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, 

 of its entrance, gives but few to 

 higher segments of the cord before 

 it terminates in the posterior column 

 nuclei. Sherringtoh 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 sta- 

 tion for the nerve fibres of the ante- 

 rior 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 ac- 

 quire 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 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. 



besides these collaterals in the neighbourhood 



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. 



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 a complete insensibility the two adjacent nerve roots must 

 be divided, in consequence of the overlap of fibres at the periphery. In the 

 limbs the segmental distribution of the sensory fibres is distinguished with 

 more difficulty. Each limb must be regarded as made up from a series of 



