Observations were made on 27 dogs, the sinus in most cases being opened for inspection 6 or 8 times. The drainage of the frontal sinus of a dog was effected by means of the movement of a lining film or web of mucin motivated by ciliary action. This film was extremely thin but of high viscosity, and had sufficient tensile strength to sustain a considerable degree of traction, which was a very important factor in the drainage. The direction of motion was that of a flat, lateral spiral so arranged that there was but little tension from conflicting currents in the film of mucin. There was a speed-gradient of fairly uniform acceleration (when numerous tests were averaged); the speed at the ostium was 4-6 times as great as that on the mesial wall. Several different materials were used to test the time required to raise small volumes from the deepest recess of the sinus upward to the level of the ostium. AH were moved more or less successfully, but thick, mucilaginous mucin was moved most completely and rapidly. Roughly, 1 or 2 hrs. were required to move out 1 cc. of thick mucin. A longer time was required to remove creamy pus, and still longer to remove watery substances. Probably the rate of drainage was greater and more uniform in the unanesthetized animal with unopened sinuses. These tests were made across a steeply slanting surface. The time required to move these materials (in considerable volume) was much less when there was no need to lift them against gravity [long dash]a concept entirely distinct from the fact that a thin film moves as fast against gravity as with it. When a thin film of ink, just sufficient to stain the lining of mucin, was sprayed into the cavity it disappeared almost completely in about 30 min., and sometimes in half that time.