324 



PHYSIOLOGY 



1 



THE WAY IN 

 We may now consider the possible ways open to a nerve impulse entering 



the cord. Each posterior root on entering the cord divides into two bundles. 



The smaller bundle passes to the outer side of the tip of the posterior horn 



where its fibres bifurcate (Fig. 159), giving 

 rise to fibres which pass up and down the 

 cords in a small longitudinal band of fibres 

 known as Lissauer's tract. The fibres run 

 only a short distance before turning into the 

 grey matter, and terminate in arborisations 

 round the cells of the substantia gelatinosa 

 in the head of the posterior horn. By far 

 the greater number of the posterior root- 

 fibres pass to the inner side of the posterior 

 horn into Burdach's or the postero-external 

 column. Here they also divide into two 

 main branches, one running up and the 

 other down in the white matter. ^ The 

 descending branch passes through two or 

 three segments before turning into the grey 

 matter of the posterior horn of a lower 

 segment. Of the ascending branches, some 

 end at different levels of the cord, but a cer- 

 tain proportion of the fibres from every 

 root traverse the whole length' of the cord 

 in the posterior columns to terminate in the 

 posterior column nuclei (nuclei gracilis and 

 cuneatus) in the medulla oblongata. As we 

 proceed up the cord the entering posterior 

 root-fibres displace the long fibres of those 

 below towards the middle line, so that in a 

 section through the cord in the upper cervical 

 region the posterior median column, or 

 column of Goll, is made up almost exclu- 

 sively of fibres from the hind limb, while 

 the postero-external column 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. 160). Five groups of these collateral branches can be 



distinguished, i.t. t 



(1) Fibres which arborise round cells in the posterior horn of the same 



side. 



(:4) Fibres which pass through the dorsal grey commissure ta the grey 



matter of the opposite side of the cord, 



Fio. l.v.i. Longitudinal section 

 of spinal cord of chick, showing 

 bifurcation of dorsal root- 

 fibres, and the passage of their 

 collaterals into the grey matter. 

 Three cells of the dorsal horn 

 are also seen sending their 

 axons into the dorsal columns. 



(CAJAL.) 



