OF NERVE CENTRES. 185 



These different classes of nerve fibres have been 

 represented in figures 1, 3, and 4, Plate II. It will 

 be found that the results of investigation into the 

 structure of terminal organs, the fact of the divisions 

 of the trunks of nerves as they pass towards nerve 

 centres or towards their peripheral distribution, as 

 well as the arrangement of nerves and nerve cells in 

 the nerve centres, strongly support the conclusion 

 that nerves do not in any case divide into single 

 fibres, each having a terminal extremity, or end by 

 becoming continuous with other tissues. Numerous 

 facts indicate that nerve fibres never end. 



Of Nerve Centres. 



Next, I must draw attention very briefly to the 

 minute structure of nerve centres, which invariably 

 contain a vast number of " nerve fibres " and " nerve 

 cells," often of large size. These cells, in some cases, 

 have a highly complex structure. The "nerve centre," 

 in fact, exhibits the essential structures characteristic 

 of the peripheral portion of the nervous system, except 

 that in the nerve centres of man and the higher 

 animals, the elementary parts or " cells " are, for the 

 most part, much larger than those found in peri- 

 pheral organs. In the nerve centre a great amount 

 of tissue is compressed into a comparatively small 

 space, so as to form a collection, knot or ganglion. It 

 has been shown that, in the lowest animals in which 

 nerves are to be demonstrated, there is not this dis- 

 tinction between the central and peripheral parts of 

 the nervous system. The nerve tissue seems almost 

 uniformly distributed. In the higher invertebrata 

 and in the vertebrata, however, it is probable that 

 the nerve tissue collected in the nerve centres exceeds 

 in amount that which is spread out amongst the 

 tissues in all other parts of the organism. 



24O. Central nerve cells. Each central nerve 

 cell consists of a mass of bioplasm surrounded by 



