2 Old and new models of the body
2.1 The sensible body
For centuries, and well into the early modern period, sense experience, including seeing, hearing and touching, as well as bodily movement, had been explained according to the precepts of Galenic physiology – that is, as the result of the action of animal spirits flowing along the nerves between the brain and the periphery. Nerves were understood as hollow ducts that distributed animal spirits to sustain sensation and motion. In his groundbreaking model of the body as a machine, Descartes retained elements of this theory, though his emphasis on mechanical principles altered the Galenic understanding of nerves and muscles. His work also encouraged others to pursue further research into the notion of animal spirits and the anatomy of nerves, muscles and brain in order to account for movement and sensation.
In the second half of the seventeenth century, the structure of the nerves and the brain became the object of minute exploration. The Englishman Thomas Willis, for example, claimed that different parts of the brain performed different tasks and argued that the muscles contracted as the result of a chemical reaction occurring in the animal spirits. Gradually, the notion of animal spirits came to be replaced with that of a ‘nerve fluid’, which was consistent with the hydraulic model of the body prevailing at the time. The nerve fluid was conceived by some as the subtle and highly mobile secretion of the brain, which was widely thought to act like a gland. Sensation was due to the impact of external stimuli on the sentient extremities (skin, eyes, ears) and was transmitted by this fluid back to the brain.
The nature of this nervous fluid remained, however, a controversial topic in medical and scientific circles. Following Newton, many argued that the nerves were solid (not hollow), and that sensation and motion were the result of the vibrations (not the flow) of an ethereal medium – that is, an imperceptible, subtle and very elastic fluid, which was diffused throughout the universe, including animal and human bodies. Throughout the eighteenth century, physicians and philosophers argued over these different models, and sometimes combined them, for example by retaining the idea of the glandular function of the brain but conceiving of nervous transmission as a vibration rather than a flow (Jackson, 1970).
A major change in making sense of how the nerves worked occurred when, after extensive physiological experiments, which involved cutting and stimulating the nerves of hundreds of animals, the Swiss anatomist Albrecht von Haller argued that the nerve fibres possessed an intrinsic and exclusive quality, which he called ‘sensibility’ (Figure 2). This quality, the capacity of the nerves to perceive outside stimuli, was located in their inner core. Tissues that possessed a rich network of nerves, such as the skin, also possessed sensibility to a high degree. Muscles too had a reactive property – they contracted in reaction to stimuli. Haller characterised this property as ‘irritability’. Published in 1752, Haller's findings were enormously influential and led to a major reorientation in medical theory. For example, they shaped the thinking of Scottish anatomists, such as Alexander Monro II (1733–1817) (Figure 3), and of the renowned professor of medicine at Edinburgh University, William Cullen.
In the physiological model Cullen taught to his Edinburgh students, he described the body as a complex and highly integrated mechanism composed of solid organs, nervous system and fluids. While the proper functioning of each part was crucial to health, in Cullen's view it was in the nervous system that the ‘generative source of life lay’ (Stott, 1987, p. 133). He regarded sensibility and irritability as the most important qualities in an individual. For Cullen, sensibility was the capacity of nerves to receive sensation and transmit will: different people showed different degrees of sensibility. Irritability was a kind of nervous power possessed by the muscles and was quite distinct from their vigour. In fact, irritability and muscle strength were to be found in inverse relation to one another, so that a very strong person would also be prone to torpor, and a debilitated person would have a high degree of irritability. Health was now characterised by a balance between nerves' sensibility and muscles' irritability, while disease was the result of the deficiency or excess of these qualities. A certain degree of ‘excitement’ in the body was required for the nerves to transmit the impulses necessary to ensure bodily functions, but outside agents could either stimulate or depress this ‘excitement’. Remedies would either bring it back or reduce its excess. In locating the ultimate cause of disease in alterations of the nervous system, Cullen sneered at the importance traditionally attached to the fluids and their affections: ‘When I was first acquainted with Physic, I found Physicians reasoned very boldly, they spoke of thickening or thinning the blood with as much clearness as a Scotch maid would speak of making pottage thicker or thinner’ (quoted in Stott, 1987, p. 139). It was to the nervous system that physicians were now advised to direct their medical treatment.
Cullen's extensive correspondence with his wealthy patients provides numerous examples of the way in which he applied these new theories to his medical practice:
Your nervous system, originally weak, has received some shocks and your complaint is entirely from disordered nerves affecting both mind and body … You have got into a very relaxed state of nerves … I suspect your constitution originally has been strong but intemperance has been especially to blame and your first step is to avoid this for the future.
(quoted in Risse, 1993, p. 149)
If most patients continued to perceive diseases as the product of a blockage of the humours or extremes of heat or cold, some were gradually convinced that nerves were to blame: ‘Dr. Gem, Physician to ye Embassy, has exerted all his skills and knowledge … I have no Fever at present, I have head-Ache, and Indigestion, & I have lately been convinc'd that I have Nerves’ (quoted in Porter and Porter, 1988, p. 70). As a result of the shift from a hydraulic to a nervous model of the body, the notion of sensibility gained great popularity.