STRUCTURE OF THE NERVOUS SYSTEM 



729 



divide, then join, and again subdivide in such a complex manner that the individual 

 funiculi become interlaced most intricately; so that each branch leaving a plexus 

 may contain filaments from all the primary nervous trunks which form the plexus. 

 In the formation also of smaller plexuses at the periphery of the body there is a 

 free interchange of the funiculi and primitive fibers. In each case, however, the 

 individual fibers remain separate and distinct. 



It is probable that through this interchange of fibers, every branch passing off 

 from a plexus has a more extensive connection with the spinal cord than if it had 

 proceeded to its distribution without forming connections with other nerves. 

 Consequently the parts supplied by these nerves have more extended relations 

 with the nervous centers ; by this means, also, groups of muscles may be associated 

 for combined action. 



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Epineurium 



Perineurium 





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Fio. 636. Transverse section of human tibial nerve. 



The sympathetic nerves are constructed in the same manner as the cerebrospinal 

 nerves, but consist mainly of non-medullated fibers, collected in funiculi and enclosed 

 in sheaths of connective tissue. There is, however, in these nerves a certain admix- 

 ture of medullated fibers. The number of the latter varies in different nerves, and 

 may be estimated by the color of the nerve. Those branches of the sympathetic, 

 which present a well-marked gray color, are composed chiefly of non-medullated 

 nerve fibers, intermixed with a few medullated fibers; while those of a white color 

 contain many of the latter fibers, and few of the former. 



The cerebrospinal and sympathetic nerve fibers convey various impressions. 

 The sensory nerves, called also centripetal or afferent nerves, transmit to the nervous 

 centers impressions made upon the peripheral extremities of the nerves, and in this 

 way the mind, through the medium of the brain, becomes conscious of external 

 objects. The centrifugal or efferent nerves transmit impressions from the nervous 

 centers to the parts to which the nerves are distributed, these impressions either 

 exciting muscular contraction or influencing the processes of nutrition, growth, 

 and secretion. 



Origins and Terminations of Nerves. By the expression "the terminations of 

 nerve fibers" is signified their connections with the nerve centers and with the parts 



