36 
to by each stimulus until at last they reach the necessary 
minimum required to displace ions from the ion-proteid. 
The same principle explains idio-muscular swellings — fixed 
waves of contraction of small extent ; these are due to the 
kations set free not being sufficient to cause a discharge by 
displacement, but sufficient to augment the P.D. between 
hyaloplasm and spongioplasm, and so cause local contraction 
— while the same principle, together with the presence of 
anions, explains the local extension at the anode seen in some 
muscles ;'^ anions are liberated, as in ordinary electrolysis, at 
the anode — the P.D. between the hyaloplasm and spongio- 
plasm is diminished, and the muscle extends ; but, as kations 
are predominant, the anions are not strong enough to cause 
displacement, and so the excitation does not travel. Some- 
times the P.D. is so far reduced that the muscle extends so 
much as to break at the anode ;t such a result could not, of 
course, take place unless the muscle had, normally, a good 
deal of "tone" — that is, there is considerable room for exten- 
sion and free kations are numerous. 
This leads us directly to the consideration of the 
^Hhre^hold number" of a tissue — that is, the number of ions 
necessary to cause a discharge in a given tissue (the inverse 
of which is proportional to the ''sensibility" of the tissue). If 
we call this number per unit cross section /? , it is evident 
that /3 must vary considerably in different tissues, and that 
the greater f3 is the slower will be conduction of excitation, 
for at each successive point more time must be allowed for 
the ions to gather. Since in non-medullated nerves the rate 
of conduction is much lower than in medullated nerves (8 
metres per sec. in the former, 27 per sec. in the latter'l) we 
may state provisionally that f3 is greater in non-medullated 
nerves than in medullated. This is confirmed by the fact 
that non-medullated nerve re-acts better to stimuli of prolonged 
duration than to short induction shocks, ^ for more time is 
required by the electric current to liberate ft ions iii non- 
medullated nerves than in medullated, in which extremely 
short current duration is sufficient. || The conductivity of 
medullated nerve, and. indeed, of all excitable tissues, is lower- 
" Biedermann : Electro-physiob'^y : Trans, by F. A. Welby, 
vol. ii., page 236. 
t Ihid., vol. ii., page 239. 
tGotch: Schafer's Textbook of Physiology, vol. ii., pages 
455 and 482. 
§ IhhL, vol. ii., page 284. 
I Ihid., vol. ii., p«ge 475: and Biederaiaiin : Electro-physi- 
ology' : Trans, bv F. A. Welby, vol. ii... pages 121 and 122. 
