THE SPINAL CORD AS A REFLEX CENTRE 325 



(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 neighbour- 

 hood of its entrance, gives but few 

 to higher segments of the cord 

 before it terminates in the pos- 

 terior column nuclei. Sherrington 

 suggests that the cells 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 

 periods at which they acquire 

 their myelin sheath. Among the 

 earliest to acquire a sheath are 

 the fibres which end in the pos- 

 terior 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 until they have 

 acquired a medullary sheath, 

 we must conclude that the reflex 



responses affecting the segment in which the fibres enter are developed 

 earlier than those which involve also the activity of the cerebellum and 

 medulla. 



The primitive segmental character of the central nervous system is 

 retained in its pure form only in the segmentation of the dorsal spinal root 

 ganglia. Each of these ganglia or afferent roots consists Of the fibres from 

 the sense-organs in a segmental area of the body surface as well as from 

 the muscular and visceral apparatus in the same segment. Section of one 

 dorsal posterior nerve-root will cause a diminution of sensibility over a 

 band-like area corresponding to the distribution of the fibres of the root, 

 though to produce complete insensibility the two adjacent nerve-roots must 



FIG. 160. Chief collaterals of dorsal column 

 fibres from new-born mouse. (CAJAL.) 

 A, intermediate nucleus ; B, anterior (ven- 

 tral) cornu ; c, dorsal or posterior cornu ; 

 c, substance of Rolando. 



