TRANSACTIONS OF SECTION I. 649 
3. Blocking Nerve Impulses aroused in Cut and Intact Nerves, and in 
Strychninised and Normal Frogs. By Cuartes M. GRuBer. 
The object of the investigation, of which a brief sketch follows, was to study 
the practical value of the tripolar block upon impulses aroused by different kinds 
of stimuli, and to compare its efficiency and advantages with the nerve blocks 
produced by other methods upon cut and intact nerves in the frog. 
From many investigations it has been demonstrated that nerve conduction 
may be inhibited by the same forces, but of different degrees, as those that 
arouse nerve impulses and effect stimulation—namely, mechanical (compression), 
chemical (cocaine and magnesium sulphate), thermal (freezing), and electrical 
(either faradic or galvanic bipolar or tripolar currents). 
The tripolar galvanic current is usually used for stimulation, though it has 
in a few instances been employed to block vagus impulses. In this investigation 
the tripolar current was brought to the nerve bv interpolating between the non- 
polarisable electrodes and the nerve zephyr fibres moistened in normal salt 
solutions. 
It was proved from the first that the bipolar method was less efficient than 
the tripolar, and it was therefore abandoned. 
The stimulating current consisted of the minimal faradic strength that 
caused a maximal contraction of the gastrocnemius muscle. Such contractions 
instantly ceased when the block circuit was closed, and reappeared immediately 
when it was broken. 
The strength of the current necessary to block the impulses varied from 
eight to twenty-five volts, depending upon the kind of stimuli and also upon 
the condition of the frog. The strongest tetanic contractions produced by stimu- 
lating the sciatic nerve with the faradic current-in frogs in good condition 
required, as a rule, about twenty volts in the block circuit. 
Strychnine tetanus was inhibited with weaker currents, as were also efferent 
eae reflexly produced by stimulating either the foot or nerve of the oppo- 
site leg. 
Any of the legs can be severed without causing reflex contractions in the leg 
to which the block-current was sent, or by placing the tripolar block on its 
innervating nerve above the cut. 
The tripolar block can be applied repeatedly both to cut and intact nerves, 
but with slight, if any, after-effects upon the functions of the nerve. 
Afferent impulses were inhibited by currents of only one-tenth of the strength 
necessary for efferent fibres. 
In every case the efficiency of the block was proved also by the absence of 
action currents beyond the block. 
The advantages of the tripolar block, as compared with the others that were 
investigated, are readily seen when we consider that it requires about an hour 
before magnesium sulphate or cocaine produce an efficient block, and they have 
an injurious after-effect upon the function of the nerve. However, they act 
alike on both cut and intact nerves; and that the freezing mixtures, especially 
liquid air, require several minutes before they produce blocking, and many 
more before their effect passes off, although the freezing may be applied 
repeatedly without injurious after-effect upon intact nerves. 
But it is interesting to note that freezing the nerves, either before or after 
cutting them, destroys their function at the blocked area. This fact is one of 
the conspicuous differences between the different blocks experimented with. 
The tripolar galvanic block-current therefore proved sunerior to the others 
in several respects. It may be employed indefinitely on both cut and intact 
nerves, acts instantaneously, and its etfect is just as quickly removed without 
injury to the function of the nerves; and what is also of importance is, that we 
have in it an effective, ready power, by means of which we can exclude the 
conductivity of the afferent fibres in the mixed nerve trunk. 
It is possible that it may prove a practical and valuable aid in surgery and 
for experimental purposes, 
