CONDUCTION IN SPINAL ROOTS 98 



the right hand engage the point of one blade of the oblique-jawed bone-cutting 

 pliers under the posterior edge of the lamina of that vertebra well to one side 

 of the root of its spinous process, and proceed to cut through the lamina. This 

 can only be done by a succession of small cuts because of the risk of pressing 

 the under blade of the forceps upon the spinal cord in the vertebral canal ; 

 with care, the risk of this is small. The cutting through of the lamina is next 

 repeated on the opposite side of the spinous process. The median piece of the 

 lamina thus freed, the spinous process is felt to yield more to the lifting pull 

 of the left hand and can be cut free with strong scissors and removed. The 

 fat covering the spinal theca in the vertebral canal is then seen. The 

 manoeuvre is then repeated for the 6th and 5th and posterior part of 4th 

 lumbar vertebrae, becoming easier as the opening enlarges. 



The operator, changing over to the right side of the preparation, cuts out 

 carefully with the tips of the cutting-pliers the lamina of the 1st sacral vertebra. 

 The opening in the vertebral canal (PI. VII, fig. 3) is next widened by removing 

 with the cutting-pliers the overhanging stumps of the articular processes. In 

 doing this, tiresome bleeding may occur. This can be largely escaped by 

 avoiding wrenching with the pliers, and by keeping the spinal column lifted 

 somewhat by the oblique-billed forceps holding the spinous process of the 4th 

 lumbar vertebra, thus preventing the abdomen of the preparation being 

 pressed against the table. The bleeding is venous, and when it occurs can be 

 lessened by raising the pelvis so as to lift the abdomen slightly from the 

 table. It is a little head ward of the truncated stump of each articular process 

 that each corresponding ganglion will be found. 



The fat in the canal is carefully pushed aside so as to lay bare the spinal 

 theca. Through this transparent membrane the spinal cord with the dorsal 

 rootlets and median longitudinal blood-vessel (PI. YII, fig. 3) is seen lying in 

 the clear cerebro-spinal fluid. 



With a blunt seeker carefully explore the left lateral edge of the theca 

 and make out the roots of the spinal nerves. The ganglia lie close outside the 

 spinal theca ; each is easily found by following a spinal root from the latter's 

 point of emergence from the theca. A guide to identification of the seg- 

 mental numbers of the roots is that the 7th lumbar root issues from the theca 

 about 3 mm. headward of a line drawn across between the headwardmost 

 points of right and left iliac crests, which are easily felt. The ganglion of 7th, 

 into which the root plunges just after leaving the theca, lies near the transverse 

 level of the crests. Also there is a difference in size between the roots, the 7th 

 being rather thicker than the 6th and the 6th much thicker than the 5th. 



A point of the theca at the headward end of the wound is then carefully 

 picked up with the fine dagger-tipped forceps and is snipped open by fine 



