Neurology in the 1700s

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Neurology in the 1700s

Overview

Neurology is the study of the body's brain and nervous system. Although modern neurology was not established until the twentieth century when surgery, antibiotics, and imaging techniques were well known, the cornerstones for neurology were laid two hundred years earlier. During the eighteenth century, scientists in England and Europe began studies on the function of nerves and why they made muscles contract. Claims of "animal electricity" traveling through the nervous system sparked new discoveries in physiology and technology, and influenced the prevailing literature and culture. Eventually, eighteenth century scientists contemplated the spark of vitality itself, debating its natural, spiritual, or electrical origin in the perception of the human brain.

Background

Prior to the eighteenth century, the fledgling science of neurology, named by English anatomist Thomas Willis (1621-1675), consisted mostly of anatomical observations. Willis was the first to try to link the structure of the brain to its function. By removing the brain from the cranium, Willis was able to describe the brain more clearly, especially illustrating the arterial supply. Willis surmised that the disease of epilepsy and many paralytic conditions were linked with brain or nervous system dysfunction. With vague ideas of brain function, Willis attempted to map particular areas of the brain as responsible for certain mental functions such as vision, hearing, and touch. Willis also observed basic reflexes and attributed them to brain function.

The first scientific studies of nerve function were undertaken in the early 1700s by the English clergyman Stephen Hales (1677-1761). By 1730, Hales had discovered a connection between the spinal cord and reflex action. An avid animal experimenter, Hales was the first to measure blood pressure in an animal by attaching a goose's trachea to a living horse's carotid artery, then observing how far up a glass tube the force of the heart pumped the horse's blood. The science of the day included the belief that muscle contraction was also caused by the force of the blood and its pressure in the muscles. Hales measured blood pressure in smaller and smaller blood vessels, and determined that blood pressure was so reduced in muscles it could not possibly be responsible for their movement. Instead, Hales suggested that muscle movement was electrical in origin. Turning to smaller animals for this experimentation, Hales studied muscle movement in decapitated frogs. By pricking the frog's skin and stimulating the frog's reflexes, Hales noted that the nerve action continued without the frog brain. Hales concluded that this action was related to the frog's extended nervous system which communicated with the spinal cord.

As to what force sparked the nervous system, it was thought at the time to be the domain of the soul. In his 1751 publication On the Vital and other Involuntary Motions of Animals, English physician Robert Whytt (1714-1766) explained the soul as the vital force from a rational and natural stance, rather than a religious one. This soul, Whytt reasoned, was equally distributed throughout the nervous system and was responsible for communicating sensory stimulus to muscles during a reflex action. Whytt demonstrated for the first time that the entire intact spinal cord is not necessary for a reflex action, but that only a small segment is necessary and sufficient to produce a reflex arc. Whytt also showed that certain reflexes could be destroyed (contraction of the pupils to light, known as Whytt's reflex) when cranial nerves were severed. In the clinical setting, Whytt observed and reported spinal shock and was the first to describe tuberculosis meningitis in children.

Swiss physiologist Albrecht von Haller (1708-1777), professor of medicine, anatomy, surgery, and botany at the University of Göttingen, made essential contributions to the developing study of neurology by delineating muscle action from nerve action. Haller's animal experimentation on the irritability (contractility) of muscles and the sensitivity of nerves was extensive—Haller himself performed almost two hundred of over five hundred experiments. Based on the theory of general irritability proposed earlier by English scientist Francis Glisson (1597-1677), Haller showed in 1752 that contractility is a property inherent in muscle fibers, while sensibility is an exclusive property of nerves. By this experimentation, the fundamental division of fibers according to their reactive properties was established. Haller concluded that muscle fibers contracted independently of the nerve tissue around it. Haller thus explained why the heart beats: contraction of muscular tissue stimulated by the influx of blood. Even after the electrophysiology pertaining to the heart was understood more than one hundred years later, Haller's basic suppositions regarding tissue contractility remained essentially correct. Haller's experiments also showed that nerves were inherently unchanged when stimulated, but caused the muscle around it to contract, thus implying that nerves carry impulses which cause sensation. This distinction between nerve and muscle action illustrated by Haller provided the framework for the advent of modern neurology.

After an accidental discovery of the relationship between electricity and muscle movement while studying frogs, Italian physician Luigi Galvani (1737-1798) proposed a theory of "animal electricity." By suspending the legs of frogs with copper wire from an iron balustrade, Galvani noticed that the frog feet twitched when they came in contact with the iron balustrade. While dissecting frogs for study, Galvani also noticed that contact between certain metal instruments and the specimen's nerves provoked muscular contractions in the frog. Believing the contractions to be caused by electrostatic impulses, Galvani acquired a crude electrostatic machine and Leyden jar (used together to store static electricity), and began to experiment with muscular stimulation. Galvani also experimented with natural electrostatic-static occurrences. In 1786, Galvani observed muscular contractions in the legs of a specimen while touching a pair of scissors to the frog's lumbar nerve during a lightning storm. The detailed observations on Galvani's neurophysiological experiments on frogs, and his theories on muscular movement were not published until a decade after their coincidental discovery. In 1792, Galvani published De Viribus Electricitatis in Motu Musculari (On Electrical Powers in the Movement of Muscles). The work set forth Galvani's conclusions that nerves were detectors of minute differences in external electrical potential, and that animal tissues possessed electricity different from the "natural" electricity found in lightning or an electrostatic machine. Galvani later proved the existence of bioelectricity by stimulating muscular contractions with the use of only one metallic contact—a pool of mercury.

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The accomplishments of eighteenth century scientists such as Hales, Whytt, Haller, and Galvani laid the foundation for the independent discipline of neurology. The fact that it was a new field, largely due to the recent knowledge of the underlying anatomy and physiology, made it an attractive choice for many outstanding clinicians of the early nineteenth century. From 1790 until the 1840s, hardly a year lapsed between major discoveries of the structure or physiology of the brain and nervous system. Aided by the eighteenth century invention of the microscope, neurons (nerve cells) were some of the first cells clearly described (in 1837 by J.E. Purkinje). Once the anatomy and elementary physiology were understood, scientists began to link various disease states solely to the brain and nervous system.

The fascination with "animal electricity" continued, although the minute electrical activity in the body was later proved to hold no special animal properties. Alessandro Volta (1745-1827), professor at Pavia, showed that a muscle could be made to contract continuously (as in tetany) by applying excessive electrical stimulation. Volta also developed the Voltic pile, the first electrical battery, in 1799, by following Galvani's example of alternating metallic conductors. In the late 1700s, some physicians used static electricity to treat a variety of ailments. The treatments were both fad and fake. The works of scientists such as Galvani and Volta inspired science-fiction fantasies. English author Mary Shelley (1797-1851) published an account of a monstrous life created with the aid of an electrical spark in Frankenstein, published in 1818.

The animal experiments of Hales, Galvani, and Haller were at times performed while the animals were still alive, provoking protest among some scientific and social contemporaries. Although a live animal was fundamental to some experiments, (Hale's measurement of blood pressure, for example) news of the protests against vivisection (live animal dissection) reached the scientists. The anti-vivisectionists argued that the experimentation had not resulted in a cure for any maladies known to man. Even the straightforward and occasionally terse English writer Samuel Johnson (1709-1784) softened to the animal cause, claiming vivisection as cruel and denouncing the doctors who "extend the arts of torture." Haller, also a poet, later refrained from further animal research and turned his interests to compiling vast bibliographies on botany and physiology.

The prevailing philosophy of the Enlightenment encouraged most experimentation and reasoned intellectualism, both of which made acceptable the scientific study of the nervous system. Experimentation prompted new consideration for the nature of vitality. The prevailing philosophy of French physician René Descartes (1594-1650), held that the body functioned mechanically as a result of stimulus and reflex. Religion held that the soul supplied by God superseded and influenced the mechanical body. The new naturalism of the Enlightenment challenged both of these philosophies. Haller's work delineating the function of nerve and muscle tissue led to his assertion that man possesses a physical body whose function is explained by properties of matter and forces, and a soul which is spiritual rather than material. Whytt disagreed, fearing the new naturalism would encourage atheism. The philosophical arguments regarding man's brain and it's thought processes occupied much of the scientists' energy, perhaps explaining why, in an age of experimentation, few advances in clinical neurology were made.

BRENDA WILMOTH LERNER