Part 29

A very important figure in the scientific world of the thirties and forties of the nineteenth century was the German Justus von Liebig (1803-73), professor of Chemistry at Giessen. He was a convinced mechanist, and over the door of the University Laboratory which he founded he had inscribed the dictum God has ordered all His Creation by Weight and Measure. His great achievement was his application of chemical knowledge to physiology. He did much to introduce laboratory teaching, and certain apparatus which he invented is still in constant use.

Liebig greatly improved the methods of organic analysis and notably he introduced a method for determining the amount of urea in a solution. This substance is found in human blood and urine, and was the first organic compound to be ‘synthetized’, that is to say, built up from inorganic materials. It is of very great physiological importance. This is due to the fact that it is regularly formed in the body in the process of breaking down the characteristic nitrogenous substances known as proteins. Along with his colleague, Friedrich Wöhler (1800-82), who had already synthetized urea, Liebig wrote a famous paper (1832) in which he showed, for the first time, that a complex organic group of atoms--or ‘radicle’ as it is called--is capable of forming an unchanging constituent through a long series of compounds, behaving throughout as though it were an element. This discovery is of primary importance for our conceptions of the chemical changes in the living body.

From 1838 onwards, Liebig devoted himself to attempting a chemical elucidation of living processes. In the course of his investigations he did pioneer work along many lines that have since become well recognized. He taught the true doctrine, then little recognized, that animal heat is the result of combustion, and is not ‘innate’ (compare p. 156). He classified articles of food with reference to the functions that he conceived they fulfilled in the animal economy. An outcome of this was his food for infants and his extract of meat. Very important was his teaching that plants derive the constituents of their food, their carbon and nitrogen, from the carbon dioxide and ammonia in the atmosphere, and that these compounds are returned by the plants to the atmosphere in the process of putrefaction. This discovery made possible a philosophical conception of a sort of ‘circulation’ in Nature. That which is broken down is constantly built up, to be later broken down again. Thus the wheel of Life goes on, the motor power being energy from without, derived ultimately from the heat of the sun.

It was very unfortunate that Liebig conceived and adhered to a totally wrong view of the nature of putrefaction and fermentation, which it took Pasteur long years to displace.

(c) Nervous Physiology in the Earlier Nineteenth Century.

From Chemical Physiology we turn to glance at the knowledge of the Nervous System. Charles Bell and his contemporaries (p. 145) had attained to a clear distinction of the nature of motor and sensory nerves and their separate origin from the two spinal roots (Fig. 98). The next fundamental contribution was made by Marshall Hall (1790-1857), who established the difference between volitional action and unconscious reflex (1833).

The fundamental ideas in the conception of reflex action had already been adumbrated by Descartes. In the view of that philosopher, any stimulus is transmitted along nerve-fibers to the central nervous system. There, on account of existing nervous connections, it gives rise to a fresh impulse which passes along outgoing nerve-fibers to the active organ, muscle, or gland, which is thereby excited to activity (p. 128). Thus, every action of the organism, and its life as a whole, conforms to definite laws. These laws must be directed to its preservation, or organisms would cease to exist. It is thus possible to look on organisms simply as elaborate mechanisms. Except that we know that we ourselves think and feel, we might eliminate mind from our consideration of the action of beings other than ourselves. Such was the view taken by the mechanists and other members of the iatro-physical school (pp. 127-131), which followed, to a greater or less extent, the teaching of Descartes. The course of physiological advance may be described, briefly, as the expulsion of the mental element from process after process associated with vital activity. This avenue leads on to a philosophical discussion whither we shall not now follow. It will suffice, at the moment, to remind the reader that only through the channel of his own thinking and feeling is he able to follow these physiological discussions at all.

The conceptions of Descartes and of his successors were greatly extended by Marshall Hall. Interest was lent to Hall’s work by the contemporary discovery by French observers of what was regarded as a special center governing respiration--a very important reflex--in the lower part of the brain. Hall’s work gave ‘reflex action’ a permanent place in Physiology.

An afferent impression from a sense organ to the spinal cord may give rise to an efferent impulse by a purely intra-spinal process. This impulse may be of the nature of a complex and balanced muscular act involving a whole system of muscles, some of which may be antagonistic to each other. All this may take place not only unconsciously, without any intervention from the higher nerve-centers in the brain, but even in an animal from which the brain has been removed. On the other hand, channels exist (and are indicated in the diagram) for passage of impressions to and impulses from higher centers. These higher centers in many cases control or modify the resulting muscular or other action to a greater or less degree.

Since Hall’s time there has been a great extension of the conception of reflexes. It has been shown that, besides the simple nervous arc (Fig. 99), there are more complex nervous arcs which depend for their action on the integrity of an elaborate mechanism. The nervous system is ‘integrated’ under higher and higher centers, till at last the highest centers of the brain are reached (pp. 308-11). Many of the ordinary acts of life, sneezing, coughing, standing, walking, even breathing, are expressible as reflexes. The attempt has also been made to press the ‘instincts’ into the same category. But it is difficult to separate the instinctive from the volitional elements or to define either. Vast, therefore, as is the development of this department of physiology, it is a very delicate task for the historian to pass any verdict upon it. The ultimate value of all this work must depend upon the conception that the next generation attaches to the mental element in vital phenomena. There is evidence of reaction at the present time from the extreme mechanist physiological position.

Lastly, in the discussion of work on the nervous system comes the question of the localization of functions of the brain. The possibility of such localization is a very ancient speculation. The idea was developed along rational lines, in the first third of the nineteenth century, by certain Viennese workers who, having made important contributions to science, unfortunately afterwards degenerated into phrenological quacks. Later several German observers began the study of the electrical excitation of those parts of the cortex of the brain which specially control movement (Fig. 100). The work was continued and developed by a number of distinguished French and English observers, among whom Paul Broca (1824-80), Hughlings Jackson (1834-1911), and Sir David Ferrier (1843-) should be commemorated. Under their influence many operations usually regarded as involving complex mental processes, such as vision, speech, reading, writing, drawing, have been represented as depending on simple nervous relationships. Centers for the initiation of these operations have been described. Of late years, there has been reaction from this mechanical conception of the brain as an organ of the mind. The older school has, however, achieved clinical success especially at the hands of the great French physician Jean Marie Charcot (1825-93) and his pupils.

§ 4. The Experimental Foundations of Modern Medicine.

We may turn back to consider those who have been the immediate progenitors of modern Physiology. Among these, three men stand out beyond all others. In order of seniority, and perhaps of genius, they are Johannes Müller, Claude Bernard, and Karl Ludwig.

(a) The Work of Johannes Müller.

Johannes Müller (1801-58) was the greatest physiologist Germany has produced, and perhaps the greatest physiologist of all time. His genius was of the universal type and, despite his early death, he attained equal distinction in every department which he touched. Among these were Comparative Anatomy, Embryology, Physiological Chemistry, Psychology and Pathology. He was a careful scholar, well versed in the history of the subjects which he taught, and as great a teacher as he was an investigator. A very large number of the best-known men who have advanced Medicine during the nineteenth century were his pupils while he was a professor at Berlin. His lovable character was pervaded by a mystical tendency.

Müller’s text-book of Physiology began to appear in 1834. It introduced into the subject the comparative and psychological points of view, which were not fully appreciated until the generation that followed. The most remarkable generalization associated with his name--and one further developed by Ewald Hering (1834-1918)--is the ‘Law of Specific Energies’. According to this law each sensory nerve, however stimulated, gives rise to its own specific sensation and to no other. Conversely, the same stimulus applied to different organs of sense produces a different sensation in each organ--that sensation, in fact, that is its specific attribute. Thus electrical, mechanical, thermal stimulation produce only the sensation of light when applied to the optic nerve. On the other hand, any particular form of stimulation, for example electrical, produces sensations of light, smell, hearing or taste if applied to the appropriate nerves.

A moment’s reflection will enable the reader to realize the very great philosophical importance of these conclusions. They provide experimental evidence that the things of the external world are not in themselves discernible by us. Such external things we know only by the events to which they give rise acting on our senses, and yet from one and the same event utterly different sensations arise within us. To beings with senses different from ours the world also would be different. The ‘Law of Specific Energies’ is thus fundamental for our view as to the range of validity of Scientific Method.

Among other important contributions of Johannes Müller to the physiology of the nervous system were his experimental confirmation of Bell’s researches on the spinal roots (p. 145) and his experiments on the production of the voice. He launched important theories in explanation of color vision, of the mechanism of hearing, and of the phenomena of fever. He was one of the first to use the microscope in pathology and he was one of the founders of Physiological Chemistry.

Like every investigator Müller made mistakes. In 1840 he stated that the velocity of a nervous impulse could never be measured. By 1852 his gifted pupil, Hermann von Helmholtz (1821-94), had measured it. Much of Helmholtz’s work hardly comes within our department. He was, however, inventor of the instrument known as the Ophthalmoscope, by means of which the interior of the eye can be examined. This is the main factor which has enabled Ophthalmology to develop along true scientific lines (p. 319).

(b) The Work of Claude Bernard.

Claude Bernard (1813-78), the great French physiologist, was Müller’s junior by twelve years and was in almost every respect a contrast to him. His mind was of that peculiarly French type to which anything mystical is abhorrent. He had few eminent pupils who owed much to him directly, but the influence of his ideas, through his writings, can hardly be exaggerated. Especially Bernard was the founder of ‘Experimental Medicine’, that is of the artificial production of disease by chemical and physical means. This is one of the most important scientific movements within our field.

Bernard’s great discovery, which occupied him for over ten years, was that the liver has the power of building up and storing certain highly complex substances, derived from the food and brought to it by the blood. The substances thus stored, and notably that known as glycogen, are distributed to the body according to its needs, in simplified and modified form. Now Wöhler in 1828 had synthetized urea (p. 206) and it was well recognized that this substance is a final degradation product which the body manufactures by breaking down the substances derived from food. It was also recognized that from this breaking-down process the bodily energy is obtained. Bernard showed that the body could build up complex chemical substances as well as break them down. This destroyed the conception, then still dominant, that the body could be regarded as a bundle of organs, each with its appropriate and separate functions. Bernard thus introduced what we may call a ‘Physiological Synthesis’, a conception of great import for the development of medical ideas.

No less important, and bearing on the synthetic view of the working of the animal body, was Bernard’s work on the physiology of digestion. Up to the time of Bernard, an elementary knowledge of the facts of digestion in the stomach constituted the whole of digestive physiology. While Bernard was working on the glycogenic function of the liver, another worker had suggested that the secretion of the organ known as the ‘pancreas’, or sweetbread, emulsifies fats. Soon after, a German researcher showed that pancreatic juice acts on starch. Bernard now stepped in and cleared up the whole subject. He showed that digestion in the stomach is, as he described it, ‘only a preparatory act’. He proceeded to demonstrate that the pancreatic juice, passing into the intestine, emulsifies the fatty food substances there and splits them up into fatty acids and glycerin. He further demonstrated the power of the pancreatic juice to convert starch into sugar, and he showed that it has a solvent action on such ‘proteids’ or organic nitrogenous substances as have not been dissolved in the stomach.

The third great achievement of Bernard was his exposition of how the blood-supply to the different parts of the body is regulated. This we now call the ‘Vaso-Motor Mechanism’. In 1840 the existence of muscle fibers in the coats of the smaller arteries was discovered. Bernard showed that the contraction and expansion of the ‘arterioles’ is associated with a complex nervous apparatus. The reactions of this apparatus depend upon a variety of circumstances in a variety of other organs; again an illustration of the close and complex interdependence of the various functions of the body upon each other.

(c) The Work of Karl Ludwig.

Karl Ludwig (1816-95) held a series of professorships at Marburg, Zürich, Vienna and Leipzig. He was, after Müller, the greatest of German physiological teachers, and he surpassed even Müller in the number of his pupils. As a physiologist he was chiefly remarkable for his ingenuity as an inventor, for his wide and deep knowledge of the physical sciences and for his extreme generosity in handing over his work to his pupils.

Among the many lines of investigation of fundamental importance which Ludwig initiated, some of the most remarkable depended on the discovery of new methods. Just as the microscope had opened to the anatomist unexplored fields of research by bringing him into closer relation with objects which were hitherto beyond his scrutiny, so the rapid progress of physics and chemistry had placed more exact modes of observation and of measurement within reach of the physiologist. But the application of these methods was attended with great difficulty; there was no physiological laboratories, no instruments, no capable mechanicians to whom the physiologist could apply for assistance. Under such conditions, ingenuity and resource were indispensable to success, and in these qualities Ludwig was pre-eminent.

Accordingly, we find that two of the most important of the early investigations of Ludwig were as much due to his ingenuity as an inventor as to his clear grasp of the physiological questions which his inventions were intended to elucidate. The most interesting of these inventions, or rather adaptations, is the mechanically rotating drum or kymograph, as it is called. The word itself is derived from two Greek words which mean ‘wave writer’. This instrument is now widely used, not only in Physiology but in every department of Science in which permanent records of any continuous movement are desired. The most familiar instance is the self-recording barometer. The kymograph--the use of which had been suggested by Thomas Young (p. 319, and Fig. 101) in 1807--led to much wider applications of the method of automatic record. Ludwig himself applied it to indicate the movements of respiration, as well as the variations in arterial pressure. Subsequently it became further adapted to the ‘graphic method’, and it serves not only for the investigation of animal movements of every conceivable kind, but even for the transient and delicate electrical changes which are associated with vital action.