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.
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.
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.
(2) If in ether anesthesia the blood vessels leading to and from the adrenal glands are first carefully tied, and then the glands are removed, excitement four or five hours later, before the weakness that follows the removal has become prominent, does not alter the blood so that the typical inhibition occurs (see [Fig. 6]). Thus, although the animal shows all the characteristic signs of sympathetic stimulation, the blood, in the absence of the adrenals, remains unchanged.
