Adrenin has a well-known stimulating effect on the isolated heart--causing an increase both in the rate and the amplitude of cardiac contraction. This effect accords with the general rule that adrenin simulates the action of sympathetic impulses. It is commonly stated, however, that if the heart holds its normal relations in the body, adrenin causes slowing of the beat.[144] This view is doubtless due to the massive doses that have been employed, which are quite beyond physiological limits and which induce such enormous increases of arterial pressure that the natural influence of adrenin on heart muscle is overcome by mechanical obstacles to quick contractions and by inhibitory impulses from the central nervous system. Hoskins and Lovellette have recently shown that when the precaution is taken to inject adrenin into a vein in a manner resembling the discharge from the adrenal glands, not only is there increased blood pressure, but generally, also, an acceleration of the pulse.[145] At the same time, therefore, that a greater amount of work, from increased arterial pressure, is demanded of the heart, blood is delivered to the heart in greater abundance, and the muscle is excited to more rapid and vigorous pulsations. The augmentation of the heart beat is thus coördinate with the other adaptive functions of the adrenal glands in great emergencies. The Changes in Respiratory Function Also Favorable to Great Effort The urgent need in struggle or flight is a generous supply of oxygen to oxidize the metabolites of muscular contraction, and a quick riddance of the resultant carbon-dioxide from the body. The moment vigorous exercise is begun the breathing at once changes so as to bring about a more thorough ventilation of the lungs. And one of the most characteristic reactions of animals in pain and emotional excitement is deep and rapid respiration. Again the reflex response is precisely what would be most serviceable to the organism in the strenuous efforts of fighting or escape that might accompany or follow distress or fear or rage. It is known that by such forced respirations the carbon-dioxide content of the blood can be so much reduced that the need for any breathing whatever may be deferred for as much as a minute or even longer.[146] And Douglas and Haldane[147] have found that moderately forced breathing for three minutes previous to severe muscular exertion results in greatly diminishing the subsequent respiratory distress, as well as lessening the amount of air breathed and the amount of carbon-dioxide given off. Furthermore, the heart beats less rapidly after the performance and returns more quickly from its increased rate to normal. The forced respirations in deeply emotional experiences can be interpreted, therefore, as an anticipatory reduction of the carbon-dioxide in the blood, a preparation for the augmented discharge of carbon-dioxide into the blood as soon as great muscular exertion begins.[*]