1st Stage.—The pulse frequently diminishes, while the pressure of the blood rises.
2nd Stage.—Not seen when large doses are employed; pulse frequency, as well as blood pressure, abnormally low.
3rd Stage.—Pressure low, pulse beats above the normal frequency.
The slowing of the heart[571] is attributed to the stimulus of the inhibitory nerves, but the later condition of frequency to their paralysis. After the section of the vagi the slow pulse frequently remains, and this is explained by the inhibitory action of the cardiac centre. The vagus, in point of time, is paralysed earlier than the muscular substance of the heart.
[571] Slowing of the pulse was mentioned first by Withering (An Account of the Foxglove, Lond., 1785). Beddoes afterwards observed that digitalis increased the force of the circulation, the slowing of the pulse not being always observed; according to Ackermann, if the inhibitory apparatus is affected by atropine, or if the patient is under deep narcosis, the slowing is absent.
The increased blood pressure Traube attributed to increased energy of the heart’s contraction, through the motor centre being stimulated later; the commencing paralysis explains the abnormally low pressure.
There is, however, also an influence on vaso-motor nerves. What Dr. Johnson has described as the “stop-cock” action of the small arteries comes into play, the small arteries contract and attempt, as it were, to limit the supply of poisoned blood. Ackermann,[572] indeed, witnessed this phenomenon in a rabbit’s mesentery, distinctly seeing the arteries contract, and the blood pressure rise after section of the spinal cord. This observation, therefore, of Ackermann’s (together with experiments of Böhm[573] and L. Brunton[574]) somewhat modifies Traube’s explanation, and the views generally accepted respecting the cause of the increased blood pressure may be stated thus:—The pressure is due to prolongation of the systolic stroke of the cardiac pump, and to the “stop-cock” action of the arteries; in other words, there is an increase of force from behind (vis a tergo), and an increased resistance in front (vis a fronte).