264 CIRCULATION OF THE BLOOD. 



wall of the chest with more or less violence. This knock is 

 called the cardiac impulse. It is precisely coincident with the 

 complete closure of the auriculo-ventricnlar valves, and deter- 

 mines the bursting open of the sigmoid valves. If the base 

 of the heart, i. e., the roots of the great arteries, were fixed, 

 the shortening of the ventricular axis, which, as we have seen, 

 occurs at the moment of hardening, would determine a with- 

 drawal or retraction of the apex from the position occupied 

 by it in diastole. As, however, this shortening is attended 

 with lengthening of the aorta, its retractive effect is more or 

 less neutralized, so that the seat of impulse in other words, 

 the centre towards which the muscular mass of the ventricles 

 draws itself together is not far from the position occupied 

 by the apex of the heart when in a state of relaxation. This 

 can be demonstrated both in man and in the lower animals. 

 In a rabbit or dog rendered insensible by opium or chloral, a 

 number of long slender needles are introduced into the heart 

 in the following positions: No. 1 is inserted vertically into 

 ' the ventricle at the point at which its knock can be felt by 

 the finger most distinctly.- From this point a line is drawn 

 upwards and inwards towards the root of the aorta, along 

 which Nos. 2, 3, and 4 are inserted in a similar manner in the 

 intercostal spaces. In like manner, Nos. 5 and are inserted 

 at equal distances on either side of the impulse in the same 

 intercostal space. The movements executed by these several 

 needles differ according to their relation to the central one, 

 No. 1, which, although it is affected by the ascent and descent 

 of the diaphragm, is indifferent as regards the heart. Of the 

 series, Nos. 2, 3, and 4, the free end of each performs an in- 

 stantaneous upward movement, the extent of which is in pro- 

 portion to its distance from No. 1 ; and finally, Nos. 5 and 6 

 oscillate more or less horizontally, their free ends receding 

 from each other, as well as from No. 1, at the moment of the 

 impulse. From these facts we learn that, whereas that part 

 of the ventricular mass which knocks against the chest is 

 nearly stationary, the base of the heart moves downwards, 

 and to the left at the moment of the ventricular hardening, 

 t. e., of the aortic pulse; and that the other parts of the ven- 

 tricles are drawn towards the impulse in a degree proportional 

 to their distance from it. 



In man, the same facts are demonstrated with the aid of the 

 cardiograph. The word cardiograph has been applied by 

 various writers to a variety of instruments, which differ from 

 each other both in their form and in the principles on which 

 they are constructed, but agree in the purpose which they are 

 intended to fulfil. This purpose is the recording of the cardiac 

 movements of the wall of the chest by the graphic method. 



