CHAP, iv.] THE VASCULAR MECHANISM. 



233 



during which the fibres of the ventricle are undergoing contrac- 

 tion, the sudden fall from d onwards representing the relaxation 



FIG. 35 l . TRACING FROM HEART OF CAT, OBTAINED BY PLACING A LIGHT LEVER ON 



THE VENTRICLE, THE CHEST HAVING BEEN OPENED. THE TUNING-FORK CURVE 

 MARKS 50 VIBRATIONS PER SEC. 



which forms the first part of the diastole. If this interpretation 

 of the curve be correct, it is obvious that the front-to-back 

 diameter is greater during the whole of the systole than it is 

 during diastole, since the lever is raised up all this time. 



This increase of the front-to-back diameter combined with 

 a decrease of the side-to-side diameter has for a result a change 

 in the form of the section of the base of the ventricles. During 

 the diastole this has somewhat the form of an ellipse with the 



1 The vertical or rather curved lines (segments of circles) introduced into this 

 and many other curves are of use for the purpose of measuring parts of the curve. 

 A complete curve should exhibit an ' abscissa ' line. This may be drawn by 

 allowing the lever, arranged for the experiment but remaining at rest, to mark with 

 its point on the recording surface set in motion ; a straight line, the abscissa line, 

 is thus described, and may be drawn before or after the curve itself is made, 

 and may be placed above or preferably below the curve. When a tuning-fork 

 or other time marker is used, the line of the time marker or a line drawn through 

 the curves of the tuning-fork will serve as an abscissa line. After a tracing has 

 been made, the recording surface should be brought back to such a position that 

 the point of the lever coincides with some point of the curve which it is desired to 

 mark ; if the lever be then gently moved up and down, the point of the lever 

 will describe a segment of a circle (the centre of which lies at the axis of the 

 lever), which segment should be made long enough to cut both the curve and 

 the abscissa line (the tuning-fork curves or other time marking line) where this is 

 drawn. By moving the recording surface backwards and forwards similar 

 segments of circles may be drawn through other points of the curve. The lines 

 a, b, c in Fig. 35 were thus drawn. The distance between any two of these points 

 may thus be measured on the tuning-fork curve or other time curve, or on the abscissa 

 line. Similar lines may be drawn on the tracing after its removal from the recording 

 instrument in the following way. Take a pair of compasses, the two points of which 

 are fixed just as far apart as the length of the lever used in the experiment, measured 

 from its axis to its writing point. By means of the compasses find the position on 

 the tracing of the centre of the circle of which any one of the previously drawn 

 curved lines forms a segment. Through this centre draw a line parallel to the 

 abscissa. By keeping one point of the compass on this line but moving it along 

 the line backwards or forwards a segment of a circle may be drawn so as to cut 

 any point of the curve that may be desired, and also the abscissa line or the 

 time line. Such a segment of a circle may be used for the same purposes as 

 the original one and any number of such segments may be drawn. 



