94 EXERCISE XV 



scissors. The cerebro-spinal fluid gushes out, flooding the floor of the wound. 

 This fluid is sopped up with cotton- wool pledgets. The operator, standing to 

 the preparation's right, then engages the tip of one blade of fine-pointed oblique- 

 bladed scissors under the thecal membrane at the posterior end of the small 

 hole cut in it and runs the scissors along the median line backward, slitting the 

 theca for its whole exposed length (PI. VII, fig. 4), taking care to lift the 

 theca slightly on the under limb of the scissors, so as to ensure the scissors- 

 point not touching the cord itself. 



V. The 7th nerve of left side is now returned to. The set of rootlets 

 composing its dorsal root (PI. VII, fig. 5) is clearly made out tapering peri- 

 pherally, and spreading out fan-like as it passes to enter the cord at the 

 dorso-lateral fissure of the latter. A vein often runs along with it. A coarse 

 thread is passed under this dorsal root as far down towards its ganglion as 

 practicable and is tied tight. Release the 1. hind-limb and let it lie so that any 

 flexion of ankle or knee may be freely seen. Sever the root carefully distal 

 to the ligature, and lift the proximal part of the root by the ligature. Faradize 

 it close proximal to the ligature with fine electrodes of about 2 mm. separation. 

 Observe movement in 1. limb. Repeat stimulation mechanically, by ligating. 

 Dorsal root gives a central response [reflex). The ventral root of the 7th nerve can 

 then be clearly seen ; note that it is smaller than the dorsal root, and note how 

 it takes a course to reach the more ventral aspect of the cord. 



VI. Turn to the 6th nerve (PI. VII, figs. 5, 6, 7). Observe the line of 

 entrance of its fan-shaped group of dorsal rootlets into the dorso-lateral fissure 

 of the cord. With a curved blunt needle in holder pass a thread under this 

 set of rootlets near, i. e. 4 mm. from, their line of entrance into cord. Tie 

 tight the ligature, and with the fine elbowed scissors carefully cut through the 

 rootlets proximal to the ligature, unless, as often happens, the rootlets give 

 way along that line as the ligature is tied. With the ligature gently raise the 

 bundle of rootlets and trace it peripherally, freeing it where necessary. 



Lifting with the left hand the dorsal root of 6th lumbar by means of its 

 ligature, so that it lies free from all fluid and tissue, apply to it, close distal to 

 the ligature, fine-pointed electrodes of about 2 mm. separation, one point on 

 either side of the root. Faradize, beginning with weak currents and somewhat 

 rapidly running the coil up to 10 cm. or 9 cm. No response is observable, no 

 movement occurs, in result of the excitation. Repeat stimulation mechanically, 

 by ligating. 



Stimulation therefore of the dorsal root distal to its place of severance produces 

 no obvious effect either on the limb or elsewhere. 



