NORMAL RECORD OF THE MAMMALIAN HEART 



141 



adjusting the lengths of the threads and the positions of the pulleys. 

 They are then brought to the writing surface and two chronographs 

 are arranged to write vertically below them — one to give a seconds time 

 tracing, and the other to act as a signal. 



Fig. Ill reproduces a piece of tracing obtained in this way, 

 the drum moving at a slow rate. The tracing is from a rabbit's heart, 

 the upper given by the auricle, the lower by the ventricle. It gives a 

 measure of the amount of contraction of the two parts, and indirectly 



Fig. 111. — Tracing Obtained from the Rabbit's Heart, Employing the Levers 



of Fig. 110. 



of the force of the contraction, the latter especially if the tension of 

 the thread be fairly high. The variations in level of the apices of the 

 ordinates are due to the respiratory movements. Each time the lung 

 is inflated the base of the heart is a little raised, which slackens the 

 thread and the lever descends. 



One vagus is now cut and its peripheral end laid upon a pair of 

 electrodes. It is then stimulated a few times and the results recorded. 

 Pig. 112 gives the result of a fairly strong current. The stimulus is 

 seen to act both on the strength and on the frequency of the beat, and 

 to chiefly affect the auricle. Frequency and force are diminished 

 both for auricle and ventricle. After cessation of stimulation the ven- 

 tricle rapidly regains its previous condition, but the auricle recovers 



