198 



NERVE-CENTRES 



[CH. XVII. 



called protoplasmic processes, are now termed dcndrons. One branch 

 becomes the long axis cylinder of a nerve-fibre, but it also ultimately 

 terminates in an arborisation ; it is called the axis cylinder process, 

 or, more briefly, the axon. The term neuron or neurone is applied to 

 the complete nerve-unit, that is, the body of the cell, and all its 

 branches. Some observers have supposed that the axis cylinder pro- 

 cess is the only one that conducts nerve impulses, the dendrons 

 being rootlets which suck up nutriment for the nerve-cell. This 

 view has not, however, been accepted ; the dendrons may be nutri- 

 tive, but there is no doubt that they also, like the rest of the nerve- 

 unit, are concerned in the conduction of nerve impulses. A strong 

 piece of evidence in this direction is the fact that the fibrils of the 



axis cylinder may be traced 

 through the body of the 

 cell into the dendrons. 



The next idea which it 

 is necessary to grasp is, that 

 each nerve-unit (cell plus 

 branches of both kinds) is 

 anatomically independent 

 of every other nerve-unit. 

 There is no true anasto- 

 mosis of the branches from 

 one nerve-cell with those of 

 another; the arborisations 

 interlace and intermingle, 

 and nerve impulses are 

 transmitted from one nerve- 

 unit to another, through 

 contiguous, but not through 

 continuous structures. A 

 convenient expression for the intermingling of arborisations is 

 synapse (literally, a clasping). 



Fig. 212 is a diagram of the nervous path in a reflex action. 

 Excitation occurs at S, the skin or other sensory surface, and the 

 impulse is transmitted by the sensory nerve-fibre to the nerve- 

 centre, where it ends not in a cell-body, but by arborising around 

 one or more cell-bodies and their dendrons. The only cell-body in 

 actual continuity with the sensory nerve-fibre is the one in the spinal 

 ganglion (G) from which it grew. 



The terminal arborisation of the sensory nerve-fibre merely inter- 

 laces with the dendrons of the motor nerve-cell ; yet simply by this 

 synapse, the motor nerve-cell (M C) is affected and sends an impulse 

 by its axis cylinder process to the muscle (M). 



A very rough illustration which may help one in realising this 



FIG. 212. Reflex action. 



