272 PHYSIOLOGY. 



Digitalin on the frog's heart augments the duration and force of 

 ventricular systole, slows diastole, and the arrest of the heart takes 

 place in systole. This systolic stop of the heart ensues after section of 

 the vagi or after the use of atropine, and is an action on the muscle. 

 At other times the heart is slowed and arrest takes place in diastole, 

 due to an action of digitalin on the cardio-inhibitory apparatus. This 

 diastolic arrest does not ensue after the previous use of atropine. In 

 the warm-blooded heart, we have a diminution of rate of the heart-beat, 

 by digitalin and at the same time strengthening of the systolic force. 

 Hence digitalis does the heart good by slowing the heart, increasing its 

 rest in diastole and toning up its muscular structure. 



Some heart-poisons in small doses diminish the heart's action and 

 in large doses usually accelerate its movements; or the converse may 

 be the truth with regard to the doses of other drugs. 



NICOTINE. The vagus fibers arborize around the nerve-cells of 

 the crescent, the seat of Kemak's ganglia in the frog's heart and from 

 these ganglia new nerve-fibers run to the heart-muscle. The fibers 

 running to arborize around Kemak's ganglion or the crescent are 

 called preganglionic fibers; those running from the ganglion to the 

 heart-muscle are postganglionic. 



Nicotine at first slows the heart because it has a stimulant action 

 on the nerve terminals in Eemak's ganglion and on the vagus center in 

 the brain. But soon after the use of nicotine stimulation of the vagus 

 is without inhibitory effect because nicotine paralyzes the terminals of 

 the nerve or the receptive substance of the cells in the ganglion of 

 Eemak. Stimulation of the crescent after the use of nicotine stops the 

 heart because the postganglionic fibers from the crescent are still 

 active. These fibers are, however, poisoned by a dose of atropine. 

 After a dose of nicotine, stimulation of the crescent often causes marked 

 acceleration of the heart, because the accelerator fibers running in the 

 vagus are stimulated, as they have no nerve-cells in their intracardiac 

 course and go direct to their termination in the muscle. 



Stannius Heart. If in a frog's heart the Stannius ligature be 

 applied around the heart at the junction of the sinus with the auricle, 

 the auricle and ventricle stand still in diastole, and it is then called a 

 Stannius heart. 



Minimal Stimulus Causes Maximal Contraction. If you excite 

 once a minute the apex of a Stannius heart with an induced current of 

 necessary intensity, commencing with currents too weak to cause a 

 contraction, it will be found that finally the heart contracts. If the 

 current is augmented, still, a contraction ensues of the same height 



