But there are exceptions to the general statement that by adrenin the viscera are emptied of their blood. It is well known that adrenin has a vasodilator, not a vasoconstrictor, action on the arteries of the heart; it is well known also that adrenin affects the vessels of the brain and the lungs only slightly if at all. From this evidence we may infer that sympathetic impulses, though causing constriction of the arteries of the abdominal viscera, have no effective influence on those of the pulmonary and intracranial areas and actually increase the blood supply to the heart. Thus the absolutely and immediately essential organs--those the ancients called the “tripod of life”--the heart, the lungs, the brain (as well as its instruments, the skeletal muscles)--are in times of excitement abundantly supplied with blood taken from organs of less importance in critical moments. This shifting of the blood so that there is an assured adequate supply to structures essential for the preservation of the individual may reasonably be interpreted as a fact of prime biological significance. It will be placed in its proper setting when the other evidence of bodily changes in pain and excitement have been presented. REFERENCES [Footnote 80: McCurdy: American Journal of Physiology, 1901, v, p. 98.] [Footnote 81: Kaufmann: Archives de Physiologie, 1892, xxiv, p. 283.] [Footnote 82: Ranke: Archiv für Anatomie, 1863, p. 446.] [Footnote 83: Panella: Archives Italiennes de Biologie, 1907, xlviii, p. 462.] [Footnote 84: Oliver and Schäfer: Journal of Physiology, 1895, xviii, p. 240.] CHAPTER VIII THE SPECIFIC RÔLE OF ADRENIN IN COUNTERACTING THE EFFECTS OF FATIGUE As a muscle approaches its fatigue level, its contractions are decreased in height. Higher contractions will again be elicited if the stimulus is increased. Although these phenomena are well known, no adequate analysis of their causes has been advanced. A number of factors are probably operative in decreasing the height of contraction: (1) The using up of available energy-producing material; (2) the accumulation of metabolites in the fatigued muscle; (3) polarization of the nerve at the point of repeated electrical stimulation; and (4) a decrease of irritability. It may be that there are interactions between these factors within the muscle, e. g., the second may cause the fourth. Variations of the Threshold Stimulus as a Measure of Irritability