CHAPTER XXYI 

 Nerve-Roots 



Nerve-roots ; Magendie's experiment. Decapitate a large frog and 

 fix it securely in a prone position on the frog- cork. Cut away the 

 skin along the whole length of the spine. With a pair of strong but 

 fine scissors sever the neural arches on each side, working from above 

 down, and removing them so as to expose the spinal cord and the 

 nerve-roots. The dorsal roots are distinguished both by their position 

 and by the ganglia through- which they pass; they are especially large 

 and long in the lumbo-sacral region. Cut them in this region on one 

 side of the body. 



1. Tetanise the skin of the corresponding foot. No reflex move- 

 ment is produced : although, if the skin of the opposite foot be 

 stimulated, strong movements are produced in both limbs. 



2. Stimulate the distal end of one or more of the cut roots. If 

 care be taken that the current does not spread to a ventral root, no 

 movement results. 



3. Stimulate the proximal end. Strong reflex movements are 

 caused. Now cut the ventral roots on the same side in the same 

 region. Notice that on cutting them the leg muscles contract. 



4. Stimulate the peripheral end of one or more of the cut ventral 

 roots. There is strong contraction of muscles of the corresponding 

 limb. 



5. Stimulate the central end of the same. No effect is observed. 

 The excitation used for the roots may be mechanical, such as a 



pinch or snip of the scissors near the cut end. In this case errors 

 which with electrical stimulus may arise from spread of current are 

 obviated. But if the Helmholtz arrangement is used, and only weak 

 induction shocks employed, the risk of spread is much reduced. 



