258 THE HUMAN BODY. 



cells from which gray fibres are given off to the rest of the 

 heart, mingled with the original gray fibres derived from the 

 sympathetic : in the ventricle and bulb only non-medullated 

 fibres are found. 



The Beat of the Frog's Heart. When both cardiac 

 nerves are cut in a frog the heart continues its regular rhyth- 

 mic beat, as it does also when carefully removed from the 

 body of the animal: this makes it clear that whatever initi- 

 ates the beat lies in the heart itself, which must therefore be 

 regarded as an automatic organ; but leaves it still uncertain 

 whether the exciting cause of each beat is to be sought in the 

 nervous elements of the heart or in the cardiac muscle itself. 

 Arguing from the analogy of ordinary striped muscle, which 

 is not automatic, one would be inclined to ascribe to the 

 nerve-cells of the isolated heart the origination of nervous 

 impulses for the myocardium, and certain experiments tend 

 to support this view; but cardiac muscle differs considerably 

 from the skeletal muscles in its histology, so it is unsafe to 

 argue from one to the other, and some experiments show that 

 we must ascribe to it, in addition to contractility, a certain 

 amount of automaticity and of conductivity and co-ordinating 

 power. In physiological properties it combines the character- 

 istic properties of fully differentiated nerve-cell and nerve- 

 fibre with those of muscle-fibre. 



/ Each beat of the heart of the frog can be seen to com- 

 mence where the great veins enter the venous sinus, and from 

 there to spread rapidly over the whole sinus; then there is a 

 brief check, and the atrium beats; then another check, fol- 

 lowed by the beat of the ventricle; finally, again after a very 

 short pause, comes the contraction of the arterial bulb : then 

 the series of phenomena is repeated in the same unvarying 

 order as long as. the heart is in good condition and is left to 

 itself. The fact that each cycle of contractions begins at 

 the mouths of the venae caves and the sinus, where nerve-cells 

 are very numerous, and passes on to the ventricle, where they 

 are few, and to the bulb, where there are none, has been taken 

 as an evidence of the origination of each beat through stimuli 

 developed in cardiac nerve-cells; and this opinion gains sup- 

 port from what is usually seen on an excised heart when it is 

 gradually dying. The bulb and ventricle cease to beat first, 

 then the auricles, last the sinus, and this although the ven- 

 tricle may still be contractile and able to give a good beat or 



