ARRANGEMENT OF THE NERVOUS SYSTEM 



569 



subject to structural variations in consequence of changes in the bodily 

 activities. In old age, certain retrogressive alterations appear which 

 present themselves in the main as a reversal of the processes observed 

 during the growth of the cell. The cytoplasm decreases in volume, 

 the nucleus becomes smaller, the pigment increases and the different 

 processes decrease in number and mass. In fact, in some cases vacu- 

 oles develop which finally lead to the complete disappearance of the 

 cell. 



A most interesting picture is presented by nerve cells which have 

 been fatigued. Hodge, 1 Mann 2 and Lugaro 3 state that a normal 

 neuron, when stimulated, first increases in size, because its metabolism 

 is augmented thereby. Excessive activity, however, diminishes the 

 amount of its cytoplasm as well as that of its nucleus until the chro- 



A. 



FIG. 283. SPINAL GANGLION CELLS FROM ENGLISH SPARROWS, TO SHOW THE DAILY 

 VARIATION IN THE APPEARANCE OF THE CELLS CAUSED BY NORMAL ACTIVITY. 

 A, Appearance of cells at the end of an active day; B, appearance of cells in the morn- 

 ing after a night's rest. The cytoplasm is filled with clear, lenticular masses, which are 

 much more evident in the rested cells than in those fatigued. (Hodge.) 



matic substance has been used up in its entirety. The Nissl's 

 granules gradually lose their conspicuousness and finally disappear 

 altogether. If long continued, the exhaustion of the reserve supply 

 of energy-yielding material manifests itself in a vacuolization of the 

 cytoplasm and a degree of disintegration from which the cell cannot 

 recover. But if the fatigue is not carried beyond a certain normal 

 limit, the chromophil substance is replenished in time. Very similar 

 changes have been observed in the ganglion cells of birds after long 

 continued flight, for example, in the anterior horn cells of the sparrow 

 and in the antennary lobes of bees at the end of an active day. These 



1 Jour, of Morphology, vii, 1892. 95. 



2 Jour, of Anat. and Physiol., xxix, 1894. 100. 

 3 Lo sperim. giornale medico. Biol., F2, 1895. 



