T h e hypothesis t h a t the liberation of potassium from the epithelial cells of frog's s k i n is responsible for the adaptation of the tactile endings (Hoagland, 1934(Hoagland, , 1935 raises the question whether potassium is actually liberated from the skin on stimulation. T h e following experiments were devised to test this.A piece Of skin 1 x 2 cm. was cut from the back of a frog and placed !n Ringer's solution for 5 minutes, The Ringer's was continually agitated in the region of the skin so as to insure complete washing away of electrolytes released from the cut epithelial cells. The skin was then mounted, inner side down, on a glass microscope slide by means of two rubber bands and again washed with Ringer's to eliminate electrolytes that might have been squeezed out of the skin as a result of the manipulation involved in mounting it. Several pieces of fine glass rod were cemented onto the microscope slide lengthwise, giving a corrugated effect, to allow washing the under side of the skin with a stream of Ringer's delivered by a medicine dropper, the tip of which was pulled out to a small diameter. The slide with the skin was placed vertically in a small glass container and the air jet used in the stimulating experiments previously described (Hoagland, 1933) was directed on the skin. 10 cc. of Ringer's solution was placed in the vessel and the air jet was turned on, producing repetitive stimulation of a spot at a frequency of 140 per second with a nozzle pressure of 400 mm. Hg. During stimulation lasting for 3 minutes the 10 cc. of Ringer's was repeatedly streamed down the under side of the skin by means of the 2 cc. medicine dropper. Some twenty such washings were delivered during the 3 minutes that the stimulation lasted. The "wash" was then collected in a dean vial and tightly stoppered to be analyzed later.Control experiments were carried out in the following manner. A sample of skin was washed and mounted on the slide in the same way as previously described.

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for advice and guidance in completing the dissertation upon which this research is based. Thanks are also due to Cleveland State Hospital for the use of its facilities.

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It is well known that under constant stimulation an end organ, such as a muscle spindle or tactile receptor, fails to respond to the stimulation after a period of time. This phenomenon-called adaptation-has been studied by many investigators, but no specific mechanisms for the process were suggested until that of Hoagland ('34). Based on direct studies of action potentials in single nerve fibers supplying single tactile receptors in frog's skin (Adrian, Cattell and Hoagland, '31 ; Cattell and Hoagland, '31 ; Hoagland, '33), and on a variety of other evidence, he suggested the following hypothesis: On mechanical stimulation of the skin the free nerve endings in the epidermis, which constitute the tactile receptors, are excited t o set up impulses over the cutaneous nerve fibers. At the same time, however, potassium is expressed from the epithelial cells which are in close proximity to the free endings. This T< accumulates around the endings and thus reduces their responsiveness by decreasing the excitability determining ratio of I< within and without the nerve fibers, KJK,,, by augmenting KO. (For a complete review of the evidence for this hypothesis, compare Hoagland, '35 a, '36 b.) According to this notion adaptation is not a property of the sense organ (in this case free nerve endings), but rather a function of its epithelial environment. This hypothesis has been substantiated by recent work (Hoagland, '35 b, '36 a ; Hoagland and Rubin, '36 ; Rubin, '36).

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