COLLATERAL FIBRES OF THE POSTERIOR ROOTS. 



21 



postero-lateral white column, with the longitudi- 

 nal fibres of which its fibres become continuous ; 

 from this column many of them sweep with 

 bold curves into the adjacent grey matter. At 

 their entrance into the cord their direction like 

 that of the bundles of fibres of the anterior roots 

 is in most regions of the cord obliquely upwards. 



Fig. 18. LONGITUDINAL SECTION OF THE POSTERIOR COLUMN 



OP THE SPINAL CORD OF A CHICK ON THE 8lH DAY OF 



INCUBATION (Ram6n y Cajal). 



This figure shows the course of 5 entering fibres of the 

 posterior root . and some of the longitudinal fibres of the 

 posterior column. 



A, A, fibres of the posterior root ; B, bifurcation of 

 one of them in form of a Y ; C, D, origin of collateral 

 branches ; E, fibres of Groll's tract, also giving off colla- 

 terals. 



Collateral fibres of the posterior roots 

 and of the white columns. It has been 

 definitely shown by Ramon y Cajal, whose 

 researches into this subject have been confirmed 

 by Kolliker and v. Gehuchten, that the fibres of 

 the posterior roots as they enter the cord join by 

 a Y" or T~ sna P e( l junction with longitudinal 

 fibres of the posterior column ; in other words 

 that they bifurcate as they enter the cord into 

 two principal branches which run upwards and 

 downwards in the posterior white column or in 

 the adjacent part of the posterior cornu. It 

 has also been proved by the same observers 



Fig. 1 9. TRANSVERSE SECTION OF THE CORD OF A CHICK ON THE QTH DAY (Ramon y Cajal). 



This figure shows collaterals passing into the grey matter from all the white columns of the cord. 

 They are seen to form an especially close plexus in the posterior horn, just within the substance of 

 Bolando, and in the posterior commissure. 



c, central canal ; v, anterior and d, posterior surface of the cord ; p, p, posterior roots. 



