266 CIRCULATION OF THE BLOOD. 



now the cardiograph is shifted towards the sternum, the 

 character of the tracing is entirely altered. (See Fig. 2326). 

 The ventricular hardening is still,' indeed, indicated by a jerk 

 upwards of the lever; but this is immediately succeeded by a 

 descent of such a character as to afford evidence that at the 

 point investigated the thoracic wall, instead of bulging, is re- 

 tracted during the systolic effort. This phenomenon, which 

 is well known to pathologists, being so marked in some con- 

 ditions of disease that it is easily appreciated by the unaided 

 hand or eye, has been called the " negative impulse." It means 

 that the heart, which, when gradually filling with blood applies 

 itself to the whole prsecordia, gathers itself from all directions 

 towards the centre of impulse in bedside language, commonly 

 miscalled the apex. If the cardiographic tracing of the im- 

 pulse is compared with that obtained manometrically by a 

 method to be immediately described, it is obvious that the two 

 correspond with each other very closely ; so that we are per- 

 fectly safe in assuming, as has been done above, that the ascent 

 denotes the beginning, the descent the end, of the ventricular 

 effort. We can thus determine with the greatest precision the 

 moment at which the mitral and tricupsid valves close. The 

 moment of the closure of the arterial valves is not so certain, 

 for it does not coincide with the end of the systole. It is 

 sometimes marked by an up-and-down movement of the lever, 

 due to the vibration into which the chest wall is thrown at the 

 moment that the curtains of the aortic valve come together. 

 The auricular contraction is often indicated by a slight eleva- 

 tion, which precedes the impulse by a distinct interval. 



61. Investigation of the Sounds of the Heart. The 

 sounds of the heart can be studied both in man and in the 

 lower animals. The first or dull sound coincides with the 

 hardening of the ventricles, the complete closure of the 

 auriculo-ventrieular valves, and the bursting open of the arte- 

 rial orifices. 



It is caused principally by the sudden distension of the ven- 

 tricles, but can bq proved experimentally to be also in part of 

 the same nature with the noise made by all muscles in the act 

 of contracting against a resistance. The second or sharp sound 

 is coincident with and caused by the closure of the sigmoid 

 valves. This is proved by the observation that if the valve is 

 injured, or prevented from closing by mechanical means, the 

 sound is no longer heard. In studying the sounds of the heart 

 in the lower animals, particularly in the dog, the student o.f 

 medicine should direct his attention specially to the modifica- 

 tions of the sounds under known conditions e. g., in dysp- 

 no2a, when the heart is distended with blood ; after hemorrhage, 

 when the ventricles are insufficiently filled in diastole ; after 

 section of the vagi, when the frequency of the contractions is 



