388 



SCIENCE. 



[N. S. Vol. XII. No. 298. 



body of the nerve cell is the physiological 

 center of the unit. 



The cell plasm occupies both the body of 

 the nerve cell and its processes. The inti- 

 mate structure of the plasm has, by im- 

 proved methods of observation introduced 

 during the last eight years by Nissl, and 

 conducted on similar lines by other investi- 

 gators, become more definitely understood. 

 It has been ascertained that it possesses two 

 distinct characters which imply difi"erent 

 structures. One of these stains deeply on 

 the addition of certain dyes, and is named 

 chromophile or chromatic substance ; the 

 other, which does not possess a similar 

 property, is the achromatic network. The 

 chromophile is found in the cell body and 

 the dendritic processes, but not in the axon. 

 It occurs in the foi-m of granular particles, 

 which may be scattered throughout the 

 plasm, or aggregated into little heaps which 

 are elongated or fusiform in shape and ap- 

 pear as distinct colored particles or masses. 

 The achromatic network is found in the 

 cell body and the dendrites, and is con- 

 tinued also into the axon, where it forms 

 the axial cylinder of the nerve fiber. It 

 consists apparently of delicate threads or 

 fibrillse, in the meshes of which a homo- 

 geneous material, such as is found in cell 

 plasm generally, is contained. In the nerve 

 cells, as in other cells, the plasm is without 

 doubt concerned in the process of cell nu- 

 trition. The achromatic fibrillse exercise 

 an important influence on the axon or 

 nerve fiber with which they are continuous, 

 and probably they conduct the nerve im- 

 pulses which manifest themselves in the 

 form of nerve energy. The dendritic proc- 

 esses of a multipolar nerve cell ramify in 

 clpse relation with similar processes branch- 

 ing from other cells in the same group. 

 The collaterals and the free end of the axon 

 fiber process branch and ramify in associ- 

 ation with the body of a nerve cell or of its 

 dendrites. We cannot say that these parts 



are directly continuous with each other to 

 form an intercellular network, but they are 

 apparently in apposition, and through con- 

 tact exercise influence one on the other in 

 the transmission, of nerve impulses. 



There is evidence to show that in the 

 nerve cell the nucleus, as well as the cell 

 plasm, is an effective agent in nutrition. 

 When the cell is functionally active, both 

 the cell body and the nucleus increase in 

 size (Vas, G-. Mann, Lugaro) ; on the other 

 hand, when nerve cells are fatigued through 

 excessive use, the nucleus decreases in size 

 and shrivels ; the cell plasm also shrinks, 

 and its colored or chromophile constituent 

 becomes diminished in quantity, as if it 

 had been consumed during the prolonged 

 use of the cell (Hodge, Mann, Lugaro). It 

 is interesting also to note that in hibernat- 

 ing animals in the winter season, when their 

 functional activity is reduced to a mini- 

 mum, the chromophile in the plasm of the 

 nerve cells is much smaller in amount than 

 when the animal is leading an active life in 

 the spring and summer (G. Levi). 



When a nerve cell has attained its normal 

 size it does not seem to be capable of repro- 

 ducing new cells in its substance by a proc- 

 ess of karyokinesis, such as takes place 

 when young cells arise in the egg and in the 

 tissues generally. It would appear that 

 nerve cells are so highly specialized in their 

 association with the evolution of nerve en- 

 ergy, that they have ceased to have the 

 power of reproducing their kind, and the 

 metabolic changes both in cell plasm and 

 nucleus are needed to enable them to dis- 

 charge their very peculiar function. Hence 

 it follows that when a portion of the brain 

 or other nerve-center is destroyed, the in- 

 jury is not repaired by the production of 

 fresh specimens of their characteristic cells, 

 as would be the case in injuries to bones 

 and tendons. 



In our endeavors to differentiate the func- 

 tion of the nucleus from that of the cell 



