188 



NERVE-CENTRES 



[CH. XVI. 



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

 branches. The fibrils of the axon may be traced through the body 

 of the cell from 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 anastomosis 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 con- 

 tinuous structures. A convenient expression for the intermingling 

 of arborisations is synapse (literally, a clasping). 



Fig. 184 is a diagram of the nervous path in a spinal reflex action. 

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



A.C.C 



FIG. 184. Reflex action. 



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

 nervous system. It does not become anatomically connected to any 

 of the cells of the central nervous system. The only cell-body in 

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

 ganglion (G) from which it grew. On entering the spinal cord, the 

 main fibre conveys impulses upwards which ultimately reach the 

 brain, but in the spinal cord it gives off fine side branches or 

 collaterals which terminate in branches that arborise around one or 

 more cell bodies and their dendrons; these cells are small ones 

 situated in the posterior cornu of the spinal grey matter ; one only 

 (P.C.C.) is shown in the diagram. The short axon of this cell similarly 

 terminates by a synaptic junction with one or more of the large 



