ON THE MAMMALIAN NERVOUS SYSTEM. 



459 



the surface immediately above with the galvanometer circuit in the manner described. 

 The objections to this experiment consist in the great shock to the animal which the 

 necessarily prolonged exposure of the roots necessitated by the procedure seems to 

 cause, and in the fact that the posterior root does not hold out as long as the sciatic 

 nerve, its excitability being more readily influenced by falling temperature, drying, &c. 



We have however succeeded in three different animals (Cat) in obtaining a series 

 of readings of the value of the root effect, the details of the experiments being as 

 follows : 



In all three animals, the left 7th lumbar posterior root was selected, this being 

 the largest of the posterior roots in the Cat which receives afferent fibres from the 

 sciatic nerve, as shown in the accompanying reproduction of a photograph of the 

 plexus. (See fig. 18.) 



Fig. 18. 



The cord was first exposed in the lower dorsal region and prepared for excitation at 

 the level indicated in the table, and then the cauda equina, and thus the roots, laid 

 bare. The 7th lumbar root was then exposed from origin to ganglion, ligatured near 

 the ganglion and divided. It was raised in air by the ligature, and cables placed 

 round its ligatured cut end and its surface 1 centim. above. The roots, as stated in 

 Chapter IV., were remarkable in exhibiting a comparatively large resting electro- 

 motive difference between the surface and the cut section. 



The different columns of the cord were then excited, first with "minimal" and then 

 with maximal stimulation ; it was found that the electrical effect in the root resembled 



3 N 2 



