470 T IIE XERVOUS SYSTEM. 



Bundles, or fasciculi of fibres, run together in the nerves, 

 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 undivided 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 different fashions 

 to be immediately described. The white or medullated 

 nerve-fibres (fig. 1 3 3), moreover, lose their medullary sheath 

 or white substance of Schwann before their final distribu- 

 tion, and acquire the characters more or less of the pale or 

 grey fibres (fig. 134). 



At certain parts of their course, nerves form 2^ exiises > in 

 which they anastomose with each other, and interchange 

 fasciculi, 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 with the spinal cord than it would have if it 

 proceeded to its destination without such communication 

 with other nerves. Thus, each nerve by the wideness of 

 its connections, is less dependent on the integrity of any 

 single portion, whether of nerve-centre or of nerve-trunk, 

 from which it may spring. By this means, also, each part 

 supplied from a plexus has wider relations with the 

 nerve-centres, and more extensive sympathies; and, by 

 means of the same arrangement, as Dr. Gull suggests, 

 groups of muscles may be associated for combined actions ; 

 every member of the group receiving motor filaments from 

 the same parts of the nerve-centre. 



The terminations of nerve-fibres are their modes 

 of distribution and connection in the nerve-centres, 

 and in the parts which they supply : the former are^ 

 called their central, the latter their peripheral termina- 

 tions. 



