THE HEAET. 



161 



onward representing the relaxation which forms the first part of the diastole. 

 If this interpretation of the curve he correct, it is obvious that the front-to- 



FIG. 52. 



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 second. 



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 long axis from side to side, but with the front 

 part of the ellipse much more convex than the back, since the back surface 

 of the ventricle is somewhat flattened. During the systole this ellipse is by 

 the shortening of the side-to-side diameter and the increase of the front-to- 

 back diameter converted into a figure much more nearly resembling a circle. 

 It is urged, moreover, that the whole of the base is constricted, and that the 

 greater efficiency of the auriculo-ventricular valves is thereby secured. 



As to the behavior of the long diameter from base to apex observers are 

 not agreed. Some maintain that it is shortened, and others that it is prac- 

 tically unchanged. If any shortening does take place, it must be largely 

 compensated by the elongation of the great vessels, which, as stated above, 



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, ar- 

 ranged 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 be- 

 low 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 backward and forward similar segments of circles may be drawn 

 through other points of the curve. The lines a, b, c in Fig. 52 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 backward or forward 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. 



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