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Fernel: the origins of ‘neurophysiology’

The concepts of physiology and neurophysiology originated at the time of Vesalius. Although some have credited Aristotle in his biological writings as having dealt with physiological subjects and have conceived of Galen’ s On the Usefulness of the Parts as the earliest separate treatise dealing with human physiology, it is the sixteenth-century physician and scholar Jean Fernel (1495–1558) who should be credited with the first formal treatment of physiology. Fernel’ s De naturali parte medicinae, published in Paris in 1542, contains the word ‘physiologia’ for the first time. In it, physiology was first defined: ‘Physiology tells the causes of the actions of the body.’⁴⁰ Fernel distinguishes anatomy, which indicates only where processes take place, from physiology, which studies what the processes or functions of the various organs are. The book was renamed Physiologia in the 1554 edition, and was soon regarded as the major treatise on the subject, a position it held for more than a century.

Aquinas’ s influence on Fernel

Fernel’ s empirical observations, as well as his general reflections, are accommodated within the framework of late medieval Aristotelian thought as modified by Christian thinkers (in particular Thomas Aquinas’ s great synthesis). Like Aristotle, Fernel holds that plants and animals have a soul (anima ), or principle of life. Possession of a rational soul (i.e. a soul that includes the powers of the intellect and will) is distinctive of man. Unlike Aristotle, but like Aquinas, Fernel conceived of the rational soul of man (as distinct from his nutritive and sensitive soul) as separable from the body, and as immortal.⁴¹ It is, to be sure, far from clear whether the Aristotelian conception of the psuchē as an array of powers and capacities (second-order powers) – can be coherently married with Christian doctrines concerning the immortality of the soul. But that was precisely what Aquinas had endeavoured to do (and in the course of his endeavour, he had reified the intellect and confused the incorporeality of powers (which are abstractions) with the alleged incorporeality of the soul, conceived as a non-physical part of a human being).⁴² Other scholastic philosophers also contributed to the attempted synthesis of Christianity with Aristotelian philosophy. Fernel was heir to this confused tradition.

‘Physiology’ as the study of organ function: Fernel

Physiology, according to Fernel, is concerned with the processes that produce a healthy body and soul. He comments that ‘In all animate beings, and most in man, the body has been created for the sake of the soul (gratia animae ). It is for that soul not only a habitation (diversorium ) but an adjusted instrument for use by its (the soul’ s) inherent powers.’⁴³ This was an Aristotelian doctrine. As we have noted, the relation of the soul to the body, according to Aristotle, is analogous to the relation of sight to the eye. The eye exists for the sake of sight (DA 412^b17–24); that is its point and purpose. So, too, the body exists for the sake of the soul – that is, for the sake of the powers and capacities of which the soul consists. Without these, and their actualization in the behaviour of the living animal, there would be no point to the existence of the body (DA 415^b15–21; De Partibus Animalium 645^b19). The explanation of the actions of the parts of an organism must be in terms of their contribution to the optimal functioning of the whole of which they are the parts.

Fernel conceived of perception as produced by the transmission of images from the sense-organs to the common sensorium in the brain, where they are apprehended by the internal sense. Memory and imagination are two subordinate faculties of the sentient soul, and they enable the sentient animal to apprehend what is pleasant or unpleasant, beneficial or harmful. Appetite causes a movement towards a pleasing or beneficial object, or a movement away from a displeasing or harmful one. This is effected by the contraction of the brain forcing the animal spirits from the front ventricle into the fourth (rear) ventricle, and thence down the spinal cord and out along the nerves into the muscles.

The idea of an organ (muscle) reflex

All this was received doctrine. What is important for our present concerns is Fernel’ s observation that some of our acts occur without the action of the will or intent or any other directive of the mind. Such behaviour, he held, is exemplified by certain movements of the eyes and eyelids, of the head and hands during sleep, as well as by the movements associated with breathing. According to Fernel, these muscular movements do not involve an act of will, and therefore can be regarded as reflexes. Fernel was emphatic that muscular movement can occur without the will initiating a voluntary act; that is, there are motor acts in which thinking plays no part.⁴⁴ This insight marks the beginning of an investigation that was completed only with the work of Sherrington in the twentieth century.

Physiologia went through several editions and had an influence that lasted for a century. However, it could not continue as the definitive text in physiology beyond the middle of the seventeenth century, inasmuch as the Aristotelian concepts and conceptions on which it was based were no longer held to be viable. What, above all, made them obsolete was the rise of Keplerian physical astronomy and Galilean mechanistic physics. The spectacular success of the new physics led to the rapid demise of Aristotelian teleological science, and to the replacement of teleological explanations of natural phenomena by mechanistic explanations. This was equally evident in the advances in the biological sciences as in those of the physical sciences. First, Harvey showed that the heart was a mechanical pump. Second, Descartes argued persuasively that the activities of the body, the subject matter of physiology, could be considered in purely mechanical terms.

Descartes: the beginning of the end of the ventricular doctrine

Descartes (1596–1650) marks a profound upheaval in European thought. Although some aspects of his philosophy are still rooted in scholastic Aristotelian thought (and others in Augustinian thought), the novelty of his philosophical reflections is the starting point for modern philosophy. Much of his neuroscientific research proved wrong, but it provided a crucial impetus and shift of direction for neuroscience. Descartes agreed with the Aristotelian scholastics that the intellect can operate independently of the body, that the soul or mind is incorporeal, that it can exist independently of the body, and that it is immortal. However, he broke with them radically over the following four matters.

Four marks of the Cartesiantransformation of the concep-tion of the mind or soul

First, he held that the mind is the whole soul. The scholastics, by contrast, conceived of the mind (understood as the intellect) as merely a part of the soul (the immortal part that is separable from the body). The other parts of the soul – namely, the nutritive and sensitive functions – are to be conceived, according to the scholastics, in Aristotelian fashion, as the form of the body. Descartes disagreed radically. Unlike Aristotle, he conceived of the soul not as the principle of life, but as the principle of thought or consciousness. The functions of the Aristotelian nutritive soul (nutrition, growth, reproduction) and of the sensitive soul (perception, physiologically conceived, and locomotion) are not essential functions of the Cartesian mind, but of the body. All the essential functions of animal life are to be conceived in purely mechanistic terms. This was to have profound effects on the further development of neurophysiology.

Second, Descartes redrew the boundaries of the mental. The essence of the Cartesian mind is not that of the scholastic-Aristotelian rational soul – that is, intellect alone – but rather thought or consciousness. A person is essentially a res cogitans, a thinking thing – and Descartes extended the concepts of thought and thinking far beyond anything that Aristotle or the scholastics would have ascribed to the rational soul. The functions of the rational soul, according to the scholastics, included the ratiocinative functions of the intellect and the deliberative-volitional functions of the will (rational desire), but excluded sensation and perception, imagination and animal appetite (concupiscence). By contrast, Descartes understood thought as including ‘everything which we are aware of as happening within us, in so far as we have awareness of it. Hence thinking is to be identified here not merely with understanding, willing, imagining, but also with sensory awareness.’⁴⁵ Thought, therefore, was, in a revolutionary step, defined in terms of consciousness – that is, as that of which we are immediately aware within us. And consciousness was thereby assimilated to self-consciousness inasmuch as it was held to be impossible to think and to have experiences (to feel pain, seem to perceive, feel passions, will, imagine, cogitate) without knowing or being aware that one does. The identification of the mental with consciousness remains with us to this day, and casts a long shadow over neuroscientific reflection (we shall investigate the contemporary debate concerning consciousness and self-consciousness in Part III).

Third, he held that the union of the mind with the body, though ‘intimate’, is a union of two distinct substances. Contrary to scholastic thought, according to which a human being is a unitary substance (an ens per se ), Descartes intimated that a human being is not an individual substance, but a composite entity. The person (the ego ), on the other hand, is an individual substance, and is identical with the mind. To be sure, because the human mind is united with the body, it has perceptions (psychologically understood). But perceptions thus understood are conceived of as modes of thought or consciousness, produced by the union of the mind with the body. Indeed, it is precisely by reference to the intimate union of mind and body that Descartes explained the non-mechanical perceptual qualities (i.e. colours, sounds, tastes, smells, warmth, etc.) as being produced in the mind in the form of ideas consequent upon psychophysical interaction. Similarly, the mind, because it is united with the body, can bring about movements of the body through acts of will. Hence neuroscience must investigate the forms of interaction between the mind and the brain that produce sensation, perception and imagination (which are ‘confused’ forms of thought), on the one hand, and voluntary movement on the other.

Fourth, just as he conceived of the mind as having a single essential property – namely, thought – so too he conceived of matter as having a single essential property – namely, extension. He conceived of the principles of explanation in the physical and biological sciences alike as purely mechanical, save in the case of the neuropsychology of human beings, who are unique in nature in possessing a mind.

Descartes contributed substantially to advances in neurophysiology and visual theory.⁴⁶ Although his theories proved to be largely wrong, they were essential steps on the path to a correct understanding. Moreover, his conviction that fundamental biological explanation at the neurophysiological level will be in terms of efficient causation has been triumphantly vindicated by the development of neurophysiology since the seventeenth century.⁴⁷

The ventricles conceived as the source of ‘animal spirits’; animal spirits con-ceived as neural transmitters

Descartes replaced the conception of Aristotle, and in particular that of Galen, in which the psychic pneuma was generated in the ventricles, with the hypothesis that the ventricles are the site of generation of corpuscles or particles that directly participate in mechanical phenomena. These are the animal spirits that are conducted by nerves and transmitted to muscle cells, and so effect action. As to the origin of these corpuscles: ‘The parts of the blood which penetrate as far as the brain serve not only to nourish and sustain its substance, but also and primarily to produce in it a certain very fine wind (that is “composed of very small, fast-moving particles”), or rather a very lively and pure flame, which is called the animal spirits.’⁴⁸ This is an unfortunate name as it is not an apt descriptive term for components of a mechanical theory, for the word ‘spirit’ can be interpreted as a principle of life that animates the body or as the active principle of a substance extracted as a liquid. However, Descartes is quite explicit that ‘animal spirits’ are material: namely, ‘a certain very fine air or wind,’⁴⁹ and that

what I am calling ‘spirits’ here are merely bodies; they have no property other than that of being extremely small bodies which move very quickly, like the jets of flame that come from a torch. They never stop in any place, and as some of them enter the brain’ s cavities, others leave it through the pores in its substance. These pores conduct them into the nerves, and then to the muscles. In this way the animal spirits move the body in all the various ways it can be moved.⁵⁰

Descartes thus argued that the flow of animal spirits from the ventricles (in the case of motor action) involved the opening of particular valves in the walls of the ventricles, with a consequent flow of spirits into the appropriate motor nerve and contraction of muscle. In the case of involuntary behaviour associated with, for example, a pinprick, this would lead to a tension on just those filaments which open the appropriate valves in the walls of the ventricles to release the animal spirits into the motor nerves that contract the muscle to move the limb away from the point of the indentation.

Transmission involves inhibitory and excitatory processes

Descartes used the word ‘reflex’ only once in developing his conception of non-human animals as automata, although it is implied throughout his descriptions of animal behaviour and human non-volitional reactions. Although Descartes does not quote Fernel’ s Physiologia in his Treatise on Man, it is clear that his development of the doctrine of mindless motor behaviour in humans and animals has for its foundations the concept of the reflex first enunciated by Fernel.⁵¹ Treatise on Man argues that such motor behaviour requires not only an excitatory process, but also an inhibitory one, a speculation that was later to be confirmed experimentally by Sherrington and analysed at the cellular level by his student John Eccles. Descartes then argued that the excitatory and inhibitory processes, when acting together, allow animals and the bodies of human beings (when functioning independently of the intervention of the mind) to be described as automatons.

These suggestions of Descartes’s about the mechanism of reflex or involuntary movement, involving as they do the animal spirits stored in the ventricles, raise the question of the mechanism of voluntary movement. Here Descartes departed fundamentally from the ventricular doctrine. He denied that the ventricles are the seat of the sensitive and rational (including volitional) powers of human beings. He also denied that non-human animals have any sensitive powers in the sense in which human beings do, inasmuch as they lack consciousness. And he held that the human mind or soul interacts with the body in the pineal gland, which he incorrectly placed inside the ventricles.⁵²

The pineal gland as locus for the sensus communis and point of interaction of mind and body

It is interesting to note the reason (or part of the reason) why Descartes concluded that the pineal gland is the locus of the sensus communis and of interaction between the body and the soul. It was because it is located between the two hemispheres of the brain and is not itself bifurcated. Consequently, he reasoned, it must be in the pineal gland that ‘the two images coming from a single object through the two eyes, or the two impressions coming from a single object through the double organs of any other sense [e.g. hands or ears] can come together in a single image or impression before reaching the soul, so that they do not present it with two objects instead of one.’⁵³ These images or figures ‘which are traced in the spirits on the surface of the gland’ are ‘the forms of images which the rational soul united to this machine [i.e. the body] will consider directly when it imagines some object or perceives it by the senses’.⁵⁴

It is noteworthy that Descartes warned that although the image generated on the pineal gland does bear some resemblance to its cause (immediately, the retinal excitation; mediately, the object perceived), the resultant sensory perception is not caused by the resemblance. For, as he observed, that would require ‘yet other eyes within our brain with which we could perceive it’.⁵⁵ Rather, it is the movements composing the image on the pineal gland which, by acting directly on the soul, cause it to have the corresponding perception.

Descartes’ s conceptual error of attributing seeing to the soul rather than to the person or living being

The warning was apt, but the caution insufficient. Descartes was, of course, wrong to identify the pineal gland as the locus of a sensus communis, and wrong to think that an image corresponding to the retinal image (and hence to what is seen) is reconstituted in the brain. These are factual errors, and it is noteworthy that they still have analogues in current neuroscientific thought – in particular, in the common characterization of the so-called binding problem (discussed below, §5.2.3). But Descartes was right to caution that whatever occurs in the brain that enables us to see whatever we see, our seeing cannot be explained by reference to observation of such brain events or configurations. For, as he rightly observed, that would require ‘yet other eyes within our brain’. Nevertheless, he was confused – conceptually confused – to suggest

1 that images or impressions coming from double organs of sense must be united in the brain to form a single representation in order that the soul should not be presented with two objects instead of one;

2 that the soul ‘considers directly’ the forms or images in the brain when it perceives an object; and

3 that it is the soul, rather than the living animal (human being), that perceives.The first error presupposes precisely what he had warned against, for only if the images or impressions were actually perceived by the soul would there be any reason to suppose that the ‘two images’ would result in double vision or double hearing. The second error is the incoherence of supposing that in the course of perceiving, the soul or mind ‘considers’ anything whatsoever (no matter whether forms or images) in the brain. And the third is the error of supposing that it is the soul or mind that perceives. (It is instructive to contrast this idea with Aristotles’ denial that it is the psuchē that is angry, pities, learns or thinks.) We have already noted the much earlier occurrence of this confusion in Nemesius. The mistake is a form of a mereological fallacy (mereology being the logic of the relations between parts and wholes). For it consists in ascribing to a part of a creature attributes which logically can be ascribed only to the creature as a whole.⁵⁶ The particular form which this mereological fallacy took in Descartes consisted in ascribing to the soul attributes which can be ascribed only to the whole animal. We shall discuss this matter in detail in chapter 3.

By the middle of the seventeenth century, Descartes had replaced the ventricular doctrine localizing psychological functions in the ventricles of the brain with his idiosyncratic interactionist doctrine localizing all psychological functions in the pineal gland, which he conceived to be the point of interaction between mind and brain. This is how he met Vesalius’ s objection that it is difficult to reconcile the idea that the different ventricles are associated with different cognitive and cogitative powers when the ventricles of humans are so similar to those of other mammals. Furthermore, he had replaced psychic pneuma by animal spirits as the medium through which the pineal gland produces its effects. This amounted to replacing the fluid derived from the pneuma described by Aristotle with mechanical corpuscles that possessed special properties. However, his contemporaries were soon to point out that the pineal gland is not inside the ventricles and, furthermore, that since other mammals too possessed this gland, his response to Vesalius was inadequate.

Descartes’ s primary contribution

Nevertheless, Descartes had made the fundamental contribution of opening up all animal activity to mechanical analysis – that is, to what became physiology and neuroscience. Furthermore, by associating the psychological capacities of humans with the pineal gland, he had moved consideration of their physical dependence from the ventricles (filled now with particles of the animal spirits rather than with the psychic pneuma of Galen) to the matter of the brain – in his case the pineal gland. This shift of attention from the ventricles to the substance of the brain became definitive as a result of the work of a young man who was but twenty-nine when Descartes died, Thomas Willis.