GANGLION-CELL AND NEBVB. 



39 



(I, i). Note that the nerve-trimk (3 a) contains nerve-fibers which pass into 

 the ganglion and those which come from other ganglia of the same side or 

 the opposite side. How many possibilities of association are given in this 

 simple ganglion! 



Every cell and every fiber can, through the profuse terminal ramifica- 

 tion, come into relation with innumerable other cells and fibers. Besides 

 that, most cells stand in combination with tracts from distant centers and 

 also from the periphery. 



There are also cells — already demonstrated, at any rate, for all classes of 

 vertebrates — which stand in no direct relation to the outer world and are 

 adapted only for Joining one central cell more intimately with that of an- 



Fig. 10. — Isolated nerve-fibers from the spinal cord of a dog. ca, Axis- 

 cylinder, mg, Medullary sheath, g, Outer sheath, c, Nucleus and protoplasm, 

 to be seen occasionally on the surface of fibers. {After Ranvier.) 



other. These cells are called Association-cells. Such cells are very widely 

 disseminated. Nowhere does their significance become more readily clear 

 than in the cornu Ammonis: a portion of the olfactory cortex. Fig. 9 shows 

 a section through this portion of the cortex. 



Below the layer of large pyramidal cells, which make up the principal 

 cell-layer of this region, note the small cells which send their neuraxons 

 near to the pyramidal cells or even through the layer, after which it divides 

 into fine transverse branches, from which great terminal ramifications push 



