THE NERVE CELL 



109 



C. KARYOCHROME NERVE CELLS, which also present but little 

 cytoplasm, but whose nucleus is large, being equal in size to that of 

 the average nerve cell. Such cells are also found in the granular 

 layer of the cerebellar cortex. 



Those nerve cells in which the Mssl substance is abundant are 

 said to be in a pyknomorphous, those in which it is scanty in an 

 apyknomorpJious condition. The Nissl granules are apparently 

 used up during functional activity of the nerve cell. 



The majority of the coarse granules of stainable substance of 

 Xissl present an irregular spindle shape ; this is particularly true 

 of those which show a linear ar- 

 rangement. They are sometimes so 

 grouped as to form two characteristic 

 structures, the nuclear caps and the 

 division wedges. Nuclear caps, as 

 their name indicates, are accumula- 

 tions of Nissl substance, which occur 

 at the poles of the nucleus ; one, two, 

 or three of these caps may be in re- 

 lation to a given nucleus. Division 

 wedges occur in the dendrites, at the 

 point of their division. The wedges 

 are of a triangular shape, their con- 

 cave base spanning the angle formed 

 by the two subdivisions of the den- 

 dritic process. 



Concerning the finer structure of 

 the unstainable substance of Nissl 

 comparatively little is known. With 

 varying methods of fixation this por- 

 tion of the cellular cytoplasm has 

 been found to show very fine fibrils 



(Schultze, Flemming, Apathy, Bethe) and fine acidophile granules 

 (neurosomes of Held). Besides these structures there remains a 

 homogeneous ground substance or hyaloplasm, which, though of 

 extreme physiological importance, in the usual histological prepa- 

 rations 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 contains a characteristic 

 brownish-yellow pigment, whose fine granules have a tendency to 



FIG. 107. NERVE CELLS OF THE 

 CEREBELLAE CORTEX. 



1, cell of Purkinje ; arkyochrome 

 type ; #, nerve cell of the granular 

 layer ; caryochrorae type with a 

 large nucleus ; #, a granule cell of 

 the granular layer ; cytochronie type 

 with a small nucleus equal in size to 

 that of a leucocyte. Nissl's stain, 

 x 1200. 



