THE STANNIUS LIGATURE 105 



recorded are mechanical and due to the process of tying. The 

 auricle and ventricle beats stopped at once, and the recorded line only 

 shows slight undulations, which were caused by the contraction of the 

 sinus. The rhythm of the sinus is seen to be at first slowed. At the 

 point l> the ventricle was stimulated by a prick of a needle. It gave 

 a single contraction. 



Prick the auricle. With each stimulus the quiescent part gives a 

 single beat. Repeat on the ventricle. The same result is obtained. 

 Next stimulate with an induced shock. A single contraction is also 

 obtained in this way. 



The Stannius ligature is of great importance because it enables us 

 to study a single contraction after the same method adopted in studying 

 a single twitch of a muscle, whereas while the heart is still beating 

 rhythmically this is impossible. 



Experiment 3.— Test the response of the heart to electrical stimuli of 

 different strengths. Attach the apex of a heart brought to a standstill by the 

 Stannius ligature to the recording lever, as in fig. 83. Remove the secondary 

 coil to such a distance from the primary that neither make nor break shocks 

 excite a contraction. Bring the writing point to the surface of a drum at 

 rest, so as to record heights of twitch only. Move the secondary coil gradu- 

 ally up to the primary, stimulating the heart with each new position until one 

 is found at which a contraction occurs at break. Move the drum 1 or 2 mm. 

 by hand. Bring the secondary 1 cm. nearer the primary, and, allowing 

 fully 30 seconds to elapse, stimulate once more. The height of the con- 

 traction is the same as before. Further increase the strength of the stimulus 

 and record the contractions. There is no increase in the height. 



Therefore a minimal stimulus causes a maximal contraction. If a 



heart contracts at all it contracts to its full power. 



The reason for allowing an interval of from 30 to 60 seconds to 

 elapse in the preceding experiment before applying a second stimulus 

 is because a second stimulus of even the same strength is found to 

 produce a greater effect than the first if it follow the first within a 

 second or so. This can be shown in the following way : — 



Experiment 4. — Arrange the drum to rotate at a very slow rate, about 

 2 cm. per minute. Apply electrical stimuli every five seconds, recording the 

 contractions on the moving surface. Record some twenty contractions in this 

 way. 



Fig. 86 shows the result obtained. The second contraction is seen 

 to be higher than the first, the third than the second, and so on up to 

 the fifth, from which time they all reached practically the same height. 

 The increment of height of the second over the first is greater than the 

 increment of the third over the second, and this increment gradually 

 decreases until it vanishes. This increase of effect on repeating the 

 stimulation, keeping to the same strength, is spoken of as the staircase 



