CHAP, iv.] 



THE VASCULAR MECHANISM. 



147 



Tracings of the movements of the ventricles themselves, corre- 

 sponding to the cardiac impulse and so to a certain extent to the 

 variations of internal pressure, may also be taken directly by 

 bringing a light lever to bear on the outside of the ventricles, the 

 chest having been previously opened and artificial respiration kept up. 

 A curve 1 taken by this method is shewn in Fig. 24. 



FIG. 24. Normal heart curve shewing changes in the antero-posterior diameter of 

 the ventricle obtained from the cat by a light recording lever moved by a button 

 which pressed gently on the anterior surface of the ventricle. The time curve gives 

 50 double vibrations per sec. and lines have been drawn to shew the duration of the 

 different phases of the ventricular movement, a to 6 corresponds to the distension of 

 the ventricle including the auricular systole, the wave-like rise during this period being 

 due to the increase in the diameter of the ventricle resulting from the entrance into it 

 of the contents of the auricle. The period from 6 to c corresponds to the time from 

 the commencement of the ventricular contraction to the moment when the organ 

 has completed its change in shape from a flattened to a more rounded form. The 

 highest part of the curve corresponds also in time with the opening of the semilunar 

 valves as well as the firm closure of the auriculo-ventrlcular valves. The duration of this 



1 The majority of cardiograph ic, sphygmographic and other tracings shew certain 

 points which can be understood at a glance, but many characteristics can only 

 be learned by "measuring out the curve" as it is termed. This is done as follows. 



Every tracing ought to bear on it an abscissa line, marked by a point which 

 remains motionless while the recording surface is travelling. Moreover, either 

 before or after taking a curve, while the paper or recording surface is at rest, the 

 point of the lever should be always moved up and down so as to describe a segment 

 of a circle of which the axis of the lever is the centre. 



The tracing thus prepared, when it has to be measured, is pinned out on aboard, 

 and, by means of a pair of compasses, the distance between whose points has 

 previously been made equal to the distance between the axis and the point of 

 the lever used in making the experiment, the centre of the circle of which the 

 curved lines previously made as directed are segments is found and marked on the 

 paper. Through this centre, which of course corresponds to the position of 

 the axis of the lever, a horizontal line is drawn parallel to the abscissa line. 



Keeping one of the compass points on this line, segments of circles are drawn in 

 succession through various points of the curve, the distance between the points of 

 the compass being fixed, but the centre of the circle described being shifted 

 backwards and forwards along the horizontal line. The points where these 

 segments cut the horizontal line are marked upon it, and the distances between them 

 measured as, for example, in Fig. 29, p. 166. If the curve of a tuning-fork, the 

 point of whose recording style was carefully placed on the same vertical line as the 

 point of the lever, be also present, the segments of circles may be continued until 

 they cut this, and the time corresponding to distances between them (as, for 

 instance, in Fig. 24 the intervals between a, 6, c, d,) thus directly measured off. 



102 



