NERVE-CENTRES. 



[CH. XVII. 



accepted ; the dendrons may be nutritive, but it is believed 

 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 inde- 

 pendent of every other nerve-unit. There is no anastomosis of 

 the branches from one nerve-cell with those of another ; the 



Fig. 202. Cerebral cortex of mammal, prepared by Golgi's method. A, B, c, D, r, nerve- 

 cells ; E, neuroglia-cell. (Ramon y Cajal.) 



arborisations interlace and intermingle, and nerve impulses are 

 transmitted from one nerve-unit to another, but not by continuous 

 structures. The impulses are transmitted through contiguous, but 

 not through continuous structures. A convenient expression for 

 the intermingling of arborisations is synapse (literally, a clasping). 

 The figure on page 198 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 nerve-cell, but by 

 arborising around a nerve-cell and its dendrons. The only 



