168 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



from fibers that arise 



S^'^Cenical 



ythUorsal 



^'^Lunbar 



i&Lumla 



Fig. 77. — Diagrams to 

 snow course of upward de- 

 generation of fibers of poste- 

 rior funiculi after section of 

 a number of posterior roots 

 of the nerves forming the 

 lumbosacral plexus. — (Mott.) 

 It will be noted that in the 

 cervical regions the degener- 

 ated area is confined to the 

 fasciculus gracilis. 



from tract cells in the gray matter of the 

 cord itself. It is convenient to speak 

 of the former group as exogenous fibers, 

 using this term to designate nerve fibers 

 which arise from cells placed outside 

 the cord; and the latter group as endo- 

 genous fibers — that is, fibers that have 

 their cells of origin in the gray matter of 

 the cord. If we omit a consideration 

 of their collaterals the course of the 

 exogenous fibers is easily understood^ 

 They come into the cord at every pos- 

 terior root, enter into the fasciculus 

 cuneatus, and pass upward. The fibers 

 of this kind that enter at the lower 

 regions, sacral and lumbar, are, however, 

 gradually pushed toward the median 

 line by the exogenous fibers entering at 

 higher levels, so that in the upper tho- 

 racic or cervical regions the fasciculus 

 gracilis is composed mainly of exogenous 

 fibers that have entered the cord in the 

 lumbar or sacral region. These fibers 

 continue upward to end in two groups 

 of cells that lie on the dorsal side of the 

 medulla oblongata, and are known, 

 respectively, as the nucleus of the 

 fasciculus gracilis (or nucleus of GoU) 

 and the nucleus of the fasciculus cunea- 

 tus (or nucleus of Burdach). Their 

 path forward from the medulla is con- 

 tinued by new neurons arising in these 

 nuclei, and will be described latero The 

 course of these fibers in the cord may be 

 shown beautifully by the method of 

 secondary degeneration. If one or more 

 of the posterior roots of the lumbar 

 spinal nerves are cut or, better still, if 

 the posterior funiculi are severed in this 

 region, the degeneration will affect the 

 exogenous fibers throughout their 

 course to the medulla, and it will be seen 

 that in the cervical region the degen- 

 erated fibers are grouped in tlie area of 

 the fasciculus gracilis (see Fig, 77). The 



