176 MAN, THE ANIMAL 



There Is no question of fact about whether 

 stimuli actually do pass from one structure to 

 another, the question is how do they pass. Until 

 more is known, science can only describe as clearly 

 as possible the relations existing between such 

 structures. These fall Into three classes: (a) In 

 a general way one may say that most receptors are 

 relatively simple cells that have minute dendritic 

 branches in contact with them. The best tech- 

 nique has failed to reveal a continuous structural 

 union between such cells and the nerve fibers that 

 enclose them. They are in contact only. No 

 one has explained how a sound stimulus Is able to 

 pass from the sound receptor on to the dendrites 

 of the auditory nerve, (b) A similar difficulty is 

 met after the stimulus has begun to travel toward 

 the brain, after it has been passed by the receptor 

 on to the dendrite and In all of those cases where 

 dendritic branches act as receptors a similar con- 

 dition exists. There are no single neurones that 

 extend from a receptor to the cerebrum. The 

 message has to be carried forward by a second 

 neuron, a third, a fourth, etc. Each of these suc- 

 cessive neurones is merely In contact, the end 

 branches of one touching the receiving branches of 

 the neuron next in the route. There are numer- 

 ous breaks in the conducting fiber tracts from skin 

 to brain that are easily recognized by the student 

 of structural neurology, (c) In all cases of 



