THE ROOTS OF THE PERIPHERAL NERVES, ETC. 167 



The anterior commissure is composed of the central fibres 

 from the root-nuclei to the opposite anterior pyramidal tract, the 

 fibres from the roots to the corresponding cells of the opposite 

 side, and of fibres from the posterior horn. 



All these fibres are surrounded and enclosed by an extra- 

 ordinarily close tangle of fibres, the unraveling 1 of which lias 

 never been accomplished. This mass of fibres, without doubt, 

 contains other nerve-tracts of which we know nothing. That 

 portion of this net-work, which is medullary at birth, is correctly 

 depicted in Fig. 98. 



The investigation of the relations of the posterior roots 

 presents peculiar difficulties. We never see a fibre connect 

 unmistakably with a cell-process : there is always a net- 

 work between the two. We must bear in mind, too, that the 

 greater part of these roots is made up of central fibres ; that is, 

 of fibres which have already passed their first terminal station 

 (the spinal ganglion). We have made out the course of some 

 of these central fibres by the study of secondary degeneration. 

 We have seen that they pass directly into the column of Bur- 

 dach along with the root-fibres, and that they show a tendency 

 to pass more and more to the median line, so that in levels 

 above their point of entrance they lie nearer the middle line in 

 the column of Goll. But we have also seen that, while they 

 were working toward the median line, they were continually 

 giving off fibres to the gray matter. 



If I am to succeed in giving you a clear explanation of the 

 complicated relations existing at the entrance of the posterior 

 roots, I must particularly request you to study the following- 

 text with continuous reference to Fig. 102. 



On a transverse section of the spinal cord we see that the 

 posterior root divides into at least four parts. The most median 

 bundle (1, in Fig. 102), which consists of coarse fibres, passes 

 almost entirely and directly into the white posterior columns at 

 the same level with their point of entrance. These arc tho 

 before-mentioned central tracts ; along with these, fibres of equal 



