In no instance did blood from the inferior vena cava of the quiet normal animal produce relaxation. On the other hand, blood from the animal after emotional excitement showed more or less promptly the typical relaxation. In Fig. 4 is represented the record of intestinal muscle which was beating regularly in Ringer’s solution. At _a_ the Ringer’s solution was removed, and at _b_ “excited” blood was added; after the preliminary shortening, which, as already stated, occurs at the first immersion in blood, the muscle lengthened gradually into complete inhibition. At _c_ the “excited” blood was removed, and at _d_ “quiet” blood was added in its place. The muscle at once began fairly regular rhythmic beats. At _e_ the “quiet” blood was removed, and at _f_ the “excited” blood was again applied. The muscle lengthened almost immediately into an inhibited state. In this instance the “excited” blood was taken after the cat had been barked at for about fifteen minutes. [Illustration: Figure 4.--Alternate application of “excited” blood (at _b_ and _f_) and “quiet” blood (at _d_), from the same animal, to intestinal muscle initially beating in Ringer’s solution.] The increase of effect with prolongation of the period of excitement is shown in Fig. 5. _A_ is the record of contractions after the muscle was surrounded with “quiet” blood serum. _B_ shows the gradual inhibition which occurred when the muscle was surrounded with defibrinated blood taken when the animal had been excited eleven minutes. And _C_ is the record of rapid inhibition after fifteen minutes of excitement. In other instances the effect was manifested merely by a lowering of the tonus of the muscle, and a notable slowing of the beats, without, however, a total abolition of them. [Illustration: Figure 5.--The effect of prolonging the excitement. _A_, the record in “quiet” serum; _B_, in defibrinated blood after eleven minutes of excitement; and _C_, in serum after fifteen minutes of excitement.] The inference that this inhibition of contraction of the intestinal muscle is due to an increased amount of adrenal secretion in the “excited” blood de la Paz and I justified on several grounds: (1) The inhibition was produced by “excited” blood from the inferior vena cava anterior to the mouths of the adrenal veins, when blood from the femoral vein, taken at the same time, had no inhibitory influence. Since blood from the femoral vein is typical of the cava blood below the entrance of the kidney veins, the conclusion is warranted that the difference of effect of the two samples of blood is not due to any agent below the kidneys. But that blood from the kidneys does not cause the relaxation is shown in Fig. 3. The only other structures which could alter the blood between the two points at which it was taken are the adrenal glands, and the material secreted by them would produce precisely the inhibition of contraction which was in fact produced.