556 Sensory Masking 



blocking nerves with procaine. The results of certain experiments, mostly early 

 ones, before our technique was reliably developed, were rejected, either be- 

 cause the block was imperfect, or because it began to pass off before the re- 

 quired observations were finished. Our criterion of a complete block was 

 anesthesia to all types of sensation in the cutaneous area supplied by the nerve. 

 The successful blocks were induced with 1 cc. of 2 per cent procaine contain- 

 ing 0.04 mg. of adrenalin. 



For this study the sense of pressure was chosen, because its threshold is 

 readily raised and easily measured (as described by Thompson and co- 

 workers"). In each experiment the question to be answered was: Under these 

 conditions, is the pressure threshold significantly raised or not? For this pur- 

 pose experience led us to define a significant rise as one of four increments or 

 greater (see Thompson and co-workers^). Hence the following rises are con- 

 sidered significant: a rise of 4 gm. or more above an initial threshold of less 

 than 10 gm.; a rise of 8 gm. or more above an initial threshold of 10 to 50 gm.; 

 and a rise of 20 gm. or more above an initial threshold higher than 50 gm. 

 This may seem a severe criterion of significance, but it is not unnecessarily so 

 in view of the strong stimulating currents employed. These were alternating 

 currents that just failed to cause distress; they were generated and applied as 

 described by Thompson and co-workers." 



(1) The radial nerve being blocked with procaine in the lower part of the 

 forearm, the skin supplied by that nerve was quite anesthetic; hence superficial 

 pressure was abolished; but upon pressing through this insensible skin the 

 sensation of pressure in the deep structures (especially the first dorsal interos- 

 seous muscle) could be aroused, the impulses evoking the sensation ascending 

 along the ulnar nerve; the threshold of this deep pressure was measured. 



(2) The radial nerve being strongly stimulated distal to the block, the deep- 

 pressure threshold was again measured at the same spot as before. This was 

 done twenty-eight times (three subjects): a significant rise in the deep-pressure 

 threshold occurred three times, a drop twice, and no significant change took 

 place on the remaining twenty-three occasions. Clearly, stimulation of this 

 cutaneous nerve distal to a procaine block produced no consistent effect upon 

 the deep-pressure threshold beneath the anesthetized skin. 



(3) Under similar circumstances, the same (radial) nerve was stimulated 

 proximal to a complete block forty-three times (four subjects): five times there 

 was no significant change in the deep-pressure threshold; but on the remaining 

 thirty-eight occasions that threshold was significantly elevated. 



(4) The dorsal branch of the ulnar nerve was stimulated proximal to a pro- 

 caine block eighteen times (two subjects): every time, there was a significant 

 rise in the pressure threshold in the foiuth dorsal interosseous muscle and 

 other structures lying beneath the anesthetized skin and supplied by the deep 

 branch of the ulnar nerve. 



Thus stimulation of the radial nerve and of the dorsal branch of the ulnar, 



