. 21.— Diagram of the elastic manometer: .4, auricle; V, ventricle ; D, drum of the kymograph, 

 revolving in the direction of the arrow, and covered with smoked paper; L, recording lever in contact 

 with the revolving drum. (The working details of the instrument are suppressed for the sake of clear- 

 ness.) 



liquid only into the portion of the system nearest the blood ; the terminal 

 chamber, and most of the rest of the system, containing only air. In every 

 case the blood in the vessel or in the heart is in free communication, through 

 the mouth of the tied-in cannula, with the cavity common to the tubes and 

 to the terminal chamber. At every rise of blood-pressure a little blood enters 

 this cavity, room being made for it by a displacement of liquid or of air, 

 which in turn causes a slight bulging of the elastic disk. At every fall of 

 blood-pressure a little blood mixed with liquid leaves the tubes as the elastic 

 disk recoils. If the disk is of the right elasticity, its rise and fall are directly 

 proportional to the rise and fall of the blood-pressure, and can be used to 

 measure it. With the centre of the disk is connected a delicate lever which 

 rises and falls with the disk. The point of this lever traces upon the 

 revolving drum of the kymograph a curve which records the fluctuations 

 of the disk and therefore those of the blood-pressure. The elastic disk 

 and the contents, together, of such an apparatus possess less inertia than mer- 

 cury, and therefore follow far more closely rapid fluctuations of pressure. 

 Such instruments maybe called "elastic manometers," and are often called 

 " tonographs," i. e. "tension-writer-." They are of several forms. 



