BY DR. MICHAEL FOSTER. 403 



consciousness are intact, and the ordinary tokens of sensation 

 are used to determine whether the impulses caused by stimu- 

 lation of the peripheral terminations reach the conscious cen- 

 tral nervous system or not. But the facts may also be readily 

 shown in the absence of the brain, when reflex action is taken 

 as a proof of a centripetal impulse having reached the spinal 

 cord. In the former case, the frog should be placed under 

 chloroform during the laying bare of the roots. In the latter 

 the medulla should be previously divided in the neck (.s<v 

 Chap. XXXIII.). 



The frog being placed on its belly, make an incision in the 

 middle of the back, from the upper end of coccyx to the level 

 of the limbs, see fig. 266 g h. Having hooked back the flaps 

 of skin, carry the median incision down to the spines of the 

 vertebrae, and dissect away the longitudinal muscles on either 

 side, so as to lay bare the bony arches, and then hook back 

 the muscles on either side, or cut them away altogether. 



With a small but strong blunt-pointed pair of scissors, cut 

 through, on either side, the arch of the last (eighth) vertebra 

 (be careful not to thrust the scissors in too deep), and remove 

 the piece so loosened. Proceed then to the next arch above, 

 and so remove three arches. The roots of the nerves will be 

 seen lying in the spinal canal. Snip away the remains of the 

 iarches on each side, until the last three (or four) roots arc 

 quite clear, being very careful not to touch the nerves with 

 the scissors. The bleeding ma)' be disregarded. The posterior 

 roots lie superficially, are large, and hide the anterior roots. 

 The several roots may be separated from each other by pass- 

 ing with great care the blunt seeker lengthways between them. 



Very gently pass a fine aneurism needle, armed with thin silk 

 (ex. gr., a fine sewing-needle, with the head slightly bent, and 

 the point fixed in a handle), under a conspicuous posterior root 

 which seems to be the last. This will be the ninth ; the tenth 

 is much smaller, and runs closer to the filum terminale, see. fig. 

 295. The seventh, eighth, and ninth form the ischiatic, from 

 which the crural, Ne t and sciatic, Ni, nerves are given otf, the 

 seventh supplying most of the fibres of the crural. Tie the silk 

 loosely round the nerve, near its entrance into the cord. Care- 

 fully avoid compressing the nerve. 



Obs. I. The frog being completely at rest, draw the ligature 

 tight, observing the frog all the while. If the animal be in good 

 condition, some movements will be visible in some parts of the 

 body as evidence either of sensibility or reflex action. Now cut 

 the nerve between the ligature and the cord ; some movement 

 will probably be again witnessed. 



06s. II. Lift the peripheral stump of the nerve carefully up 

 by means of the ligature, and slip it upon the curved shielded 

 electrodes (fig. 271) which may be held in the hand, or, better, 



