There is no major medical opus written by Linnaeus himself where he sums up his own research and gives an account of his impact as a physician. His observations and conclusions are documented in a great variety of letters, protocols, and writings. It was the custom of the time to present research findings in the form of dissertations, that is, written accounts whose content was to be defended publicly. Linnaeus often served as president (preses) when his students defended their dissertations. These dissertations provide us with knowledge about his medical findings, opinions, and advice regarding how the new knowledge should be applied. They were written in Latin, but many of them are available in Swedish translation  (Valda avhandlingar av Carl von Linnaeus i översättning, published by the Swedish Linnaeus Society). The dissertations are of two types: Pro exercitio, a kind of exercise or practice dissertation, and Pro gradu, which was a doctoral dissertation in the proper sense. The candidate, or respondent, was responsible for giving an account of a set of material, hypothesis, and conclusions. In most cases they were notes from Linnaeus’ lectures on the subject. It is said that it also happened that the material was dictated by the teacher and then arranged in accessible form by the candidate. The student publicly defended the content before his own teacher – an intriguing educational concept. (Those who would like to know how it went for Linnaeus’ students are recommended to read Ingvar Svanberg's book on the subject, in Swedish). These dissertations are a source of knowledge about Linnaeus’ views on medical issues, systematic pathology (nosology), the study of medicines (pharmacodynamics), and dietetics.

Some examples of Valda avhandlingar av Carl von Linnaeus i översättning (Selected dissertations by Carl von Linnaeus in Swedish translation), published by the Swedish Linnaeus Society.

Some Examples of the Contents of Dissertations

a. Dietetics
The importance of food substances for health was treated in the dissertation “Det förändrade köket” (The new cuisine, 1757). Here we find a review of what previously constituted people’s diets, that is, prior to the 18th century, and how certain plants had been replaced by others, either beneficially or detrimentally. The contribution of various animals to diet is also included in the catalogue. Hot spices accelerate aging, and the amounts of sugar render fibers thin and weaken the body’s tissues. We can thank the Hindus for tea, coffee, and chocolate, but Americans are to be blamed for the poisonous tobacco that has plagued the entire world. This was thus something that was realized as early as the 18th century.

In the dissertation “Ätliga Frukter” (Edible fruits, 1763) it is pointed out that humans are most closely related to apes and have the same kind of hands and teeth suitable for picking and eating fruit: four front incisors to take a bite of the fruit, two canine teeth to shred it, molars to grind it. Ripe fruits are put forward as healthful food if they are eaten in proper amounts. The dissertation provides an inventory and systematization of berries, drupe fruits, apples, siliquae (pod fruits), grains, and nuts.

b. Pathology
The dissertation “Hinder för läkekonsten” (Obstacles to medicine, 1752) treated the need for detailed knowledge of diseases and medicines. It is claimed that many medicines are neglected owing to ignorance. According to the author, the reasons medicines are not made use of are:

  1. routine
  2. differing theories rooted in different times
  3. neglected determination of the nature of a disease
  4. hasty and unfounded notions about poisons and distinguishing them from medicines that differ from poisons only in their different strength
  5. the mistakes of quacks
  6. physicians’ cautiousness and fear of doing harm
  7. overly small doses of medicine
  8. the lack of botanical knowledge among pharmacists that often leads them to sell false instead of genuine plants
  9. use of composites, that is, mixtures of non-compatible medicines
  10. ignorance of natural classes
  11. non-use of household remedies
  12. lack of travel to Europe
  13. neglecting to study botanical writings
  14. carelessness in producing medicines
  15. recklessness in regard to the proper season for collecting medicinal plants and storing them in the proper manner

c. Intoxicating Substances
A review of various intoxicating substances in terms of their effects is found in the dissertation “Berusningsmedel” (Intoxicating substances, 1761). Discussed here are the effects of fermented and distilled spirits and of intoxicating substances that do not require preparation but rather can be taken directly from nature. The effects of opium, seeds from thorn apples, belladonna, yarrow, saffron, tobacco, etc, are described. An interesting observation involves addiction and abstinence among those who drink alcohol for extended periods. Linnaeus clearly depicts “the miserable vodka drinker who gets out of bed in the morning depressed, shaking, melancholy, and sick in all his limbs…” but when he replenishes the vodka, he goes on, the individual becomes content and healthy “until the vodka has dissipated.” Abstinence and side effects are described in detail. He has theories about how the body is affected by alcohol, but they are wrong. Linnaeus notes that yarrow (Achillea millefolium) sometimes replaced hops in brewing beer, entailing that “people would become very dizzy and crazed.”

The Constituent Parts of the Body and Pathology

As Linnaeus saw it, the body, in a functional sense, we must assume, is made up of the nervous system and fibrous tissue. The fibrous system is constituted not only by musculature and support tissue but also blood circulation. He states that the nervous system has two functions: sense perception and movement. What we today call the cognitive system, that is, regulation of memory, consciousness, emotions, etc, is not mentioned. The nerves’ regulation of muscle activities includes the elimination and secretion of products, as well as breathing and blood circulation.

To reverse a disease, Linnaeus maintained that it is necessary to bring about a change in the bodily fluids or the solid parts that are contrary to the condition that produced the disease (contrariorum e contrariis curatio). The human body can be ”considered a balance that, when it is in equilibrium, makes us feel good, but as soon as there is any excess on either side, we are ill.” (Linnaeus’ lecture, 1771, see O. E. A. Hjelt). The notion that the balance of nature is the result of a struggle between opposites is central to Linnaeus’ thinking.

Equilibrium is a concept that often appears when he discusses sickness. Traces of ancient humoral pathology (the relationship between the fluid content of the body and diseases) can be discerned in Linnaeus’ views on the genesis of diseases.

Cataloguing Diseases (Genera morborum)

According to Linnaeus’ Clavis medicinae (1766), the study of diseases (pathology) comprised etiology, semiology (the study of symptoms), diagnostics, and prognostics. Was there anything distinctive in what Linnaeus contributed to medical research? Linnaeus placed high demands on medical research. He regarded it as part of natural science, and it should methodologically follow the same rules, that is, diagnosis and treatment should be based on experiments and observations. Preconceived notions and matters of faith should be countered by knowledge of the structure and functions of the body. Rational treatment and prevention of diseases are drawn from facts based on anatomy, botany, chemistry, and physiology, as Linnaeus saw it. In his practice as a physician, his goal was to sharpen his ability to observe scientifically. At the same time, he emphasized the importance of tolerance, in that the interpretation of cause and effect required some leeway as long as there was a gap in knowledge and a hope to gain new knowledge.

Each disease should be viewed on the basis of its:

  1. signs (signa), that is, symptoms,
  2. essence (essentia), which referred to the origin of the symptoms (e.g. inflammation of the lungs),
  3. cause (causa), what produces the disease.

To understand the cause of diseases, Linnaeus felt it was necessary to have a system in which various diseases can be grouped. This grouping can be done in regard to symptoms and observations made in autopsies. Linnaeus had experience from botany in cataloguing and setting up systems. He used this experience to create a system for diseases (Genera morborum). Here he listed diseases in orders, classes, and families with regard to certain diagnostic symptoms. When Linnaeus arranged his system, he was influenced by two contemporary scientists that he had close contact with: the French researcher Francois Boissier de Lacroix, (1706–1767; he called himself Sauvages), who had set up a “Nosologia methodica” and the Dutchman Herman Boerhaaven (1668–1738).

It should be pointed out that the grounds of classification, viewed from the perspective of today, were not accurate, but they did stimulate observers to pay attention to details in the pattern of symptoms and also provided an overview of pathology, constituting an attempt to order the principles of treatment. Linnaeus’ system was primarily based on patterns of symptoms. For fevers he followed another basis for classification (mode of transmission, course of disease): fevers with rashes, critical fevers, and fevers stemming from inflammation.

The Effect and Use of Medicines (Materia medica, Clavis Medicinæ)

Besides a system of diseases, Linnaeus also set up a catalogue of the properties of medicines. He divided medicines by taste (tasteless, sharp, bitter) and by smell (pleasant and strong). He maintained that the former affect the fibrous system, whereas those that smell, that is, aromatic substances, have an impact on the nervous system.

Clavis Medicinæ is a book on systematizing medicine and is
regarded as perhaps the most influential of Linnaeus’ writings
in the field of medicine.
Source: Carl von Linnés betydelse såsom naturforskare och
läkare. Skildringar utgifna af Kungl. Svenska Vetenskapsak-
ademien i anledning af tvåhundraårsdagen af Linnés födelse 
(Carl Linnaeus’ importance as a natural scientist and physician.
Accounts published by the Royal Swedish Academy of
Sciences on the occasion of the bicentenary of Linnaeus’ birth).
Almquist&Wiksell, 19077

Substances are divided into classes and orders, and within each order compounds are collected primarily from the plant kingdom. This is accounted for by the candidate J. T. Fagraeus in the 1758 dissertation ”Strong-smelling Medicines.” The publication Materia medica (1733) and the dissertation ”Observations in Materia medica” (1772) provide an inventory of plants that are used for medicinal purposes. The compilation takes up facts about their use and effects. The information had been gathered from a number of countries outside of Europe. The plants are given with their genus and their trivial names.

Pharmacopoea Svecica

A compilation of available medicines and their preparation is called a pharmocopoeia. There was no such compilation in Sweden in the 16th century, and the need for one was not felt until the 17th century, which saw the rise of the first apothecaries. Apothecaries came from Germany at that time, and they brought with them pharmacopoeias. When the Collegium Medicum was established with its powers and responsibilities in 1663, one of the first priorities was to create a pharmacopoeia. A Pharmacopoea Holmiensis (i.e. Stockholm) was put together (1686). It was not widely distributed in the country, and the need for a more complete compilation led to court physician A. Bäck being commissioned ”to compose a new pharmacopoeia.” He was assisted by Linnaeus, who primarily contributed to the section on the effects and uses of medicines. This document, (Materia medica),  contained some 380 simple substances, the section on “Praeparata et Composita” some 400 chemical preparations – as well as a list of substances to induce vomiting (emeticorum), compounds containing mercury, opium, and an alphabetical index. The pharmacopoeia also described how plant drugs should be dried, pulverization, etc. Pharmacopoea Svecica appeared in 1775, that is, three years before Linnaeus passed away.

Pharmacopoea Svecica from 1775.
Source: Uppsala University Library

Control of Pharmaceutical Activities and Taking Inventory of Medicinal Plants

From the very outset, when the first pharmacies were established in Sweden, there were regulations governing their operations. This control took the form of inspections, that is, visits from qualified individuals certified for this purpose. They were solemn occasions. Linnaeus’ inspection of the Academy Pharmacy in Uppsala (1741) was attended by the members of the faculty, the county governor, the university vice chancellor and secretary, and the mayor of the city. Linnaeus was the unquestioned authority in this context. His works (Systema Naturae 1735, Genera Plantarum 1737, Upsats på de medicinalväxter som i Apotheken bevaras och hos oss i fäderneslandet växa (Essay on the medicinal plants kept by apothecaries and growing in the country of our fathers) 1741, Materia Medica 1749, Philosophia botanica 1751, Species Plantarum 1753, Plantae officinales 1753) were indeed weighty textbooks in the field. Also important were the journeys Linnaeus had undertaken in the country to take inventory of the prevalence of plants that could be used for the preparation of medicines. During the 1740s this was a pressing issue in parliament, which had also allocated funding for Linnaeus’ inventories. Linnaeus had also submitted to the Academy of Sciences a compilation of the plants found at pharmacies (see above “Upsats på de medicinalväxter…” 1741), and in 1753 a dissertation appeared, “Plantae officinalis,” dealing with the question of how to make it possible to cultivate medicinal plants. Though only to a limited extent, medicinal plants had in fact been cultivated by an interested apothecary (J.Ferber, in Agerum outside of Karlskrona, had a garden of eight acres in 1711, where a great many of the plants were medicinally useful). In 1739 400 different plants were cultivated and, with the assistance of Linnaeus, were placed on a list (Hortus agerumensis). But it was only under the influence of Linnaeus that more substantial domestic cultivation got underway, enabling the country to produce a major portion of the medicinal plants it needed in the 18th century.

Materia medica, Nature Products and Synthetic Pharmaceuticals

In the past all medicines were products of nature, that is, a whole plant, parts of a plant, or extracts from a plant. The Latin term for the science of these products was Materia medica. Linnaeus’ system for classifying plants and his studies of the use of nature products for medicinal purposes, medicinal plants, contributed to our knowledge of the subject. However, humans had used plants and animal products as medicines long before Linnaeus. Opium, ginseng, and rhubarb are said to have been used 5,000 years ago. In modern times, many medicines are synthetic products. Often the active substance is isolated from a nature product. Linnaeus did not have access to the knowledge of analytical chemistry that we have today. His contribution was to describe the medicinal properties of plants. The connections between effect, cause of disease, and the chemically active compound (structure and activity studies) are the result of more recent research.

In this connection a distinction should be made between what we call natural pharmaceuticals and nature products. Natural pharmaceuticals are medicines whose constituent parts are made up of a plant or animal part, bacteria culture, mineral salt, or salt solution. These products are regulated (Medical Products Agency) regarding their quality, effect, and safety. Nature products are not subjected to the control prescribed for natural pharmaceuticals, but they must be approved by the Medical Products Agency. Natural concoctions with medicinal claims that are not natural pharmaceuticals or nature products are illegal. It is sometimes difficult to know where to draw the line between natural pharmaceuticals and dietary supplements.

Linnaeus and the Therapeutic Use of Electricity

Through A. Bäck in 1745, Linnaeus learned about the therapeutic use of electricity in Europe. It was through Sauvages in Montpellier that Linnaeus would develop a deeper interest in electricity. In a letter to Linnaeus, Sauvages described how he had managed to cure a number of paralyzed patients with the help of electricity. Other diseases, such as rheumatism, chilblains, and epilepsy, had also been treated. Linnaeus replied that he would immediately start using electricity. Linnaeus and Rosén got underway in the 1750s. The treatment was performed by Professor Strömer (electro-medical private practice), who was assisted by Linnaeus’ student Pehr Zetzell. The findings were reported in the work “Försök med Elektricitetens verkan på människans kropp” – Experiments on the effect of electricity on the human body, 1752), and the treatment grew to be so popular that an electricity machine was purchased for Nosocomium in Uppsala. It was operated by Zetzell, who also wrote a dissertation about the findings, “Elektriskt – medicinska satser” (Electric – medical theses, 1754). Prior to this Johan Lindhult had defended his dissertation under Linnaeus in 1752 on a subject from Materia medica (“Sjukdomar som igenom Electriciteten blifvit lindrade eller lyckeligen botade” – Diseases that have been alleviated or successfully cured by electricity). Linnaeus wanted to bring together the effects of electricity on the nervous system with his theories about the importance of smells. “Electricum exists in the air, is transported via inhaling to the lungs, from whence it is led by the blood to the brain and is stored as in a charge jar to be distributed via the nerves to the various parts of the body.” Linnaeus seems to be adumbrating the action potentials of nerve cells. This Materia electrica can also convey bodily sensations to the brain. Linnaeus has a rather murky conception of this.

It became fashionable to use electricity treatment, and there were probably suggestive elements in the effects.

Last modified: 2022-02-16