118 



by another lig-ature from the auricles, seems to show that it 

 also contains motor centers. The hypothesis has been 

 advanced that both sinus venosus and ventricle contain motor 

 centers, while the auricles contain inhibitory centers. 



So longf as the auricles are in connection, both with the 

 sinus venosus and the ventricle, the motor centers in the latter 

 two parts are supposed to be sufficiently powerful to overcome 

 the resistance offered by the inhibitory centers, and thus the 

 cardiac rhythm is maintained. Wlfen the motor centers of 

 the sinus are removed, the inhibitory centers of the auricle 

 are supposed to be so powerful as to keep both it and the 

 ventricle in a state of rest. 



369. Cardiac Delay or Latent Period op Cardiac 

 Muscle. In the case of skeletal muscle, the muscle is at rest 

 and a stimulus excites it to contraction ; cardiac muscle has 

 the power of contracting- rhythmically ; it will therefore be 

 necessary to stop the heartbeat by the application of a 

 " Stannius " lig-ature. 



Arrange the apparatus for single induced shocks, and in- 

 clude in the primary circuit a signal magnet to mark the exact 

 time the stimulus is applied. Use also a time-marker record- 

 ing in half seconds. 



After exposing the heart apply the " Stannius ligature " 

 to stop the beat. Attach the apex of the ventricle to the 

 heart lever. Arrange the three levers, heart, signal magnet 

 and time marker so that their writing points will all be 

 exactly in the same vertical line. Let the drum revolve. 

 Stimulate the ventricle with a single induced shock. When 

 the circuit is made or broken the lever of the signal magnet 

 will immediately respond and shortly after the heart lever will 

 also respond. The interval represents the "latent period" 

 and may be about half a second, depending upon temperature 

 and other conditions. 



Stimulate an auricle in the same way and note the longer 

 "delay;" the wave of contraction travelling slowly and de- 

 laying at the groove. 



Compare the cardiac latent period with the latent period 

 of skeletal muscle. 



370. Maximum Contractions only. Find the weakest 

 stimulus that will cause contraction of the ventricle. Increase 



