THE NERVE ELEMENTS. 147 



with the advancing position of the animal in the zoo- 

 logical scale, as well as with the advancing development 

 of the individual cell. 



Although these relations are easily recognised, it is 

 not yet clear in how far all the branches of the neuron 

 are alike ; whether, for instance, each acquires the sheath 

 of myeline which characterises the medullated nerve 

 fibres, though it has been shown that at least some of 

 these branches do become thus medullated (Flechsig). 



The current conception of the nervous system de- 

 mands an arrangement for the following physiological 

 processes. In some way impulses must reach the cell- 



FlG. 22. Spinal ganglion of an embryo duck, X 30 diameters. 

 Composed of dineuric nerve cells. (Van Gehuchten.) 



bodies within the central system. These bodies are 

 masses of irritable cytoplasm, from which the discharge 

 of energy may be much greater than that represented 

 by the exciting stimulus. This discharge must be 

 carried off by some pathway other than that by which 

 the stimulus arrived. The simplest arrangement by 

 which this is accomplished is the one found in Fig. 22. 



This shows a spinal ganglion, containing typical 

 bipolar or dineuric cells. In the usual disposition of 

 such cells, one neuron extends to the periphery, and 

 there terminates in such a way as to be exposed to 

 some form of external stimulus. The stimulus acting 



