THE EVOLUTION AND ANATOMY OF THE CEREBELLUM

Observing that the ‘hemispheres’ are essentially a mammalian structure, Edinger (1910) applied the term ‘palaeocerebellum’ to the vermis and flocculus and the term ‘neocerebellum’ to the remainder (Fig. 3). Hausman (1929), elaborating on this concept, stated that a neocerebellar equivalent has been provided for each lobule of the vermis except the lingula. Although useful and expressive, the terms neo- and palaeocerebellum have been used by many different authors to designate somewhat different cerebellar parts, and unless qualified or limited have at present no certain meaning. Further, as Winkler (1923) and others have emphasized repeatedly, new acquisitions to the cerebellum take the form of growth of preexisting parts rather than altogether new structures superimposed on the old. The identification of the vermis, as defined by Edinger, as a morphological entity is misleading and lacks morphological or functional support. Hausman's identification of all lateral cerebellar parts with the neocerebellum seems hardly justified when we realize that both ontogenetically and phylogenetically the cerebellum had a bilateral origin, and that even in man one of the lateral parts, the flocculus, appears very early in the development of the cerebellum. The term neocerebellum has some meaning if reserved for those parts which in the higher mammals have come to be dominated by cortico-pontine connexions; but such a definition allows for no hard and fast delimitation by particular fissures. If the rest of the cerebellum is then designated palaeocerebellum, it must be recognized that it contains subdivisions of varying phylogenetic age. From a functional point of view probably the most important single contribution to cerebellar morphology is that of Sven Ingvar (1918) (Fig. 4). In his earlier monographic work he combines comparative morphological observations, fibre connexion studies, pathological and physiological investigations in an attempt to subdivide correctly the cerebellum. He was impressed with the uniformity of the lobules anterior to the fissura prima and posterior to the fissura prepyramidalis and with the wide variation in the lobules between these two deep and constant fissures in many species of birds and mammals. He noted the presence of two fissures which appeared to correspond to those in the alligator. A case of cerebellar atrophy which was restricted largely to the lobes lying between the primary and prepyramidal fissures was described, and the distribution of the spino-cerebellar fibres rostral and caudal to these fissures was also pointed out. Using all this evidence, Ingvar divided the cerebellum into three lobes, anterior, middle and posterior. This division, however, was less illuminating than a second concept which he emphasized later (Ingvar, 1928). This second classification was based on the distribution of afferent fibre connexions. He presented it in a novel way by comparing the cerebellum to a three-story house. The ‘basement’ consisted of the lobes receiving direct vestibular root fibres, which included the nodulus, uvula, lingula and flocculus. The next story he called the spinal floor. It consists of the lobules anterior to the fissura prima, the pyramis and paraflocculus. The remaining lobes which received predominantly ponto-cerebellar fibres he called the ‘cerebral floor’. This last division has in recent years been frequently identified by the term ‘neocerebellum’. Summary A review of previous work which has influenced cerebellar terminology is presented. An effort is made to harmonize the various sets of terms applied to lobes and fissures of the mammalian cerebellum with each other and with the terminology of human anatomy. The points in which agreement cannot yet be found and on which homologies with human anatomy are not clear are emphasized. The phylogenetic development of the cerebellum is described with particular attention to the contributions of Larsell in Amphibia, reptiles and primitive mammals. A schema of the folial pattern which seems best suited for the whole vertebrate system is presented, which is designed to show fundamental morphological principles and at the same time to conserve those terms now in most frequent use. The embryological development of the mammalian cerebellum is described in the light of the findings in lower forms. The phylogenetic and ontogenetic importance of the postero-lateral fissure of Larsell and the fundamental difference between the flocculus and paraflocculus of the mammalian cerebellum is stressed. A detailed description of the histology of the cerebellar cortex in mammals is given. Recent advances in the study of the afferent fibre connexions to the cerebellum are brought out and contributions of oscillographic studies emphasized. Each afferent system is analysed in detail both as to origin and termination in the cerebellum. Particular attention is given to the important cortico-ponto-cerebellar system of the higher mammals and the most recent analysis of the olivocerebellar connexion by Brodal. The functional localization in the cerebellum on the basis of the afferent fibre connexions is pointed out. It is emphasized that the termination of the afferent fibres does not favour topographical localization within the respective vestibular, spinal and cortico-pontine parts of the cerebellum. The common features of the vestibulo-cerebellar, spino-cerebellar and pontocerebellar systems are mentioned as well as the fact that the olivo-cerebellar system seems to be distinct from the other afferent systems. A suggestion is made that the mossy fibre endings are from the vestibular, spinal and pontine systems, while the olivo-cerebellar system terminates as climbing fibres. Available evidence, both for and against this conception, is discussed in some detail. Recent findings on the cerebellar cortico-nuclear connexions are presented. The development of the mammalian cerebellar nuclei and their homologies with the nuclei of the human cerebellum are discussed. It is pointed out that until such homologies are better established than at present it is impossible to transfer data from animal experiments to man in so far as the connexions of the globose nucleus is concerned. The efferent connexions of the deep cerebellar nuclei, as determined experimentally, are described in detail. The lack of available anatomical evidence for topographical localization on the basis of efferent cerebellar connexions is pointed out.