Von Haller’s health suffered under this long strain, and he was therefore more than justified in asking the Regency of Hanover for permission to resign and take up his residence in Switzerland. His request was unhesitatingly granted; and, after resting for a few months from his recent labors, von Haller resumed those quiet literary undertakings which he loved so keenly and which enabled him to publish such famous works as the following:— “PRIMAE LINEAE PHYSIOLOGIAE” (First Lines of Physiology), Goettingen, 1747 (also 1751)—the very first systematic treatise on physiology of which we have any knowledge. A German edition was not published until 1759–1776. Commentaries on Boerhaave’s “INSTITUTIONES MEDICINAE,” 1739–1744. “ELEMENTA PHYSIOLOGIAE CORPORIS HUMANI,” 8 vols., Lausanne, 1757–1766; the most important of all his works. “BIBLIOTHECA ANATOMICA,” 2 vols., 1774–1777. “BIBLIOTHECA CHIRURGICA,” 2 vols., 1774–1775. “BIBLIOTHECA MEDICINAE PRACTICAE,” 4 vols., 1776–1788. “EXPÉRIENCES SUR LES PARTIES SENSIBLES ET IRRITABLES,” 4 parts, Lausanne, 1759. These titles represent only a small part of the numerous books and elaborate essays published by this tireless worker. In 1749 he was ennobled by the Emperor, thus gaining the right, so highly esteemed in Germany, of placing a “von” before his name. In the middle of the eighteenth century there appears to have existed considerable confusion in the minds of scientific men regarding the distinction between the terms “sensibility” and “irritability,” and to von Haller is due the credit of having once and for all defined the correct meaning of these words. As early as in 1747, when he published the first edition of his treatise on physiology, von Haller taught that the contractile force of muscles is supplied by the nerves, and that in this way they acquire _irritability_, a force which they cannot exercise except through the influence of the nerves. Irritability, therefore, is not a characteristic that originates in muscular tissue but is conferred upon it by the nerves. Von Haller’s experiments reveal the fact that the heart possesses the maximum degree of irritability. Next in order come the intestines and the diaphragm, the ordinary red muscles possessing a lesser degree of irritability. In order that the reader may form at least some idea of von Haller’s manner of treating physiological topics I give below a rough translation[6] of the first three paragraphs which occur in Chapter XX of his _Primae Lineae Physiologiae_ (edition of 1751):— SLEEP