162 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



endogenous fibers that 



8 tk Cerulcal 



2nd 



Fig. 72. Diagrams to 

 show course of upward de- 

 generation of fibers of poste- 

 rior columns after section of 

 a number of posterior roots 

 of the nerves forming the 

 lumbosacral plexus. (Afott.) 

 It will be noted that in the 

 cervical regions the degener- 

 ated area is confined to the 

 column of Goll. 



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 posterior root, enter into the col- 

 umn of Burdach, 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 thoracic or 

 cervical regions the columns of Goll are 

 composed mainly of exogenous fibers that 

 have entered the cord in the lumbar or 

 sacral region. These fibers continue up- 

 ward 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 Goll) and the nucleus of the 

 fasciculus cuneatus (or nucleus of Bur- 

 dach). Their path forward from the 

 medulla is continued by new neurons 

 arising in these nuclei, and will be de- 

 scribed later. 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 columns are 

 severed in this region, the degeneration 

 will affect the exogenous fibers through- 

 out their course to the medulla, and it 

 will be seen that in the cervical region the 

 degenerated fibers are grouped in the 

 area of the column of Goll (see Fig. 72). 

 The endogenous fibers, so far as they are 

 ascending, represent afferent paths in 

 which two or more neurons are con- 

 cerned. The posterior root fibers con- 

 cerned in these paths end in the gray 

 matter of the cord and thence the con- 

 duction is continued by one or more 

 tract cells. The conduction by this set 



