Exploring the dual meanings of homunculus and its role in the history of neurology.

By Peter J. Koehler

Figure 1. 19th century engraving for Goethe’s “Faust,” depicting an alchemist creating a homunculus (public domain).

The German internist and Paracelsus biographer Dieter Kerner (1923-1981) wrote: “Neither Paracelsus nor Goethe inaugurated the homunculus motif; rather, they represent stages in a long series of developments that continues to the present day.”¹ Despite this, Paracelsus and his medical alchemy are usually referred to when we are looking for the origin of the term homunculus.

Two Meanings

Figure 2. Portrait of Paracelsus, engraving (1644-1650) by Pieter van Sompel after design of naar ontwerp van Pieter Claesz. Soutman, Rijksmuseum Amsterdam, obj. no. RP-P-OB-59.692; https://id.rijksmuseum.nl/200257515 last accessed Dec. 29, 2025 (public domain).

Homunculus has two meanings. It can refer to either an artificial human created through alchemy (see Figure 1) or a tiny human being. The first meaning refers to the German physician and alchemist Paracelsus, whose real name was Theophrastus von Hohenheim (c. 1493-1541; see Figure 2). Parecelsus is supposed to have written about the “chymicall homunculus” in a treatise, “De Natura Rerum” [Of the Nature of Things]” in 1537. The book was translated into German by the German personal physician to Count Palatine of Veldenz in Pfalzburg, Lucas Bathodius (1540s-c. 1598),² in 1584.³ (See Figure 3a.)

A century later, in 1650, a translation from Latin into English was produced by “J.F. MD,” likely a reference to the English physician and chemist John French (c. 1616-1657). This book also contains a translation of the work “Novum Lumen Chymicum [A New Light on Alchemy];” (see Figure 3b) by the Polish alchemist and physician Michael Sendovogius (1566-1636).⁴

Figure 3. (a) De Natura Rerum 1584 German and (b) 1650 English translation.

There is doubt about the authenticity of “De Natura Rerum,” in particular the first chapter “Of the generations of Naturall things,” including the description of the homunculus. The issue was extensively studied and described by the Swiss physician Urs Leo Gantenbein, who noted that since ancient times, and especially since Aristotle, the spontaneous generation of certain animals from inorganic matter had been a recognized fact that was hardly ever questioned. Paracelsus, however, rejected this doctrine, undoubtedly one of the first to do so. Furthermore, in other authentic texts, such as “De Vita Longa” (1527), Paracelsus referred to the homunculus as “a small statue made of wax, intended to ward off evil spells.”⁵

At the time, the possibility of the creation of artificial life and especially of an artificial human being in the form of a homunculus had fascinated humankind, from Aristotle through the Arab philosophers to the medieval myth of the golem.⁵ The author of the first book (chapter) of “De Natura Rerum” wrote that an artificial human could be formed alchemically from human sperm, by a kind of parthenogenesis (a natural form of asexual reproduction in which the embryo develops directly without need for fertilization). “Let the Sperm of a man by itself be putrefied in a gourd glasse, sealed up, with the highest degree of putrefaction in Horse dung, for the space of 40 days, or so long untill it begins to bee alive, move, and stir, which may easily be seen… This wee call Homunculus, or Artificiall [Man?].”^4,6

Some authors go even further back and note that the homunculus was already mentioned in the Arabic “Kitāb al-Nawāmīs [Book of the Cow]” from the end of the ninth century. They incorrectly claimed that this is a work by Plato. It has been suggested that Paracelsus, or whoever wrote the first chapter of “De Natura Rerum,” had read this text. Some other medieval authors may also have referred to the homunculus, including the Catalan physician Arnold of Villanova (c. 1240-1311), who was believed to have succeeded in creating a homunculus himself.⁶ A related concept, to which Kerner was referring, goes back to biblical sources and the Hebrew term golem — a human figure from Jewish legend made of clay and brought to life by a rabbi.

Figure 4. Antonie van Leeuwenhoek, painting (1680-1686), 56 x 47,5cm, by Jan Verkolje (I), Rijksmuseum Amsterdam, obj. no. SK-A-957; https://id.rijksmuseum.nl/200108635 last accessed Dec. 29, 2025 (public domain).

The second meaning, tiny human being, is the one usually referred to in medicine. If we go back to the 17th century and the discovery of the “microworld,” the two most important persons associated with this were the English naturalist Robert Hooke (1635-1703), who used a compound microscope and published his Micrographia in 1665, and the Dutch cloth merchant Antonie van Leeuwenhoek (1632-1723; see Figure 4).

Van Leeuwenhoek, Hartsoeker, and Sperm Animals

In contrast to Hooke, van Leeuwenhoek used a seemingly primitive microscope with one lens, but his instrument could magnify many times greater than the compound microscopes of his time. He made more than 500 microscopes, mostly with a single lens clamped between two metal plates. He also made the lenses himself. They were usually ground, sometimes blown. He was able to increase the magnification from 30 to several hundred times. The strongest specimen had a magnification of 270 times.⁷

Van Leeuwenhoek corresponded about his observations with the Royal Society from 1673. In 1677, together with the student Johannes Ham (c. 1651-1723), he observed sperm cells, which he described in remarkable detail and called “zaad dierkens [sperm animals].”

Figure 5. Van Leeuwenhoek’s drawing of sperm cells as published in the Philosophical Transactions of the Royal Society.⁹

In the summer of that year, he sent their observations to the mathematician and president of the Royal Society, William Brouncker (c. 1620-1684).⁸ The observations were published in Philosophical Transactions in 1679, after he had had the text translated into Latin.⁹ (See Figure 5.) Because sperm cells move using their tails, he considered them to be carriers of life. The female reproductive organs serve only to provide nourishment to the organisms already present in the sperm. He continued his research into sperm cells and the question of reproduction for more than 20 years.

Van Leeuwenhoek may be considered a preformist, believing that “animals, plants, and humans are preformed, created by God at the Creation as a miniature of the eventual adult it will become.”¹⁰ Some believed this occurs in ovaries (ovism) while others, like van Leeuwenhoek, believed this occurs in the first male of each species (animaculism). The man’s seed alone, he reasoned, forms the fruit (fetus), and all that the woman can contribute is to receive or nourish the male seed. In 1699, he wrote that it was inconceivable “that human ingenuity will penetrate so deeply into that great secret that, by chance or by the dissection of the animalcule in the semen, we will come to see the entire man.” However, he had no doubt that the entire man — in whatever form — was there.¹¹

Figure 6: Nicolaas Hartsoeker, painting (1682), 49 x 40 cm, by Caspar Netscher, Louvre Paris, https://collections.louvre.fr/ark:/53355/cl010064577 last accessed Dec. 29, 2025 (public domain for scientific purposes).

The Dutch naturalist Nicolaas Hartsoeker (1656-1725; see Figure 6) was another proponent to animaculism. He visited van Leeuwenhoek in early 1672 or 1673 and was inspired by him to study medicine and exact sciences. He is said to have discovered living creatures in human and animal semen in 1674, using a homemade microscope. He wrote about it in his 1694 book “Essay de Dioptrique,”¹² in which he said he published on it in the 30th Journal des Sçavans of 1678 and included a figure. (See Figure 7.) He later got into a dispute with van Leeuwenhoek about the priority of this discovery, which he called animaux spermatiques.

Searching for the 30th Journal des Sçavans of 1678, there is indeed an extract of a letter discussed on Monday, Aug. 29, 1678: “Extrait d’une lettre de Nicolas Hartsoker écrite à l’Auteur du Journal touchant la manière de faire les nouveaux Microscopes, dont il a esté parlé dans le journal il y a quelques jours [Excerpt from a letter written by Nicolaas Hartsoeker to the author of the journal concerning the method of making new microscopes, which was discussed in the journal a few days ago].”

Figure 7. From Hartsoeker’s “Essay de Dioptrique” (p. 230) sperm cell, in which he assumed the presence of a tiny human being.

The two-page excerpt begins with a comment about the new microscopes that “M. Huguens” had brought from the Netherlands. This is referring to the Dutch mathematician and astronomer Christiaan Huygens (1629-1695), who became the first scientific director of the Académie Royale des Sciences in Paris in 1666. After a description of the microscope, we find how Hartsoeker found “petits animaux [small animals]” in urine.

Moreover, “Il en a trouvé dans la semence du Cocq, qui ont paru à peu prés de cette mesme figure qui est fort differente, comme l’ont voir de celle qu’ont ces petits animaux dans la semence des autres qui ressemblent, comme nous l’avons remarqué, à des grenoüilles naissantes [He found some in the semen of the rooster, which appeared to be almost identical in shape, which is very different, as we can see, from that of these small animals in the semen of other animals, which resemble, as we have noted, newborn frogs].”¹³ The image included in the excerpt is not of the homunculus, but of the new type of microscope.

Looking at Hartsoeker’s image in Essay de Dioptrique of 1694, we must realize that he and van Leeuwenhoek did not claim to have seen tiny homunculi or fetuses in human spermatozoa. This common misconception is based on the fact that both Hartsoeker and van Leeuwenhoek once included drawings of homunculi in their writings, but they did so only to suggest their possible appearance (Hartsoeker) or to refute claims that they existed (van Leeuwenhoek).¹⁴

Wilder Penfield and the Montreal Procedure

Wilder G. Penfield (1958); © NLM (unique ID: 101426096).

About 240 years later, the term homunculus was reintroduced into medicine, particularly in neurology, where it usually refers to the projection of motor and sensory functions onto brain models. For this type of homunculus, we must discuss the work of the American-born Canadian neurosurgeon Wilder Graves Penfield (1891-1976). After having spent some years with neurophysiologist Charles Scott Sherrington (1857-1952) in Oxford, neurosurgeon Harvey Cushing (1869-1939) in Boston, Otfrid Foerster (1873-1941) in Breslau (the present Wroclaw, Poland), and histologist Pío del Río Hortega (1882-1945) in Madrid, Penfield became neurosurgeon in Montreal (1928).

Later, he became chair of McGill University’s newly created Department of Neurology and Neurosurgery (1930) and director of the Montreal Neurological Institute (1933).¹⁵ He became well known for his research on epilepsy surgery and brain mapping. The latter was important for localizing the epileptogenic zone in the cerebral cortex, but also for mapping functionally eloquent areas. He applied electrical stimulation of the brain under local anesthesia (awake craniotomy) to distinguish normal and pathological tissue.

Penfield was not the first to work in this area as his teacher Sherrington had mapped brain functions of monkeys and Foerster mapped the motor functions of humans.^16,17,18,19 In 1937, Penfield, in cooperation with his resident Edwin Boldrey (1906-1988), published an important article on the subject in Brain, in which they mapped both motor and sensory functions. It was the result of a study involving 126 patients who had undergone surgery under local anesthetic between 1928 and 1936.²⁰ Although general anesthesia was applied in the early days of modern neurosurgery, awake craniotomy was generally applied beginning in the 1910s, first by Thierry de Martel (1875-1940) in 1913 and then by Cushing in 1917.

Figure 9. Homunculus, made by medical illustrator Hortense P. Cantlie-Douglas.²⁰

The homunculus Penfield and Boldrey presented in their 1937 article (see Figure 9) was generated from 170 summary maps of the number and location of stimulation points for each body part, each sketched by Boldrey from Penfield’s operation notes, photographs, and drawings. The drawing was made by medical illustrator Hortense P. Cantlie-Douglas (1901-1979), who had studied at John Hopkins University under the German medical illustrator Max Brödel (1870-1941). He worked for physiologist Carl Ludwig (1816-1895) in Leipzig and moved to Baltimore in 1894. Cantlie-Douglas had obtained a certificate in art as applied to medicine in 1926.

The Cantlie-Douglas homunculus is different from the one Penfield and Rasmussen drew for their 1950 book “The Cerebral Cortex of Man.”²¹ (See Figure 10.) That drawing was made by another medical illustrator, namely the later president of the Association of Medical Illustrators (1961-62) Eleanor A. Sweezey (1915-2007). Several more homunculi (thalamus, supplementary motor, etc.) were drawn for the book “Epilepsy and the Functional Anatomy of the Human Brain” that Penfield wrote in cooperation with clinical neurophysiologist Herbert H. Jasper (1906-1999) in 1954.²²

Information on how Penfield and his colleagues made the homunculus from the cortical stimulation experiments can be found in an article by Gandhoke et al.²³ It is important to realize that in clinical situations, the homunculus provides only a rough estimate of the likelihood that the neurosurgeon will find the activation of specific body parts. According to neuroscientist Marco Catani, there is no doubt that Penfield and Boldrey were aware of this.¹⁶

Catani compared the homunculi from 1937 and 1950 and noted that the original version was not proportionally scaled according to the dimensions mentioned in the 1937 article. He noted that, for example, the size of the tongue was clearly exaggerated in the earlier homunculus, a misrepresentation that Penfield corrected in 1950. Furthermore, he mentioned that Penfield warned against attributing “too much significance … to the shape and comparative size.”

Figure 10. Homunculus, made by medical illustrator Eleanor A. Sweezey, as represented in the 1950 book “The Cerebral Cortex of Man,” by Penfield and Rasmussen.²¹

The homunculus was criticized by Queen Square neurologist Francis Walshe (1885-1973) at an Anglo-American Symposium in London in 1958: “Even today cortical cartography is eagerly pursued as new modes of electrical stimulation uncover fragments of electrical excitability in new cortical territories. Nor are the moderns content with maps, for homunculi and simiusculi have now made their horrid appearances, lineal descendants of Lewis Carroll’s Jabberwock, purporting to depict the fair face of nature, but in fact achieving something quite unnatural.”²²

According to Catani, Penfield was in the audience. He opined that it is evident from Penfield’s publications before and after this meeting that he never tried to promote his homunculus as the bearer of a new principle of brain organization. He concluded his paper by noting that the homunculus “gained popularity as a brilliant aide-mémoire and for this reason it will probably never lose its place in textbooks.”^16,22

References:

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