CIR CULA TION. 4 ! 9 



open at the end and charged with a fluid which checks the coagulation of the 

 blood, is tied into a vessel, or, if the heart is under observation, is passed down 

 into it through an opening in a jugular vein or a carotid artery. If the chest 

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

 wound in an auricle or even through the walls 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 

 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. Upon the centre of the disk rests a delicate lever of the " third 

 order," which rises and falls with tlie disk. The point of this lever traces 

 upon the revolving drum of the kymograph a curve which records the fluctua- 

 tions 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 may be called " elastic manometers," and are often called 

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



It has been indicated already that the pressure of the blood may be 

 communicated to the disk of an elastic manometer either by means of 

 liquid or of air. A given series of fluctuations of blood-pressure may yield 

 decidedly diiferent curves according to the method of " transmission " employed 

 to obtain them ; and the controversies as to the true form of the endocardiac 

 pressure-trace turn upon the question whether such " transmission by air " or 

 " transmission by liquid " yield the truer curve. The objections to the former 

 method depend upon the readier compressibility of air; the objections to trans- 

 mission by liquid depend upon its greater inertia. 



The General Characters of the Ventricular Pressure-curve. AVhat- 

 ever kind of elastic manometer and of transmission be used, the curve 



