Hnxley and Stampfli, '49) assumed that the myelin sheath has a hiqh electrical resistance. This would iiidicate a low ioii permeability of the sheath, ivhich is consistent with its high content of fatty material. I n the present investigation, changes in the I< eoncentration in nerve and muscle under various coriditioris were determined to obtain iiiforination on the potassium permeability of the sheath.
;LI ETHOUS After removing adhering fluid with filter paper, the tissue was weighed in a platinum crucible, dried a t about 100ยฐC. to constant weight, and ashed a t 500OC. The ash was dissolved in 1.2 cm3 5y, acetic acid aiid the potassinm determination was performed in 1 cni3 of this solution, according to the directioiis of Kramer and Tisdall ('21). In figure the known coneenti-atioiis of a iiurriber of I< solutions a r e plotted against the coilcentrations determined. Down to 10 mg 7. the method is reasonably accurate. Below this concentration not all the potassium is recovered, and in solutions below 3 m g % no I< can be detected. In general the amount of K present in the tissues used, when dissolved in 1.2 em3, gave a solution of a concentration higher than 10 mg 7 ( . F o r deterniined conceii-'hided by a grant from the U. S. Public IIealth Service.