THE CENTRAL NERVOUS SYSTEM 



667 



loss of function of the axon reacts on the cell-body. When a neive- 

 fibre is divided from its cell characteristic changes are produced in 

 the latter and in its dendritic processes, and they are scarcely less 

 rapid, although usually not so far-reaching, as the degeneration in 

 the peripheral portion of the nerve-fibre. Many of the Nissl bodies 

 (Fig- 235) are reduced to a finely granular condition (chromatolysis). 

 Many of them disappear altogether. The nucleus may be displaced 

 to one side of the cell. In rabbits after division of the facial nerve 

 the alterations in its nucleus of origin have been found to reach a 

 maximum in about three weeks, after which there is a tendency to 

 recovery on the part of the majority of the cells, even when regenera- 



FIG. 



THROUGH HALF OF 

 NEURAL TUBE (BAR- 

 KER, AFTER HIS). 



The pear-shaped neuro- 

 blasts are seen migrating 

 outwards. The axons of 

 some of them are seen 

 pushing their way out 

 through the marginal veil 

 as the anterior root of a 

 spinal nerve. 



FIG. 234. 



i, spinal ganglion cells of a still-born male child ; 2, of 

 a man ninety-two years old ( X 250) N, nuclei ; 3, nerve- 

 cells from the antennary ganglion of a honey-bee just 

 emerged in the perfect form ; 4, of an old honey-bee. 

 The nucleus is black in the figure. In 3 it is very large, 

 in 4 it is shrunken, and the cell-substance contains 

 vacuoles. (Hodge.) 



tion of the nerve has been prevented by cutting out a portion of it. 

 Some of the cells may completely atrophy and disappear. Similar 

 changes have been found by Warrington in the motor cells of the 

 anterior horn after section of the posterior (dorsal) spinal roots. 

 Since in this case no anatomical injury has been inflicted on the 

 motor neurons, it has been surmised that the cause of the alterations 

 is the loss of impulses which normally reach them along their 

 dendrites. In short, we may say with Marinesco, that the functional 

 and anatomical integrity of the neuron depends on the integrity of all 

 its constituent parts, and of the neurons which carry to it functional 

 excitations, i.e., excitations connected with its proper physiological 



