CONDUCTION IN SPINAL ROOTS 95 



VII. Turn to the ventral root of the 7th nerve ; expose it down to where 

 it joins the dorsal root at the ganglion. Ligate it near the ganglion. Cut it 

 distal to its ligature. Raising it by the thread, free it well towards spinal cord. 

 Faradize it, well lifted from surrounding tissue, and observe whether or no 

 any movement results. Repeat the stimulus mechanically, by ligating. No 

 movement results. Ventral root gives no central response. 



VIII. The ventral root of the 6th nerve is exposed when the dorsal root 

 is lifted (PI. VII, figs. 6, 7). Follow the ventral root to the cord under which 

 it plunges. Ligate it as far proximal as possible ; cut proximal to the ligature. 

 Lift the root by the ligature and free it by light dissection towards the 

 ganglion. Holding the ligated end well free from fluid and tissues, faradize it 

 as in the previous observations. Note that there is contraction of muscles in 

 the left hind-limb. 



Expose the tendon of tibialis anticus to the inner side of the left foot 

 below ankle. Cut and ligate its tendon. The muscle contracts on stimulating 

 the ventral root of the 6th lumbar nerve. 



Stimulation of the ventral root distal to its point of severance produces therefore 

 contraction of certain muscles. Its fibres are therefore efferent and motor. 



Finally, cut the spinal cord across, behind the attachment of the 7th 

 nerve, and, by cutting the dorsal roots of the 7th and 6th nerves of the right side 

 close to the cord, free it. Attaching a thread to the theca at the posterior end of 

 the freed stump of cord, lift the stump by the ligature. This brings into good 

 view the ventral root of the right 6th nerve and gives favourable opportunity 

 for repeating the above observation. Ligate this close to the cord. Cut it 

 proximal to the ligature. Stimulate with faradization ; note the low threshold 

 of stimulation required to give motor effects. 



IX. A useful and interesting manoeuvre for distinguishing the motor roots Obs. 74. 

 from the afferent is Galvani's experiment with metals. This succeeds with Galvani's 

 motor roots but not with afferent— that is, it provokes a muscular twitch in the ®^?^^^°^f°i* 

 limb from the motor root, but does not from the afferent, or does so very 



rarely. The reasons for its failure to provoke a reflex contraction are not 

 far to seek. The stimulus, a brief weak constant current, is not a very suitable 

 one for evoking response through the reflex centre, even when that latter is in 

 good condition. In the preparation as made by a student, especially at 

 the first time of his making it, the condition of the reflex centre is usually 

 not very good, his procedure being rather slow and allowing considerable cooling 

 of the exposed cord, even if no further damage has been done. 



The Galvani experiment can be readily made by applying a small Y-shaped 



