284 PROFESSOR MARSHALL. 



to divide the axial cord between two of the segments by a fine scalpel, 

 but I failed, as Dr. Carpenter had done previously, owing to the fact 

 that as soon as the knife reached the axial cord the arm was at once 

 thrown off, usually at a point two or three segments nearer the disc 

 than the injury. I then tried the plan adopted by Dr. Carpenter, 1 

 i.e. burning away the dorsal half of the arm with nitric acid so as to 

 expose and divide the axial cord, and with the following results. 



Experiment 14. — An active specimen was removed from the water, 

 the dorsal surface of one of the arms carefully dried, and strong nitric 

 acid applied with a fine brash to the dorsal surface of the sixth and 

 seventh radials, which were dissolved away until the axial cord was 

 exposed and destroyed. If the arm were held during the operation it 

 was usually thrown off, but if the disc only were held and the arm 

 allowed merely to rest on the fingers, the operation was always 

 successful. The animal was then returned to the water, where it 

 assumed almost at once the normal position. After half an hour's 

 rest, the distal end of the injured arm was nipped sharply with 

 forceps; active movements of the irritated arm beyond the injury 

 ensued, but no movement whatever of either the proximal part of the 

 injured arm or of any of the other arms. 



This experiment also was repeated several times on both entire and 

 eviscerated specimens, the results being without exception as recorded 

 above. It is difficult to limit the action of the acid to the dorsal 

 surface of the arm, but by sufficient care it can be done, and on 

 several occasions the ambulacral epithelium, including of course the 

 subepithelial band, was left absolutely uninjured, responding to stimu- 

 lation in a perfectly normal manner. The experiment must, I think, 

 be considered, when taken in conjunction with Experiment 13, as 

 proving that the axial cord is the sole afferent communication between 

 the arm and the central motor mechanism, for the former experiment 

 shows that the communication is still perfect when it alone remains, 

 while the latter shows that division of the cord, other parts remaining 

 intact, destroys the communication absolutely. 



Experiment 15. — One further and very obvious experiment is worth 

 recording. One of the arms of an active specimen was cut across about 

 its middle, and the animal held in the tank so that the stump of the 

 amputated arm was just above the surface of the water ; the cut end 

 of the axial cord could then be very readily seen with the naked eye. 

 1 Carpenter, 'Proc. Royal Soc.,' 1876, p, 654. 



