HISTOLOGY OF HIGHEE ANIMALS 



59 



local aggregations, or ganglia, in various parts of the body. 

 The nerve-fibres extend outwards from the central nervous 

 system in long, slender bundles, the nerves, which are distributed 

 to the various organs. 



The body of a nerve-cell contains the nucleus and is usually 

 much branched into slender processes or dendrons (Fig. 24), 

 which are quite distinct from the nerve-fibre and are supposed 

 to afford the means of transmitting impulses between one nerve- 

 cell and another, with the dendrons of which they interlace. 



FIG. 23. -Nervous Tissue, as seen in a thin Section of the Brain (Medulla 

 oblongata) of the Monk Fish. Two large nucleated Nerve Cells are 

 shown imbedded in a Mass of smaller Cells and Fibres, X 168. (From a 

 photograph.) 



Like other higher specialized tissue-cells of the animal body the 

 neurons have lost the power of multiplication by division. More- 

 over there appears to be no provision, in some adult vertebrates, 

 for their renewal when worn out or injured. A certain number 

 are formed in the course of the development of the embryo and 

 these have to serve the animal for the whole of its life. 



All the different kinds of cells met with in the body, a few of 

 which have been thus briefly described, are derived from the 

 apparently simple unicellular ovum by repeated subdivision and 

 gradual differentiation. The functions which in an Amceba are 

 all performed by a single protoplasmic unit are in one of the 



