NERVES OF THE HEART. CARDIAC MUSCLE. 265 



feeble, and is stopped when the stimulus is more powerful; 

 and in this animal it is possible by carefully applied stimula- 

 tion to keep the heart at rest for a considerable time, during 

 which it lies distended and flabby; but nearly always it ulti- 

 mately recommences its beat even though the stimulation of 

 the nerve be continued. During its inhibition the heart is 

 irritable and contractile, for it beats if a direct stimulus be 

 applied to it: the myocardium is therefore not incapable of 

 action; but either some influence normally proceeding from 

 its nerve-cells and promoting its automatic contraction is pre- 

 vented, or the stimulation directly acts on the cardiac muscle 

 and for the time lowers or removes its spontaneity. If the 

 stimulus applied to the cardiac nerve be not strong enough to 

 completely inhibit the heart, it is usually seen that the pulsa- 

 tions are not only fewer, but more feeble; but this is not always 

 the case : the beats may be slower and not less powerful than 

 before, or they may continue with the same rhythm, but be 

 less powerful; in any case the result is to diminish for the 

 time the work done by the heart. 



In marnmalia the phenomena are essentially the same. If 

 artificial respiration be maintained in an anaesthetized rabbit 

 and its heart laid bare, and then the pneumogastric trunk be 

 divided on one side of the neck and its cardiac end stimu- 

 lated, the heart comes to rest, distended and soft to the touch; 

 or, with more feeble stimulation, the pulsations are slowed; 

 or they may be both slower and feebler, or feebler and not 

 slower; but the amount of blood driven out by the ventricles 

 in a given time is usually much less. When the beat is only 

 weakened it often happens that the effect shows itself much 

 more markedly on the auricles than on the ventricles, though 

 this of course diminishes the work done by the ventricles, as 

 they are then supplied with less blood to pump on; and occa- 

 sionally it may be seen that the auricles miss a beat, giving only 

 one for each two of the ventricles, quite contrary to the case 

 of a dying heart, in which, as we have seen, the auricular beat 

 is more prominent. This illustrates the fact that the auricles 

 are more sensitive to external nervous control than the ven- 

 tricles, and provide, so to speak, the " fine adjustment" of the 

 cardiac apparatus. 



Wh ether the heart is stopped or slowed or its beats weak- 

 ened, the result must be a fall in arterial pressure, for the 

 stretched arteries go on driving blood through the capillaries 



