*[Footnote: In some cases after injection of amyl nitrite the normal blood pressure, which was high, dropped sharply to a point below the critical region. There resulted a primary increase in muscular contraction due to the betterment in circulation caused by the dilation of the vessels before the critical region was reached. During the time that the pressure was below the critical region the muscle contraction fell. As the blood pressure again rose to normal the muscle contraction increased coincidently.] [Illustration: Figure 21.--Top record, blood pressure with mercury manometer. Middle record, contractions of the _tibialis anticus_ muscle 240 times per minute against a spring with an initial tension of 120 grams. Bottom record (zero blood pressure), injection of 0.4 cubic centimeter of adrenin (1:100,000). Time in half minutes.] [Illustration: Figure 22.--Top record, blood pressure with mercury manometer. Middle record, contractions of _tibialis anticus_ muscle 240 per minute against a spring with an initial tension of 100 grams direct load. Bottom record (zero blood pressure), time in half minutes. The arrow indicates the point at which a solution of amyl nitrite was injected.] Although the fall in arterial pressure caused by dilation of the vessels due to amyl nitrite was almost three times as great as that produced by the adrenin, yet the resultant betterment was only about one-fourth the percentage height and lasted but one-ninth the time. In all cases in which these solutions caused an _equal_ fall in arterial pressure, adrenin caused higher contractions, whereas amyl nitrite caused _no appreciable change_. The Point of Action of Adrenin in Muscle