In [Fig. 15] the original blood pressure was low—68 millimeters of mercury. This was increased in Fig. 15 A by 18 millimeters (the same as in [Fig. 14] C without effect), and there resulted an increase of 20 per cent in the height of contraction. In Fig. 15 B the pressure was raised 24 millimeters with a corresponding increase of 90 per cent in the muscular contraction; and in Fig. 15 C 30 millimeters with a betterment of 125 per cent.
Figure 15.—During the periods indicated in the time line the left splanchnic nerves were stimulated. The vessels of the left adrenal gland were tied off.
Comparison of Figs. 13, 14 and 15 reveals that the improvement of contraction of fatigued muscle is much greater when the blood pressure is raised, even slightly, from a low level, than when it is raised, perhaps to a very marked degree, from a high level. In one of the experiments performed by Nice and myself the arterial pressure was increased by splanchnic stimulation from the low level of 48 millimeters of mercury to 110 millimeters, and the height of the muscular contractions was increased about sixfold (see [Fig. 16]).
Figure 16.—The bottom record (zero of blood pressure) shows stimulation of left splanchnics; between the arrows the pressure was kept from rising by compression of heart.
Results confirming those described above were obtained by Gruber in a study of the effects of splanchnic stimulation on the irritability of muscle when fatigued. In a series of eleven observations the average value of the barely effective stimulus (the “threshold” stimulus) had to be increased as the condition of fatigue developed. It was increased for the nerve-muscle by 25 per cent and for the muscle by 75 per cent. The left splanchnic nerves, disconnected from the left adrenal gland, were now stimulated. The arterial pressure, which had varied between 90 and 100 millimeters of mercury, was raised at least 40 millimeters. As a result of splanchnic stimulation there was an average recovery of 42 per cent in the nerve-muscle and of 46 per cent in the muscle. The increased general blood pressure was effective, therefore, quite apart from any possible action of adrenal secretion, in largely restoring to the fatigued structures their normal irritability.
The Effect of Decreasing Arterial Pressure
Inasmuch as an increase in arterial pressure produces an increase in the height of contraction of fatigued muscle, it is readily supposable that a decrease in the pressure would have the opposite effect. Such is the case only when the blood pressure falls below the region of 90 to 100 millimeters of mercury. Thus if the arterial pressure stands at 150 millimeters of mercury, it has to fall approximately 55 to 65 millimeters before causing a decrease in the height of contraction. [Fig. 17] is the record of an experiment in which the blood pressure was lowered by lessening the output of blood from the heart by compressing the thorax. The record shows that when the pressure was lowered from 120 to 100 millimeters of mercury (A), there was no appreciable decrease in the height of contraction; when lowered to 90 millimeters (B), there resulted a decrease of 2.4 per cent; when to 80 millimeters of mercury (C), a decrease of 7 per cent; and when to 70 millimeters (D), a decrease of 17.3 per cent. Results similar to those represented in [Fig. 17] were obtained by pulling on a string looped about the aorta just above its iliac branches, thus lessening the flow to the hind limbs.
