822 THE NER VE SYSTEM 



The Neurone Doctrine. The results of the investigations of Golgi, Cajal, Forel r 

 and others prompted Waldeyer to enunciate a theory with regard to the nerve 

 mechanism of the neurone. This hypothesis is generally known as the neurone 

 theory and assumes that (1) each neurone is a distinct and separate entity; (2) 

 the collaterals and other terminals of the neurone form no connections among 

 themselves; (3) neurones are associated, and impulses conveyed, by contact 

 or contiguity of the axonic terminals of one axone with the cell body or dendrites of 

 another neurone. The theory postulates a nerve cell amebism analogous to 

 the extension and retraction of the pseudopodia of an ameba, and the "retraction 

 theory" has been propounded in explanation of certain functional dissociation 

 phenomena attending nerve force manifestations. 



Organ of Golgi, showing Tendon bandies, 



ramification of nerve fibrils. 

 Muscular fibres. 



FiQ. 593. Neurotendinous spindle organ of Golgi from the human tendo calcaneus (Achillis). (After Ciaccio.) 



Opposed to the "neurone theory" or "contact theory" is the more recent 

 continuity theory which is being earnestly advocated by Apathy, Bethe, and 

 Nissl. In behalf of this theory it is claimed that the neurofibrils are continuous 

 not only within the cell and its processes, but through an extracellular network 

 as well. The dispute now being waged does not, however, affect our funda- 

 mental ideas regarding the individuality of neurones with regard to their dynamic 

 condition. 



The Supporting Tissue Elements of the Nerve System. A fine meshwork of non-neural 

 tissue, more or less dense in different localities, but apparently restricted to the central axis, 

 serves to support the neurones. This sustentacular tissue is of two kinds: (1) the neuroglia; 

 (2) connective-tissue trabeculae derived from (a) the pia or (b) vascular channels. 



The Neuroglia. The neuroglia consists of glia cells of varied forms and glia fibres. Glia 

 cells are divisible into two species ependymal cells and astrocytes of long-rayed and short- 

 rayed type. 



Ependymal cells are the columnar epithelial cells which line the neural canal throughout. 

 In the embryonic condition each cell is seen to project a long multibranched filament toward 

 the periphery of the neural tube, while the free end carries a tuft of cilia. In adult life both the 

 cilia and the radial filament are apparently lost or very much reduced. 



Regarding the structure of the glia cells proper as well as of the glia fibres there is a variance 

 of opinion among different investigators. The astrocytes, as they are commonly revealed in 

 Golgi preparations, may, as pointed out by Weigert and others, be due to an extension of the 

 silver deposit upon glia-cell nuclei as well as upon adjacent filaments. Huber, 1 Hardesty, 2 and 

 others regard neuroglia tissue as a syncitium resulting from an early fusion of the protoplasm 

 of the cells of the neural tube which at first were individual and definitely bounded. The fila- 

 mentous reticulum of glia fibres ordinarily seen in adult tissues seems to result from an increase 

 of the fine threads of the spongioplasmic network of the original cell protoplasm. Neuroglia 

 occurs in both gray and white substances as an all-pervading supporting tissue. In certain 

 localities, as upon the surface of the brain and cord, the neuroglia tissue is disposed in the form 

 of a thin layer. 



1 American Journal of Anatomy, 1901, "pp. 45 to 61. 



2 Ibid., 1904, pp. 229 to 268. 



