NERVE ELEMENTS AND THEIR FUNCTIONS. 91 



central end of the nerve is necessary; without it the strand formed 

 from sheath cells degenerates. The conditions of the experiments 

 of Bethe are not such as to show what takes place in the course of 

 normal regeneration. It is not shown that the strands derived 

 from sheath cells form part of the nerve when the proximal stump 

 is undisturbed. The later researches of Cajal are opposed to this 

 "autogenetic regeneration" of Bethe and give positive evidence 

 that the nerve is regenerated by outgrowth of the proximal stumps 

 of the cut fibers. 



For the continued performance of its normal functions all 

 parts of the neurone are necessary. The fact that a neurone may 

 continue to function for some days after the cell-body containing 

 the nucleus has been cut away shows that impulses follow the short- 

 est path from dendrites to neurite. The further fact that the 

 dendrites and neurite die after a few days proves the trophic unity 

 of the neurone. The Wallerian degeneration is evidence of the 

 same. It appears also that injury to one of the processes of the 

 neurone may start destructive changes which the neurone is unable 

 to combat and the whole neurone may degenerate as the result 

 of the cutting of its neurite (Gudden's degeneration). It is quite 

 unnecessary to think that all parts of the neurone must enter at 

 once into the chemical and physical changes which constitute 

 any single act of the neurone. The notion that the cell-body 

 acts as a ganglionic center with reference to its dendrites and 

 neurite is no part of the neurone theory. Facts opposed to such 

 a conception were known long before that theory was formulated, 

 namely the unipolar form of spinal ganglion cells and the origin 

 of neurites from the dendrites of many cells in the brain. 



The neurone theory as an expression of our general ideas 

 regarding the structure and functioning of the nervous system 

 may be tentatively re-stated as follows. 



1. The nervous system is formed of cells each of which is 

 derived from a single cell in the course of development. From 

 each cell grow out one or more processes which are comparable 

 in a general way to the pseudopodia of unicellular animals. Such 

 cells may be called neurones. 



2. Although in their primitive condition, as commonly in 



