STRUCTURE OF NERVE CELLS. 



599 



amount of true conducting matter within tlie nerves, and there may besides 

 be a rapid process of reparation going on at the same time. There is, indeed, 

 insufficient basis upon which we can at present found a probable theory as to 

 the mode of origination and conduction of nervous impulses, or as to the exact 

 nature of these impulses. 



Another point of peculiarity displayed by the protoplasm of many 

 nerve cells, is the existence within it of peculiar angular granules having 

 a special affinity for basic aniline dyes, such as methylene-blue and 

 toluidin-blue. The demonstration of these granules and the evidence 

 of their importance in the physiology of the nerve cell we owe to Nissl, 1 

 and they are usually called after him the " Nissl bodies " : they are also 

 known as " chromatophil " or " tigroid " granules. They occur scattered 

 in variable amount in all parts of the cell body and in its dendrons, but 

 they are absent from the axis 

 cylinder process and also from the 

 part of the cell body to which this 

 process is immediately attached. 

 Their disposition within the cell 

 body varies. In some cells they 

 are chiefly disposed in the neigh- 

 bourhood of the nucleus, whereas 

 in others they are uniformly dis- 

 tributed or are more numerous at 

 the periphery, but whether this 

 implies differences in function or 

 in the functional condition of the 

 cells, is uncertain. 



The most important physio- 

 logical fact regarding these gran- 

 ules is that, under certain condi- 

 tions of either normal or abnor- 

 mal activity of the nerve cell, 

 they undergo a change which has 

 been termed by Marinesco chro- 

 matolysis, and which appears to 

 consist in a gradual disintegra- 

 tion of the substance of the granules, and its diffusion in the juices 

 of the protoplasm : this change is at first accompanied by a swelling 

 of the cell, together with a displacement of the nucleus to one side ; 

 subsequently the cell may diminish in size, with disappearance com- 

 plete or partial of the chromatophil substance. 



The structural changes in the nerve cell which result from its 

 excitation, whether this be direct excitation of a ganglion, or indirect 

 by means of a nerve passing to it, have been studied by Hodge, 2 Vas, 3 

 Mann, 4 and others. Hodge has also studied the changes which result 

 from natural excitation of nerve cells in the motor ganglia of insects, 

 and in the cerebellum, cerebrum, and spinal cord of birds, both before 

 and after a period of prolonged activity ; and observations of a similar 



1 Allg. Ztschr. f. Psychiat., etc., Berlin, 1892, 13d. xlviii. S. 197; see also Neurol. 

 Ccntralbl., Leipzig, 1894, Bd. xiii. S. 676. 



2 Am,. Journ. Psychol., 1887 to 1890 ; Journ. Morphol., Boston, 1892, vol. vii. p. 95 ; 

 1894, vol. ix. p. 449 ; Journ. Physiol., Caml iridge and London, 1894, vol. xvii. p. 129. 



3 Arch. f. mikr. Anat., Bonn, 1892, Bd. xl. !S. 375. 



4 Journ. Anat. and Physiol., London, 1894, vol. xxix. p. 100. 



Fig. 305 



Diagram of a nerve cell to show the 

 Nissl granules. — a, axis cylinder process ; 

 b, Nissl bodies ; c, achromatic substance 

 between them ; d, nucleus ; c, dendron, with 

 Nissl body at division. — Kamon y Cajal. 



