THE SPINAL CORD AS A REFLEX CENTRE 



367 



in the upper cervical region the posterior median column, or column 

 of Goll, is made up almost exclusively of fibres from the hind limb, 

 while the column of Burdach consists of fibres from the fore limb. 



Besides these distant connections, every entering nerve fibre 

 makes connection with all parts of the grey matter in and about its 

 level of entrance by means of collaterals (Fig. 161). Five groups 

 of these collateral branches can be distinguished, i.e., 



(1) Fibres which arborise 

 round cells in the posterior 

 horn of the same side. 



(2) Fibres which pass 

 through the dorsal grey com- 

 missure to the grey matter of 

 the opposite side of the cord. 



(3) Fibres terminating 

 round the median group of 

 cells of the anterior horn. 



(4), Fibres which end in a 

 rich basket-work round the 

 cells of Clarke's column. 



(5) The sensori -motor 

 bundle, which passes forwards 

 through the grey matter to 

 end round the cells in the 

 anterior horn of the same side 

 of the cord. 



Each entering posterior root 

 fibre, besides these collaterals in 

 the neighbourhood of its entrance, 

 gives but few to higher segments PIG. 161. Chief collaterals of dorsal column 

 of the cord before it terminates fibres from new-born mouse. (CAJAL.) 

 in the posterior column nuclei. A > intermediate nucleus ; B, anterior (ven- 



cornu 



of Clarke s column receive fibres 



mainly from the ascending 



branches of the nerve roots from the posterior limb, a corresponding station 



for the nerve fibres of the anterior limb being represented by the cells of the 



nucleus cuneatus. 



That several different systems of fibres are included in these 

 roots is shown by the different period at which they acquire their 

 myelin sheath. Among the earliest to acquire a sheath are the 

 fibres which end in the posterior horn and those which pass to the 

 anterior horn, while the long fibres in the dorsal columns do 

 not become medullated until much later in foetal life. Since the 

 nerve fibres of the central nervous system do not become functional 



