CENTRA L NER VO US S ) 'S TKM. 



177 



Maturing- of the Nerve-cell. — The maturing of the nerve-cell involves 

 several changes. First, the outgrowth of the axone or axones; next, the 

 formation of the dendrites ; and, finally, in some cases, the medullatiou of 

 the axone, while simultaneously and with greater or less rapidity the absolute 

 amount of substance in both cell-body and axone is being increased, together 

 with a chemical differentiation of the cytoplasm and the nucleus. The time 

 in the life-history of the individual at which these several events occur is 

 variable, and may be delayed beyond puberty at least, while the rate at which 

 they occur is diiferent in different cases. Furthermore, many nerve-cells 

 never develop beyond the first stage of immaturity (Fig. 70). 



Fig. 70.— A-D, showing the phylogenetic development "f mature nerve-cella in a series of vertebrates ; 

 a-e, the ontogenetic development of growing cells in a typical mammal; in both ra>.-v only pyramidal 

 cells from the cerebrum are shown : .1, frog ; /•', lizard ; ''. rut ; />, man ; o, neuroblast withoul dendrites ; 

 6, commencing dendrites ; r, dendrites further developed ; </. tirst appearance of collateral branches; >, 

 further development of collaterals ami dendrites (from S. Ram6n y Cajal). 



Form of the Axone as a Means of Classification. — Of the various de- 

 vices used to classify nerve-cells, the form of the axone is the most useful. 



Physiologically, the nerve-cell is significant as the source and pathway for 

 the nerve-impulses. The current conception of the change called the nerve- 

 impulse is that it begins at one point of the cell and travels from there to the 

 other parts ; one of the other parts is the axone. and along this the impulse 

 can be shown to pass. Indeed, the nerve-cell body stimulated at any point 

 may be responsive just an an amoeba is responsive at any portion of its sur- 

 face. When, however, the branches are formed they become the channel- 

 through which the impulses pass, and hence assume a special significance 

 Voi,. i r. — 12 



