THE NERVOUS SYSTEM. 



73 



Course. Every nerve-fibre in its course proceeds uninterruptedly 

 from its origin in a nerve-centre to near its destination, whether this be 

 the periphery of the body, another nervous centre, or the same centre 

 whence it issued. 



Bundles of fibres run together in the nerve-trunk, but merely lie in 

 apposition with each other; they do not unite: even when they anas- 

 tomose, there is no union of fibres, but only an interchange of fibres 

 between the anastomosing funiculi. 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 



FIG. 308. Small branch of a muscular nerve of the frog, near its termination, showing divisions 

 of the fibres, a, into two; 6, into three; x 350. (Kolliker.) 



subdivisions (Fig. 308) before their final ending. The medullated nerve- 

 fibres, moreover, lose their medullary sheath before their final distribution, 

 and acquire the characters more or less of non-medullated fibres. 



Plexuses. At certain parts of their course, nerves form plexuses, 

 in which they anastomose with each other, as in the case of the brachial 

 and lumbar plexuses. The objects of such interchange of fibres are, (a}, 

 to give to each nerve passing oif from the plexus, a wider connection with 

 the spinal cord than it would have if it proceeded to its destination with- 

 out 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. (b) Each part supplied from a plexus has wider relations with 

 the nerve-centres, and more extensive sympathies; and, by means of the 

 same arrangement, groups of muscles may be co-ordinated, every member 



