Geschichte der deutschen Neurochirurgie
A brief history of neurosurgery in Germany
Hartmut Collmann, Reichenberg – Jörg Skrzypczak, Leipzig
Since the middle of the 19th century, many nations have contributed to the foundations of modern surgery: general anesthesia was introduced in the United States in 1846 (Warren) and antisepsis for the prevention of wound infection in Scotland in 1867 (Lister). The theory of the topical organization of brain function was proved in Germany in 1871 (Fritsch & Hitzig) and in Scotland in 1881 (Ferrier).
Ernst von Bergmann
In Germany, surgeons became obliged to carry out brain surgery because of the large number of gunshot wounds caused by the newly developed military weapons. Ernst von Bergmann (1836-1907), who may be regarded as the father of modern German surgery, was primarily a military surgeon.
Born in Riga, Latvia, he acquired his surgical experience on numerous battlefields before being appointed Professor in Tartu (Dorpat), Estonia, in 1871. After 1878, he held the Chair of Surgery at the University of Würzburg and, in 1882, succeeded Bernhard von Langenbeck at the University Hospital in Berlin. He was the first German to carry out an intensive study of the surgical aspects of brain diseases. He soon adopted Listerâ€™s principle of antisepsis and, in 1886, together with his brilliant assistant, Curt Schimmelbusch, introduced steam sterilization. This subsequently underpinned the principle of asepsis. His military experience taught him that increased intracranial pressure constituted a specific consequence of brain trauma. In his book on brain injuries (“Die Lehre von den Kopfverletzungenâ€, 1880), he discussed these problems in detail. Over the next few years, he also studied non-traumatic brain diseases, which he reviewed in his book “Die chirurgische Behandlung der Hirnkrankheitenâ€ (1889). The extremely poor results achieved in tumor surgery led him to discourage this kind of activity because of unsolved problems such as blood loss and brain edema. His warnings brought to a temporary end a period of unbridled enthusiasm for brain surgery.
Independently from Bergmann, Fedor Krause (1857-1937) soon became the true pioneer of German neurosurgery. Brain surgery became his main field of activity, although he remained active as a general surgeon, covering the whole surgical spectrum. Krause had initially studied music – he was an excellent pianist – but he eventually changed to medicine. He received his surgical training with Richard von Volkmann in Halle, where he contributed to developments in plastic surgery with the “Krause-Lappenâ€ (Krause-Wolfe graft), an unattached full-thickness skin graft.
In 1892, after gaining his postdoctoral qualification, he was appointed Chief Surgeon in Hamburg-Altona and, in 1900, moved to Berlin to become Chief Surgeon at the Augusta Hospital, where he remained until his retirement in 1921. Krause was a highly gifted surgeon who pioneered several surgical approaches to deep-seated areas of the intracranial space. In 1893, shortly after, but independently from, the American Frank Hartley, he devised the extradural approach to the Gasserian ganglion in cases of trigeminal neuralgia. For craniotomy, he adopted the technique of a pediculated bone flap devised by Wilhelm Wagner in 1889. This technique, which ensures bone healing, has survived to this day. In 1898, he developed a method for exposing the cerebellopontine angle and, in 1900, the subfrontal approach to the sellar region which he first used to remove a bullet from the brain of a man who had shot himself because of unrequited love. In this case, he took advantage of the new x-ray technique which he had introduced into routine use as early as a few months after Röntgenâ€™s discovery. In 1908, he was probably the first surgeon to successfully remove a lumbar disc hernia, which he erroneously confused with a tumor, calling it an “enchondromaâ€.
Finally, in 1913, he developed the lateral infratentorial supracerebellar approach to the pineal region, the “Krause approachâ€. The technique was published by his partner in neurological science, Hermann Oppenheim.
In 1921, Krause was succeeded by Emil Heymann, who had been his senior assistant for many years. He was the only pupil of Krause to teach Krauseâ€™s neurosurgical methods to subsequent generations. He thus became the mentor of Georg Merrem, the founder of neurosurgery in the German Democratic Republic. Heymann (1878-1936) was a highly talented surgeon who was expelled by the Nazi regime because of his Jewish origins. Just before his planned emigration to South America, he died following a heart attack. Unfortunately, because of their poor language skills, neither Krause nor Heymann gained extensive international recognition, although many specialists from abroad visited their department. However, in Germany, Krauseâ€™s two-volume textbook on neurosurgery (1908-1911) remained a standard reference work for decades.
As neuroradiology did not exist in Krauseâ€™s time, and as he was not trained in neurology, he had to rely on his neurologist partner, Hermann Oppenheim, and his colleagues to identify where he should open the skull or the spinal canal. The topical diagnostics facilitated by detailed neurological examination made rapid progress. One of the major contributors to this development was Otfrid Foerster, some sixteen years younger than Krause.
After a two-year fellowship with Jules Dejerine and Joseph Babinski in Paris and, subsequently, with Heinrich Frenkel in Switzerland, Foerster (1873-1941) received his neurological training from Carl Wernicke in Breslau (now Wroclaw in Poland). In 1911, he became head of a small neurological unit at the municipal Wenzel-Hancke Hospital in Breslau.
While Krause may be regarded as a hands-on practitioner, Foerster was a neuroscientist who dedicated his professional life to functional neuro-anatomy. He made an extensive study of the motor and sensory tracts of the spinal cord, improved the description of the dermatomes and studied the effect of exercises on impaired nervous function, thereby promoting the idea of neuro-rehabilitation.
From his physiological knowledge, he deduced treatment strategies which he soon carried out by himself, although he had never been trained as a surgeon. Basing his methods on his studies of the motor reflex arc, he began to cut some dorsal roots to provide relief from spasticity. He then applied the same technique for treating the severe pain attacks caused by tabes dorsalis - a very common disease at that time! Some years later, he modified this kind of surgery by introducing the open dissection of the spinothalamic tract. This led to the invention of cordotomy at the same time as, although independently from, the Americans Spiller and Martin. During WW I, the lack of trained staff meant that he had to operate as a surgeon full-time. Over the course of a few years, he performed more than 500 nerve repairs, including autologous grafts.
After the war, he continued to extend his surgical activities on the central nervous system, in particular on tumor surgery. Like Krause, he also treated epilepsy. Using electrical cortical stimulation, he provided the basic data for what is now known as “brain mappingâ€.
By the mid-1920s, Foerster had become one of the most renowned neurologists in the world. He was, for example, a member of the team which treated Lenin after his stroke. Visitors from abroad, including Wilder Penfield, Percival Bailey and Paul Bucy, were fascinated by his academic skills. They also admired his delicate handling of nervous tissue during surgery, although they declared themselves shocked by his crude method of craniotomy, performed with a simple hand trephine. Like Krause, he used to divide brain operations into two separate sessions so as to allow the patient – as well as the surgeon – time to recover. In 1932, Foerster was invited to visit Yale by Harvey Cushing who appointed him “Surgeon-in-Chief pro temporeâ€.
Towards the end of his career, Foerster was able to move into a new building sponsored by the Rockefeller Foundation, the “Neurologisches Forschungsinstitutâ€ (Institute for Neurological Research) in Breslau, a building that still exists today. In 1935, together with Oswald Bumke, he edited a 17-volume Handbook of Neurology, which became a standard reference work for many years.
In 1940, Foerster and his wife fell ill with tuberculosis, to which he succumbed in June 1941. She died just two days later. Because of her Jewish origins, there was an unconfirmed rumor that she had committed suicide in order to escape torture and execution by the Nazis. Strangely enough, not a single one of the numerous contemporary obituaries mentioned either her or this remarkable coincidence.
Neurological surgery or surgical neurology?
Krause and Foerster may be regarded as the founders of the two different schools of neurosurgery described as “neurological surgeryâ€ and “surgical neurologyâ€. Foersterâ€™s example encouraged several neurological departments to run their own neurosurgical units. Foersterâ€™s senior staff members, Arist Stender, Ernst Klar, Helmut Penzholz and Hans Kuhlendahl, emphasized their neurological origins after they had become heads of newly built neurosurgical departments. Another pupil, Ludwig Guttmann, was persecuted by the Nazi regime and emigrated to England, where he became renowned for his work on the care of paraplegics and as the founder of the Paralympics.
Apart from Krause and Foerster, four pupils of Ernst von Bergmann deserve acknowledgement for their substantial contributions to the advances of neurological surgery. Friedrich Gustav (“Fritzâ€) von Bramann (1854-1913), senior assistant of Bergmann and, subsequently, Professor of Surgery in Halle, ignored the warnings of his teacher and achieved some spectacular results in brain tumor surgery. His name is mainly associated with what is known as the “Balkenstichâ€, an early (for some time even effective) type of ventriculostomy, created by puncturing the lateral ventricle via the interhemispheric fissure. His trainee, Alexander Stieda (1875-1966), also working in Halle, became one of the most experienced German brain surgeons during the 1930s. In 1906, Moritz Borchardt (1868-1948) was appointed Chief Surgeon at the Rudolf-Virchow Hospital. In 1920, he was appointed Head of the Third Surgical University Department at the Berlin-Moabit Hospital. He won international recognition for his early contributions to the removal of acoustic neurinomas and for the “Borchardt Pflugâ€, an electrical drill facilitating trepanation. Since he was a Jew, he was forced to emigrate by the Nazi regime. Nicolai Guleke (1878-1958) established a renowned surgical department in Jena, where surgery on the brain and spinal cord were paramount. His neurological partner was Hans Berger, the inventor of electroencephalography. Fritz König (1866-1952), who became Professor of Surgery in Würzburg in 1918, took a crucial step forward when he allowed his assistant, Wilhelm Tönnis, to devote himself exclusively to neurological surgery, thereby following the example set by Harvey Cushing.
Wilhelm Tönnis (1898–1978) was born in the Ruhr and, after 1924, spent his residency with König. He earned his PhD (“Habilitationâ€) for general surgery at the age of 31 and then applied for permission to specialize in neurosurgery. After an approach to Harvey Cushing had been rejected, König arranged for Tönnis to be granted a fellowship with his Swedish colleague, Herbert Olivecrona. Olivecrona himself had received some training from Erwin Payr in Leipzig and as a research fellow from the innovative Walter Dandy in Baltimore. Tönnis stayed seven months at the Serafimer Hospital in Stockholm. After his return to Würzburg, he was instantly able to lower the operative mortality rate in brain-tumor surgery from 30-60 % to around 10 %. This success was attributed not only to his exceptional surgical skills but also to the neuroradiological techniques (air encephalography and angiography) that he had learned in Stockholm.
In 1934, he was officially appointed Head of the first German Department for Neurosurgery in Würzburg. After König had retired, Tönnis had problems with his successor and looked for other facilities where he might continue his career. In 1937, with the support of Fritz König, he was appointed Associate Professor of Neurosurgery at the Charité Hospital in Berlin under Ferdinand Sauerbruch. Since the National Socialist regime had been established by then, this appointment went beyond the normal academic election procedure. Tönnis and his staff moved into the Hansa Klinik, the former neurological hospital of Friedrich Heinrich Lewy who, as a Jew, had been expelled. Also in 1937, Tönnis was appointed head of a sub-unit at the Kaiser Wilhelm Institute for Brain Research. In the same year, Tönnis became a member of the Nazi Party – as did some 50 per cent of German physicians and almost 70 per cent of brain surgeons. Other German neurosurgeons were ousted by the Nazi regime on racial grounds (because of their Jewish descent). Apart from the aforementioned Emil Heymann, Ludwig Guttmann and Moritz Borchardt this also applied to Bernhardt Badt (Berlin and Hamburg), Walter Lehmann (Universities of Göttingen and Frankfurt), Carl Felix List (Berlin), Alice Rosenstein (neurosurgeon and neuroradiologist with Karl Kleist in Frankfurt am Main) and Franz Schück (Chief Surgeon at the Urban Hospital in Berlin).
In 1936, in an attempt to establish neurosurgery as an international discipline, Tönnis founded the first neurosurgical journal in the world and arranged for an international editorial board to be appointed. He received additional support from the British Neurosurgical Society which, in 1937, decided to hold its annual summer conference with its German counterpart in Berlin and Breslau. The foundation of a national Neurosurgical Society, scheduled for October 1939, was cancelled because of the outbreak of World War II. Until then, neurosurgical activities at various levels had been undertaken at 20 different sites. Some of Tönnisâ€™ former trainees now headed their own neurosurgical units and thus represented a particular neurosurgical school.
During the war, Tönnis was consultant brain surgeon to the German Air Force and, later, to the German Army. He acquired wide experience in the treatment of brain injuries and, above all, won international recognition for establishing an air rescue system for the wounded. In addition, he promoted methods of physical rehabilitation.
Proliferation of neurosurgery in post-war West Germany
By the end of World War II, the Hansa Klinik had been destroyed, large swathes of eastern Germany had been lost and, since it was now located on Polish territory, Breslau had changed its name to Wroclaw. As a consequence of the war and of the moral catastrophe caused by the Nazi regime, Germany was divided into four occupation zones which, in 1949, developed into two separate states. For the next four decades, the antagonism between the two countries reflected the Cold War between the Western and the Eastern blocs.
During the early post-war years, in a period of reconstruction, a rapid proliferation of neurosurgical units took place, particularly in West Germany. In 1946, Tönnis was appointed Head of a large minersâ€™ surgical hospital, parts of which he assigned to a neurosurgical unit. He soon started to organize annual neurosurgical conferences and, in 1949, revitalized the “Zentralblatt für Neurochirurgieâ€, which had been closed down in 1943. In 1950, he initiated the foundation of a German Neurosurgical Society, something that he had originally scheduled for 1939. One year before that, he had been appointed to the first Chair of Neurosurgery in Cologne (Köln).
During the early post-war years, Tönnis and his team continued their research work on intracranial tumors. The surgical specimen, together with the diagnostic and follow-up data, provided the basis for a system of tumor classification which was created by his long-standing partner, Klaus Joachim Zülch, and finally led to the first WHO brain-tumor classification. Matters relating to neurotraumatology and intensive care were his other main fields of interest. In 1954, together with his mentor and friend, Herbert Olivecrona, he started work on a 12-volume handbook of neurosurgery, with articles written by internationally renowned neurosurgeons. The work was not completed until 20 years later. Tönnis deserves even greater credit for supporting neurosurgery in Germany which was, to a great extent, represented by his own former staff members such as Gerhard Okonek, Peter Röttgen, Erich Fischer-Brügge, Georg-Friedrich Häussler, Friedrich Loew, Kurt Schürmann, Hans Werner Pia, Wolfgang Schiefer, Werner Krenkel, Frank Marguth, Wilhelm Driesen and Reinhold Frowein.
In addition to the schools created by Tönnis and Foerster, a third branch is represented by Traugott Riechert, who had started his career with the neurologist Karl Kleist in Frankfurt but acquired most of his surgical experience at military hospitals during the war. In 1946, he was appointed to the second German Associate Chair of Neurosurgery. Riechert and his co-workers are renowned for the refinement of stereotaxy, but they also represent a separate neurosurgical school which includes Wilhelm Umbach, Fritz Mundinger, the first German pediatric neurosurgeon Robert Hemmer, Klaus Schmidt and Peter Christian Potthoff.
In 1956, board certification for neurosurgery was introduced in West Germany. The increasing importance of neurosurgery was reflected by the increase in the number of members of the Neurosurgical Society: from fewer than 50 members in the early 1950s, the number increased to 154 in 1962 before reaching a total of 631 members in 1990, the year of German reunification.
As another consequence of World War II and of the multiple crimes committed by German scientists, Germany was isolated from the international scientific community, and its reintegration took many years. In 1949, Arist Stender was allowed to publish a short historical article in the Journal of Neurosurgery, the journal which had been established as a response to the ban on the “Zentralblatt für Neurochirurgieâ€. In 1951, he was probably the first German to be invited to visit the United States (according to the personal testimony of Ludwig Kempe). Finally, in 1955, he joined the founding committee of the WFNS.
Tönnis and his team members consistently pursued the idea of establishing international cooperation in Europe. Their zeal was shared by representatives of other European countries such as the Frenchman Marcel David and Ferdinand Verbeek from the Netherlands. Eventually, the first European scientific conference was held in Zurich in 1959. In the early 1950s, Fritz Loew joined the editorial board of “Acta neurochirurgicaâ€, which had been founded in 1950 by the Italian Mario Miletti and the Austrian Wolfram Sorgo as an international neurosurgical journal. Loew remained Editor-in-Chief for four decades, during which time he established the “Actaâ€ as the official organ of the EANS. Hans Werner Pia was elected the first President of the EANS Training Committee. Karl-August Bushe, another of Tönnisâ€™ protégés, became a founder member of the European as well as of the International Society for Pediatric Neurosurgery.
Neurosurgery in the German Democratic Republic (GDR)
In East Germany, Georg Merrem played the same role as Tönnis had in the West when, in 1949, he was called by Arwed Pfeifer to establish a neurosurgical unit at the Neurology Department in Leipzig. Merrem had been trained by Emil Heymann and his successor, Carl Max Behrend, and so became the only surgeon to maintain the legacy of the neurosurgical school established by Fedor Krause. In 1959, he was appointed to the first and, indeed, only Chair of Neurosurgery in the GDR. His staff members ensured the spread of neurosurgery in the GDR, among them Günther Niebeling, Friedrich Weickmann, Horst Fried and Siegfried Krumbholz. In 1960, together with his son-in-law, Wolf-Eberhard Goldhahn, Merrem wrote a widely read textbook and, in 1966, an atlas of modern neurosurgical techniques.
Apart from Merrem and his staff members, several general surgeons became active in neurosurgery after having received some specialized training. These include Willi Felix, Werner Usbeck, and Hans Joachim Serfling. In their own departments, Serfling and Usbeck established neurosurgical sub-units managed by their senior assistants, Rudolf Reinhold Unger and Joachim Reichel respectively. In Magdeburg, Werner Lembcke, a certified neurologist and general surgeon, focused on neurosurgery. In addition, he introduced the emergency ambulance system as early as 1960.
In 1962, Merrem was elected President of a newly founded separate Association of Neurosurgeons in the GDR (“Vereinigung der Neurochirurgen in der DDRâ€), after the East German neurosurgeons had been forced by the political authorities to quit the joint German Neurosurgical Society. This event reflected the increasing isolation of the East German state during the Cold War. As a reminder: in 1961, the Berlin Wall had been erected in order to stem the everincreasing exodus of refugees (totaling 2.7 million). In 1963, the first meeting of the new East German Neurosurgical Society was held in Leipzig.
Compared with the situation in West Germany, the development of neurosurgery in the GDR faced considerable difficulties for various reasons. Firstly, many general surgeons persistently and successfully opposed surgical sub-specialization. Board certification for neurosurgery was introduced in 1955 and required a five-year training period which included a one-year course in general surgery and six months in neurology. However, in 1967, for hitherto unknown reasons, but probably at the instigation of the general surgeons, board certification for neurosurgery was cancelled, only to be re-established in 1974. At that time, 37 certified neurosurgeons were working in eight neurosurgical units and were responsible for a population of nearly 16 million people. By 1990, the number had increased to some 50 full-time neurosurgeons working in 13 units. They were affiliated to five universities (Berlin-Charité, Greifswald, Halle, Jena and Leipzig), three medical academies (Dresden, Erfurt and Magdeburg), four municipal hospitals (Berlin-Buch, Berlin-Friedrichshain, Chemnitz and Schwerin) and a military hospital (Bad Saarow). Apart from the single ordinary chair in Leipzig and the independent municipal department in Berlin-Buch, neurosurgical units served as sub-divisions of surgical departments. Prior to reunification, the “Gesellschaft für Neurochirurgie der DDRâ€ consisted of 130 members, including 50 certified neurosurgeons. At the same time, the West German Neurosurgical Society consisted of 630 members and 330 certified neurosurgeons affiliated to 85 units and caring for some 62 million people.
A second problem resulted from the policy of the East German regime which insisted on isolating itself from West Germany, for instance by imposing severe travel restrictions on its own citizens. Accordingly, most East German scholars remained cut off from large parts of the international scientific community. In 1976, at the request of the GDR authorities, the West German members of the editorial board of the “Zentralblatt für Neurochirurgieâ€ were dismissed. Thereafter, the number of contributions to this journal from Western “non-socialistâ€ countries decreased dramatically, and “The Neurosurgical Reviewâ€ was launched as a West German substitute. Nevertheless, East German scientists could easily become members of international neurosurgical organizations. In particular, the European Association of Neurosurgical Societies (EANS) made every effort to integrate all European countries into its organization. Consequently, in 1983, Rudolf Unger was selected to organize the 1989 European Training Course to be held in Berlin. In the event, the course was transferred to Rostock-Warnemünde and was chaired by Ungerâ€™s successor, Günter Lang.
Thirdly, because of the imposition of swingeing war reparations, its centrally planned economic system and its political and economic dependence on the Soviet Union, East Germany suffered from a shortage of foreign currency. That resulted in the supply of goods and hi-tech equipment from Western countries being limited. More importantly, a shortage of staff constituted a decade-long problem with the result that a significant number of neurosurgical beds remained empty. Despite these difficulties, daily health care was provided on an international level. Furthermore, East German neurosurgeons achieved remarkable scientific results by applying some of their own modifications to diagnostic and surgical methods. Innovative techniques included intraoperative brain-tumor staining with fluorescent tetracycline, tumor grading by analysis of electrical resistance of tissue, the application of semiconductor probes for determining tumor margins and local tissue cooling during spinal cord surgery. Detailed EEG analysis allowed for the prediction not only of the tumor site but, frequently, of the histologic type as well. In complex vascular lesions and other high-risk surgery, controlled hypotension and hypothermia had been routinely used since the early 1960s. Some units acquired broad experience in microsurgical vascular anastomoses, others pioneered interventional neuroradiology using the detachable Serbinenko balloon catheter. Wide-ranging studies covered the long-term results of the Torkildsen shunt in hydrocephalus and the Frazier procedure in trigeminal neuralgia.
In 1989, the East German people called for political changes and, finally, demanded reunification with West Germany. Their rallies ultimately led to the fall of the Berlin Wall and to the collapse of the regime. On 3 October 1990, reunification was achieved. Shortly afterwards, the East German Neurosurgical Society held its final national congress in Cottbus, where it decided to wind up its activities. As a result of the subsequent political cleansing procedures, several East German professors, accused of having actively supported the totalitarian regime, lost their posts. In many instances, West German surgeons took their place, and this resulted in some bitter feelings which lasted for many years.
Meeting of East and West German neurosurgical representatives in preparation for the reunification of the separate neurosurgical societies, Dresden, 24 February 1990
Developments in the reunited Germany
Since 1990, the neurosurgical landscape has changed dramatically: not only have 30 new neurosurgical units been established, 18 of them in eastern Germany, but many community hospitals have also been transferred from public to private ownership. In addition, an increasing number of private practices have been established, and this reflects a substantial transfer of neurosurgical health care to private care-providers. In 2010, according to a survey carried out by the German Neurosurgical Society, nearly 1600 qualified neurosurgeons were responsible for a relatively stable population of 80 million people. More than 1000 of them worked in 40 university departments and 100 community or commercial hospital units, the remainder in private practice. The total number of neurosurgical operations carried out amounts to some 320 000 per year.
The main task of the German Neurosurgical Society (“Deutsche Gesellschaft für Neurochirurgieâ€) is to promote neurosurgical science and research and to guarantee undergraduate education and postgraduate training at a high level. With this in mind, a number of committees and working groups have been established. Several of their tasks have now been combined in the Neurosurgical Academy for Further Education and Advanced Training ("Neurochirurgische Akademie für Fort-und Weiterbildung") founded in 1999. Apart from the Neurosurgical Society, a separate German Neurosurgical Academy (“Deutsche Akademie für Neurochirurgieâ€) was founded in 1996 with a view to giving specific support to progress in science and research.
The current system of postgraduate training in Germany is mainly, but not exclusively, bound to hospital services and includes a minimum four-year stint in standard diagnostic and operative techniques and patient care. Specific training in diagnostic ultrasound is required, as is a minimum half-year period in an intensive care unit. Training in other specialties is accepted to a certain extent. The German Neurosurgical Society contributes to further education by organizing annual training courses. It is hoped that Europe-wide standards of training will be adopted in the near future. A number of working groups contribute to scientific progress as well as to further education in particular fields of interest.
Since its inception, the German Neurosurgical Society has awarded honorary membership to eminent scientists, the first recipients being the surgeon Nicolai Guleke and the neurologist Karl Kleist. Other awards were created to acknowledge outstanding contributions to neurosurgical science and the profession: since 1953, the Fedor Krause and the Otfrid Foerster Medals have been awarded for outstanding contributions to methods of neurosurgery and neuroscience respectively. In 1995, the Fritz König Medal was added for activities in support of the German Neurosurgical Society. Several foundations, for example the Wilhelm Tönnis Foundation and the Foundation for Neurosurgical Research, provide individual grants for research projects and study tours.
Acknowledgement: The authors wish to thank Professor Rudolf Unger for his critical review and advice and Leslie Woodhall for revising and editing the final text.
This text last amended on 10 July 2015