AS TO THE STRUCTURE AND FUNCTIONS OF NERVE 317 



manner, we know that the gangha are centres whence that excitement 

 originates. We are therefore justified, analogically, in seeking for the 

 sources of the contractions of the cardiac muscles, in the cardiac 

 ganglia ; and the experiments which have been detailed — by showing 

 that the rhythmical contractions continue in any part of the heart 

 which remains connected with these ganglia, while it ceases in any 

 part cut off from them — prove that they really are the seats of the 

 regulative power. 



The speaker then exhibited another very remarkable e.xperiment 

 (first devised by Weber) which leads indirectly to the same conclu- 

 sion. An electro-magnetic apparatus was so connected with the frog 

 upon the table, that a series of shocks could be transmitted through 

 the pneumogastric nerves. When this was done, it was seen that the 

 index almost instantly stopped, and remained still, so long as the 

 shocks were continued ; on breaking contact, the heart remained at 

 rest for a little time, then gave a feeble pulsation or two, and then 

 resumed its full action. This experiment could be repeated at will, 

 with invariably the same results ; and it was most important to 

 observe, that during the stoppage of the heart the index remained at 

 the lowest point of its arc, a circumstance which, taken together with 

 the distended state of the organ, showed that its stoppage was the 

 result, not of tetanic contraction but of complete relaxation. 



Filaments of the pneumogastric nerve can be traced down to the 

 heart, and whenever these fibres are irritated the rhythmical action 

 ceases. The pneumogastric nerves must act either directly upon the 

 muscles of the heart, or indirectly through the ganglia, into which 

 they can be traced. If the former alternati\'e be adopted, then we 

 must conceive the action of the pneumogastric nerve upon muscle to 

 be the reverse of that of all other nerves — for irritation of every other 

 muscular nerve causes activity and not paral}'sis of the muscle. Not 

 only is this in the highest degree improbable, but it can be demon- 

 strated to be untrue ; for on irritating, mechanically, the surface of the 

 heart brought to a standstill by irritation of the pneumogastrics, it at 

 once contracts. The paralysing influence therefore is not exerted on 

 the muscles, and as a consequence, we can only suppose that this 

 " negative innervation," as it might be convenientl}' termed, is the 

 result of the action of the pneumogastric on the ganglia. 



It results from all these experiments, firstly, that nerve substance 

 possesses the power of exciting and co-ordinating muscular actions ; 

 and secondly, that one portion of nervous matter is capable of con- 

 trolling the action of another portion. In the case of the heart it 

 is perfectly clear that consciousness and volition are entirely 



