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Unzer, Johann August


(b. Halle, Germany, 29 April 1727: d. Altona, Germany, 2 April 1799)

physiology, medicine.

Unzer began the study of medicine when he was only twelve; his early teacher was Johann Juncker, an ardent disciple of Stahl. He received the M.D. in 1748, but even before then, in 1746, he had began to publish metaphysical and philosophical works on such physiological problems as life, the emotions, and sleep, in which he defended Stahl’s animistic doctrines. Nonetheless, he also seems to have been influenced by the mechanistic and electric medicine of Halle’s other leading man, Stahl’s opponent Friedrich Hoffmann, who was still teaching at the University when Unzer began his studies.

In 1750 Unzer left Halle to take up a busy medical practice, first in Hamburg and then in Altona. (Although some sources suggest that he was for a time professor at the small University of Rinteln, this cannot now be confirmed.) At the same time, he conducted research and, from 1759 to 1764, edited a popular medical weekly. Der Arzt, and a series of collections of medical writings that were translated into Dutch, Danish, and Swedish. His writings of this period indicate his gradual abandonment of the Stahlian system and his attempts to work out his own physiology, or “physiological metaphysics,” as it was later characterized. While Unzer remained interested in the basic phenomena of life, especially the higher animal functions, he turned from his earlier animism toward a more anatomical and physiological approach, directed principally to the role of the nervous system in animal functions. His Erste Gründe einer Physiologie der eigentlichen thierischen Natur thierischer Körper (1771) is the product of twenty-five years of research and reflection.

The Erste Gründe (later translated into English as The Principles of Physiology of the Proper Animal Nature of the Animal Organism) marked an attempt to establish the fundamental bases of zoology, considered as a natural science embracing all the animal kingdom, according to the “forces” of each species. In it, Unzer made use of a broad comparaitive method, which he applied particularly to nerve functions and to motion. He distinguished three types of motion–-those that are dependent on the will, and those that (although conscious) are independent of th will, and those that are wholly unconscious and involitional. He proceeded from the notion of the animal as a machine to state that some animals (beseelte Tiere) have a soul that (unbeseelte Tiere) have neither soul nor brain, and are instead moved by animal “forces”.

Unzer was led to draw this distinction by a series of observations, particularly of decapitated higher animals, which he compared to lower animals that have no brain. He concluded that the brain is the seat of the soul, although animal machines are capable of organic work without the stimulus of a brain or soul, and many animal movements occur through neural stimulus only. He recognized that external stimuli tend to be referred toward the brain but noted that they could be reflected or deviated and localized either in the brain or at a lower level in nerve crossings. He thus distinguished the afferent (aufleitend) and efferent (ableitend) nerves. He made a thorough study of the nervous reactions, and noted that external stimuli are transmitted in the nervous system by reflection. He also advanced the notion that motor phenomena may be caused by external stimuli that are not consciously perceived and emphasized the difference between voluntary and involuntary movements. Unzer’s effort toward defining a rational concept of reflex action was elaborated by Georgius Prochaska in 1784.

Although Unzer’s original contribution to science was slight, he nonetheless provided a valuable step in the development of physiology. His careful and essentially correct presentation of the mechanical and material aspects of nerve functions bridged the gap between conflicting views and became the basis of a considerable body of work on the nervous system in the nineteenth century. Unzer’s wife, Johanna Charlotte Ziegler, was also a writer on natural history.


1. Original Works. Unzer’s physiological writings include Gedanken vom Einflusse der Seele in ihrem Körper (Halle, 1746); Gedanken vom Schlaf (Halle, 1746); Neue Lehre von den Gemüthsbewegungen (Halle, 1746); Dissertatio inauguralis medica de sternutatione (Halle-Magdeburg, 1748); Philosophische Betrachtung des menschlichen Körpers überhaupt (Halle, 1750); and Grundreiss eines Lehrgebäudes von der Sinnlichkeit der thierischen Körper (Rinteln, 1768). The last was a preliminary study for his most important book, Erste Gründe einer Physiologie der eigentlichen thierischen Nature thierischer Körper (Leipzig, 1771), which was trans into English (London, 1851) and is quoted by Fearing, below, and by Edward E. Clarke and C. D. O’Malley in The Human Brain and Spinal Cord (Berkeley-Los Angeles, 1968) 342–345.

His medical books include Sammlung kleiner physikalischen Schriften; vols. I and III, Physikalische Schriften: vol, II, Zur speculativen Philosophie (Leipzig, 1768–1769): Medizinisches Handbuch, 3 vols. (Leipzig, 1770; 5th ed. 1794); Über die Ansteckung, besondersder der Pocken, in einer Beurtheilung der neuen Hoffmann’schen Pockentheorie (Leipzig, 1778); Einleitung zur allgemeinen Pathologie der ansteckenden Krankheiten (Leipzig, 1782); and Verteidigung seiner Einwürfe gegen die Pockentheorie des Hrn Geh Rath Hoffmann (Leipzing, 1783).

II. Secondary Literature. There are few biographical works about Unzer, but see T. Kirchhoff, in Deutsche Irrenärzte I (Berlin, 1921), 13–15. On his work, particularly in the development of animal physiology and on reflex action, see Georges Canguilhem Laformation du concept de réflexe aux XVIIe et XVIIIe siecles (Paris, 1955), 108–114; Franklin Fearing, Reflex Action. A Study in the History of Physiological Psychology (London, 1930), 90–93; C. F. Hodge, “A Sketch of the History of Reflex Action,” in American Journal of Psychology, 3 (1890), 149–167, 343–363; Thomas Laycock, into The Principles of Physiology of the Proper Animal Nature of the Animal Organism (his trans. of Unzer’s Erste Gründe) (London, 1851); and Max Neuburger. Die historische Entwicklung der experimentellen Gehirn- und Rückenmarksphysiologie vor Flourens (Stuttgart, 1897).

Vladislav Kruta

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