The heart, the centre of organic life, and the brain, the centre of animal life, the two culminating organs of the living machine, are in an incessant relation of action and reaction which shows itself in two principal states,—syncope and emotion; the first due to the momentary cessation of the cerebral functions through intermission in the arrival of the arterial blood; the second due to the transmission to the heart of a circulatory modification. There is always an initial impression which slightly arrests this organ (according to Claude Bernard), whence a passing paleness, then a reaction which the heart, by reason of its extreme sensibility, is the first to feel; for, as the brain is the most delicate of the organs of the animal life, the heart is the most sensitive of the vegetative vital organs. When it is said that the heart is _broken_ by grief, this expression corresponds to actual phenomena. The heart has been arrested by a sudden impression, whence, sometimes, syncope and nervous attacks. The heart’s being “big,” answers to a prolongation of the diastole, which causes a feeling of fulness and oppression in the præcordial region. The “palpitation” of the heart is not merely a poetic formula, but a physiological reality; the beats being rapid and without intensity. The facility with which the heart is emptied, the regularity of the circulation being kept up by slight pressure, corresponds to the “light” heart. Two hearts beat “in unison,” under the influence of the same impressions. In the “cold heart” the beats are slow and quiet, as if under the influence of cold; in the “warm” heart, the contrary is the case. When we tell a person that we love him “with all our heart,” this expression signifies, physiologically speaking, that his presence, or the recollection of him, awakens in us a nervous impression, which, transmitted to the heart by the pneumogastric nerve, causes in our heart a reaction of such a kind as to produce in the brain a sentiment or an emotion. In man, the brain, in order to express its feelings, is obliged to take the heart into its service.[82] Footnote 82: For further details see Claude Bernard, _La science expérimentale, Étude sur la physiologie du cœur_, 1865, and Cyon’s Address to the Academy of St. Petersburg, “The Heart and the Brain,” translated in the _Revue Scientifique_, November 22nd, 1873. Also, Mosso, _Sulla circolazione del sangue nel cervello_ (1880), and _La Paura_ (Fear, English translation, 1896).