372 



THE NERVOUS SYSTEM. 



but merely lie in apposition with each other ; they do not unite ; 

 even when the fasciculi anastomose, there is no union of fibres, 

 but only an interchange of fibres between the anastomosing 

 fasciculi. Although each nerve-fibre is thus single and undi- 

 vided through nearly its whole course, yet as it approaches the 

 region in which it terminates, individual fibres break up into 

 several subdivisions (Fig. 135) before their final ending in the 



FIG. 135. 



Small branch of a muscular nerve of the frog, near its termination, showing divi- 

 sions of the fibres, a, into two ; b, into three ; magnified 350 diameters (from K61- 

 liker). 



different fashions to be immediately described. The white or 

 medullated nerve-fibres (Fig. 133), moreover, lose their medul- 

 lary sheath or white substance of Schwann before their final 

 distribution, and acquire the characters more or less of the 

 pale or gray fibres (Fig. 134). 



At certain parts of their course, nerves form plexuses, in 

 which they anastomose with each other, and interchange fas- 

 ciculi, as in the case of the brachial and lumbar plexuses. 

 The object of such interchange of fibres is, probably, to give 

 to each nerve passing off from the plexus, a wider connection 



