THE NERVE CELL 



125 



fibrils, Fig. 141) ( Schultze, Flemming, Apathy, Bethe) and fine acidophil 

 granules (neurosomes of Held; probably mitochondria). Besides these 

 structures there remains a homogeneous ground substance or hyaloplasm, 

 which, though of extreme physiological importance, in the usual histo- 

 logical preparations presents no structure. Centrosomes and attraction 

 spheres have been frequently observed in the nerve cells of the lower 

 vertebrates, and occasionally in those of 

 mammals. 



The cytoplasm of many nerve cells con- 

 tains a characteristic brownish-yellow pig- 

 ment, whose fine granules have a tendency 

 to accumulate in the vicinity of the nu- 

 cleus. 



Mitochondria also have been reported in 

 ganglion cells of the rabbit (Schirokogoroff, 

 Anat. Anz., 43, 19 and 20, 1913). Cowtlry 

 (Amer. Jour. Anat., 17, 1, 1914) describes 

 granular and rod-like mitochondria in the 

 spinal ganglion cells of a number of verte- 

 brates, including man. They are said to 

 occur throughout the entire neuron, axon 

 as well as dendrons. They are regarded as 

 fundamental constituents of the neuro- 

 plasm. It is suggested that they are con- 

 cerned with the metabolism of the neuro- 

 cyte. 



Neurons are incapable of division ; de- 

 stroyed neurons cannot be replaced; the axon, however, may regener- 

 ate. 



End fibrils of other neurons have been demonstrated within the cyto- 

 plasm of the nerve cell. Apathy has likewise demonstrated that fibrils 

 occasionally pass from one neuron to another, so that we no longer con- 

 sider that a neuron, though a structural unit, is in all cases anatomically 

 independent of all other neurons. The present status of this much dis- 

 cussed question seems to be comparable to that of the cell, as a histological 

 unit of structure, which though formerly thought to exist independently 

 of other cell units, has since been found to be frequently connected, as 

 by the intercellular bridges of epithelium and of smooth muscle. The 

 neurons of the nervous system therefore, while being usually related to 

 one another by contiguity or by contact only, may occasionally be more 



FIG. 138. VARIOUS TYPES OF 

 NERVE CELLS OF THE CERE- 

 BELLAR CORTEX. 



1, cell of Purkinje; the cyto- 

 plasm contains large flakes of 

 Nissl substance; 2 and 3, small- 

 er nerve cells, 'granule cells.' 

 Nissl's stain. X 1200. 



