20 



HABIT AND INTELLIGENCE. 



[chap. 



Ganglia. 



Eeflex 

 action. 



Nervous 

 function 

 differenti- 

 ated from 

 muscular 

 function. 



Sensation 

 does not 

 exist at 

 first. 



for the purpose of transmitting stimuli directly from the 

 skin, or any other part which is more exposed to stimuli 

 than the rest, to the muscles that have to make the 

 response to the stimuh. In the simplest nervous system 

 that appears to be possible under the laws of life, there 

 must be at least two fibres, meeting in a ganglion, and 

 acting one on the other through it. One of the fibres 

 conducts the stimulus from the skin, or wherever its 

 outer extremity is situated, to its inner extremity at the 

 ganglion. The other fibre conducts the stimulus from the 

 ganglion to the muscle in which it terminates, and causes 

 the muscle to contract. This is what is called "reflex 

 action ;" the stimulus being, as it were, reflected from the 

 ganglion. 



As already mentioned, nervous structure is a develop- 

 ment and outgrowth of muscular structure, being deve- 

 loped by differentiation from it. This is observed, both in 

 watching the successive stages of the development of the 

 highest animals, and in comparing the various members of 

 the animal kingdom, from the simplest to the most complex 

 development of a nervous system. And it will be obvious 

 from what has been stated above, that nervous function 

 also is a development and outgrowth of muscular function, 

 being developed by differentiation from it. In the 

 Hydrozoa, there are no separate organs for the purpose of 

 transmitting stimuli, nor any organs at all which transmit 

 them, except the muscles themselves. But when a 

 nervous system is developed, the function of transmitting 

 stimuli is separated from the ordinary muscular function, 

 and assigned to the nerves. 



In the simplest and lowest development of a nervous 

 system, the action of the nerv^e-fibres on their gangUa is 

 probably unattended by any sensation ; and this continues 

 to be true of large parts of the nervous systems even of 

 man and the higher animals. Higher up in the animal 

 scale, sensation appears : the action of some — not all — of 

 the nerve-fibres on their ganglia produces sensation. We 

 cannot tell where it begins. I think it most likely, that 

 sensation begins where organs of special sense come into 



