THE RHYTHMIC AND PERISTALTIC MOVEMENTS 121 



the vagus system only. It does not follow that the maintenance 

 of rhythm in an isolated heart implies that this rhythm is de- 

 pendent entirely on its intrinsic nerve cells ; but rather that the 

 normal double tone control of the two systems can under altered 

 circumstances be carried on with greater or less efficiency by one 

 system only. 



To recapitulate : rhythmic activity is an intrinsic property 

 of involuntary muscle which becomes manifest when that muscle 

 is in a condition of tone ; this condition of tone is dependent upon 

 both suitable chemical conditions and proper nervous control. 

 The nervous control is normally dependent upon the integrity of 

 two antagonistic systems of peripheral neurons but can also be 

 maintained by one of these systems alone under certain con- 

 ditions. 



Bayliss and Starling seem to think that the combination of 

 a preceding inhibition with a peristaltic contraction necessarily 

 implies a true reflex action ; it seems to me possible to give 

 another explanation. The evidence shows that the inhibition is 

 not due to any excitation of the inhibitory fibres in the splanch- 

 nics ; but, as we know from many instances in the central nervous 

 system, inhibition may occur in two ways, either by excitation of 

 inhibitory nerves, or by inhibition of motor cells ; also according 

 to the whole of the evidence in this book the involuntary nervous 

 system is built up on the same plan as the voluntary nervous 

 system. Inhibition of motor cells in the former as well as ex- 

 citation of motor cells is brought about by the action of con- 

 nector nerves, as in the "well-known instance of the reciprocal 

 innervation of flexors and extensors either in the spinal animal 

 or on stimulation of the pyramidal tracts through the cortex. 

 So also in the case of the gut the same factors are present, 

 motor nerve cells arranged serially in the gut wall and connector 

 nerves the vagus nerves which by means of collaterals connect 

 with these serially arranged motor nerve cells. If then the ex- 

 citation of a connector nerve in the central nervous system can 

 simultaneously excite one motor neuron and inhibit another, the 

 excitation of a vagus nerve fibre in the gut may reasonably be 

 able to excite one motor neuron in the gut and simultaneously 

 inhibit another one. 



Bayliss and Starling point out that stimulation of the vagus 

 causes inhibition followed by increased movement, so that the 



