568 SIGNIFICANCE OF THE NERVOUS SYSTEM 



individual local cells which eventually coalesce to form the conducting 

 path. In accordance with the neuron concept, the different neuro- 

 blasts finally elongate and form their own axons. These changes may 

 be traced without difficulty in neurons which are made to grow out- 

 side the body in a medium of lymph. 



The histological evidence favors the neuron doctrine in a very 

 decisive manner. In the first place, it has. been proved that the 

 '^ neurofibrillar" network found in the immediate vicinity of ganglion 

 cells (Bethe), is not composed of fibrillse at all, but constitutes an intri- 

 cate system of lymphatic channels set aside for the nutrition of the 

 cell-body. In this connection, reference should also be made to the 

 fact that complexes of ganglion cells are always well supplied with 

 blood-vessels and lymphatics, while nerve fibers are not (Kollicker). 

 In addition, it should be mentioned that some ganglion cells are in 

 possession of an internal system of capillaries. A condition of this 

 kind exists in the cells of the medulla of Lophius picatorius. Further- 

 more, the cytoplasm of some nerve cells contains a hemoglobin-like 

 pigment, a fact which suggests an intense metabolism.^ 



The neuron doctrine also receives support from certain data 

 pertaining to the metabolism of the nerve cell. Thus, it has been 

 found by Langendorff^ that the gray matter readily assumes an acid 

 reaction upon activity and also becomes acid after death. In analogy 

 with the changes occurring in active muscle, it has been assumed that 

 this acidity is due to the production of lactic acid. It has also been 

 stated by Mosso^ that increased mental activity is associated with a 

 rise in the temperature of the brain. Reference should also be made 

 at this time to the fact that a nerve fiber atrophies when separated 

 from its cell-body, and that ganglion cells display very obvious histo- 

 logical changes during growth or when fatigued. Since a more de- 

 tailed account of these trophic changes will be given in a subsequent 

 paragraph, attention need only be called at this time to the fact that, 

 unlike the cell-bodies, the nerve fibers cannot be fatigued under ordi- 

 nary conditions and do not betray an intense metabolism. This fact 

 implies that the refractory period of the nerve fiber is shorter than 

 that of the nerve cell. Naturally, the only deduction to be derived 

 from these data is that the ganglion cells are the more active nervous 

 units and that they serve as the generator or supply house of nerve 

 impulses. 



Fatigue of Nerve Cells. — The development of the neuroblast into 

 its mature form manifests itself by a deposition of additional cellular 

 material, an increase in the number of its processes, an acquisition of 

 enveloping membranes and a formation of pigment granules within 

 ;the cytoplasm.* During their mature state, the neurons become 



1 Fritsch, Archiv fiir mikr. Anatomie, xxvii, 1886, 13; Holmgren, Anat. Hefte, 

 XV, 1899, and Pewsner-Neufeld, Anat. Anzeiger, xxiii, 1903. 



2 Zentralbl. der med. Wissensch., 1886. 



3 Die Temperatur des Gehirnes, 1894. 



* Vas, Arch, fur mikr. Anatomie, 1892. 



