1917] The Nervous System of Thysanura 313 



places groups are closely correlated because of this relation to 

 each other. 



Nuclei are distinct from the fibers and fibrils; they are 

 nutritive centers. The cell bodies also are important in 

 metabolism, but they are not important enough centers for the 

 mingling of many fibers. The individual fibers from cells are 

 less important in the relationship of parts than the groups and 

 masses of fibers and fibrils from many cells in conveying 

 impulses. Association of fibrils seems more important outside 

 of cells than in them. Why should not lateral as well as terminal 

 contacts be important in conveying impulses? The fibrils are 

 carried out in fibers, but the fibrils break away and are dis- 

 tributed in complex ways. It seems that an impulse may flow 

 through ganglia like floods of water through a swamp. The 

 impulses follow the lines of least resistance, if the bundle is 

 large the direction is more definite, if small, of less importance. 

 The nervous system of invertebrates might be compared to the 

 heart and circulation of insects; it, like the heart, receives and 

 passes on, but the distribution is not definite until there is a 

 more perfect insulation. Insulation may be accomplished 

 in several ways: (1) Bundles of -fibers protect the central 

 strands with a similar destination from loss to the surrounding 

 parts; (2) The fibers in some cases remain distinct from each 

 other, or the fibers are large and the inner fibrils are protected ; 

 (3) Neuroglia cells and neuroglia nets may help a little; (4) In 

 vertebrates the more perfect insulation by means of myalin 

 seems the most efficient protection. 



SPECIAL REFERENCES. 



Bottger, O. 1910. Das Gehirn eines niederen Insektes (Lepisma saccharina L.) 



Jen. Zeit. f. Naturw. Bd. XLVI. 

 Grassi, B. 1885. I progenitori degli Insetti e Miriapodi. L'Japax e la Campodea. 



Dagli Atti dell' Accadema Gioenia di Scienze Naturali in Catania. Ser. 3, 



Vol. XIX. 

 . 1885. Contribuzione alio studio dell Anatomia del genere Maehilis. 



Lett, all ac. nella tornata. Mem. III. 



1888. Anatomia comparata dei Thisanuri. Memoria VII. Reals ac. dei 



lincei. Roma. 



Oudemans, J. T. 1887. Bijdrage tot de kennis der Thysanura en Collembola. 

 Amsterdam. 



