36f> 



PHYSIOLOGY 



an exception to the otherwise universal law of forward direction in the mam- 

 malian nervous system. 



A third exception to the law of Bell and Magendie is only apparent. It 

 is sometimes found that excitation of the peripheral end of a divided anterior 

 root gives rise to manifestations of pain or to reflex movement. This has been 



shown by Schiff to be due to the presence, 

 in the sheaths of the anterior roots, of fine 

 fibres derived from the posterior roots and 

 taking a recurrent course to end probably in 

 the membranes of the cord. This recurrent 

 sensibility is at once abolished by section of 

 two or three adjacent posterior roots. 



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. 160), giving rise 

 to fibres which pass up and down the 

 cord 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 arbori- 

 sations 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 certain 

 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 



FIG. 160. 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.) 



