Book - The Nervous System of Vertebrates (1907) 15
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Chapter XV. The Cerebellum
In the most primitive existing vertebrates the cerebellar segment shows little or no higher organization than any segment of the medulla oblongata. In man the cerebellum is, next to the cerebral hemispheres, the most complex part of the brain and is connected by a great number of tracts with nearly all parts of the nervous system. The chief facts in the evolution of this complex organ and its relationship will be dealt with in this chapter.
FIG. 112. The relations of the cerebellum, brachium conjunctivum and gustatory tracts in selachians (Scyllium}. Projection upon the median sagittal plane.
In primitive vertebrates the cerebellar segment differs from those following it chiefly in the absence of visceral nerve roots and their primary sensory and motor nuclei. The dorsal part of the segment belongs to the somatic sensory column (cf. p. 115) and receives general and special cutaneous nerve fibers. The cutaneous centers arch over the ventricle and are connected dorsally by a commissure. In the lateral wall, at least in true fishes, the visceral sensory column is represented by the secondary gustatory nucleus. In the ventral wall is the somatic motor nucleus of the trochlearis nerve. The central gray and the superficial zone of longitudinal fiber tracts with a decussation in the central raphe complete the structure of the cerebellar segment.
FIG. 113. Transverse section through the cerebellum of the sturgeon.
In cyclostomes the central gray matter presents nothing of special interest and the visceral sensory structures are as yet not studied. The somatic sensory centers consist of large and small cells. The small cells give rise to neurites of fine caliber which form a superficial fiber zone and a part of which cross to the opposite side, forming a dorsal commissure. The neurites of the large cells take the characteristic course for secondary cutaneous fibers, decussating in the ventral wall of the mesencephalon. Whether they run to the tectum mesencephali or to some other center is not certainly known. They may represent the fibers in the brachium conjunctivum of higher vertebrates which arise from the nucleus dentatus and run to the optic thalami.
In selachians the somatic sensory portion is greatly increased in size and complexity. It forms a large, sometimes enormous, arched and folded roof to the metencephalon, in which histological specialization has gone much farther than in the cerebellum of cyclostomes. The increased volume is due to a much greater volume of small cells and to a larger size and greater number of the large cells. The small cells constitute the granular layer and are known as granule cells. Their neurites constitute the greater part of the molecular layer and form the commissure of the cerebellum and the so-called cerebellar crest over the acusticum (cf. Chap. VII). The large cells are definitely arranged on the border between the granular and molecular layers and their dendrites, which expand in the molecular layer, have the characteristic appearance of Purkinje cell dendrites. This portion of the cerebellum receives in addition to somatic sensory root fibers secondary tracts from somatic sensory centers, including one from the tectum opticum. This shows that it serves both as a primary somatic sensory center and as a center of correlation for activities called forth by impulses from different organs of somatic sensation. A well developed brachium conjunctivum is present and probably consists in larger part of fibers arising in the cerebellum (Fig. 124, A, B).
FIG. 114. Transverse section through the cerebellum and secondary'gustatory nucleus of the sturgeon.
In the lateral walls of the metencephalon in selachians lie the secondary visceral sensory nuclei (p. 172 above). These nuclei receive the secondary visceral or gustatory tracts and give rise to a commissure which crosses dorsally at the junction of the cerebellum and optic lobes, in the velum medullare anterius. This is known as the decussatio veli and lies in close relation with the decussation of the IV nerve. The secondary gustatory nucleus also sends a large tract forward and downward to the inferior lobe, the tertiary gustatory tract (p. 173). The arrangement of the brachium conjunctivum and the tracts related to the secondary gustatory nucleus is shown in Fig. 112.
FIG. 115. Transverse section of the brain of the sturgeon at the junction of the cerebellum and midbrain.
In ganoids and bony fishes the somatic sensory portion of the metencephalon is not essentially different from that in selachians. It is not so large but the secondary tracts from other somatic sensory centers external arcuate fibers and tractus tecto-cerebellaris are more highly developed. The connections of the cerebellum with the secondary centers for the skin, ear and eye become more important relative to its primary connections than in lower fishes. The visceral nuclei (Figs. 114, 115) are situated somewhat farther ventrally and are both actually and relatively much larger than hi selachians owing to the greater development of the gustatory organs. The position of the commissure between the gustatory nuclei is very different from that in selachians. Instead of lying in the roof it lies deeply imbedded in a massive median structure the valvula cerebelli which largely fills up the ventricle of the cerebellar and mesencephalic segments. The position of the commissure is shown in Figure 91, which represents a sagittal section of the brain of a teleost a few days after hatching (see also Figs. 115 and 116). The form of the adult cerebellum of teleosts is essentially the same as this. The place of crossing of the commissure and of the IV nerve is homologous with the velum in selachians and it is clear that the valvula has been formed by an infolding of the roof in the region of the velum. Since the structure of the valvula is essentially that of the cerebellum it may be assumed that the typical layers of the cerebellum have spread forward into the valvula. The infolding of the roof, however, came about undoubtedly on account of the great size of the secondary gustatory nucleus and it is probable that the nucleus has invaded the valvula along the commissure. The dorsal limb of the valvula is thin and belongs to the tectum mesencephali. The arrangement of tracts related to the gustatory nucleus in a ganoid is shown in Figure 117.
FIG. 116. Transverse section through the midbrain of the sturgeon.
The cerebellum in the higher classes of vertebrates does not continue to increase in size and complexity as it does through the several classes of fishes. On the contrary, in amphibia and reptiles it is relatively very small, owing to the comparatively sluggish habits of these animals and the consequent reduction in the number of cutaneous and gustatory sense organs. The reduction of the cerebellum in amphibia as compared with selachians is chiefly a decrease in size only. The somatic sensory portion constitutes lateral lobes which are simply arched over the ventricle, not folded, but in these the typical granular and molecular layers and Purkinje cells are as clearly marked as in selachians. Similarly the secondary visceral nuclei are present in the lateral walls but no valvula is developed. The size of the cerebellum is closely related to the degree of activity of the animals. In active forms such as the frog and crocodile the somatic sensory lateral lobes are greatly increased in size. This increased size is not due chiefly to a greater number of sense organs or to a richer cutaneous innervation, but to an increase in the secondary tracts from the cutaneous, auditory and optic centers to the cerebellum, and to a greater number of Purkinje cells whose fibers go to motor nuclei. In other words, the cerebellum in the more highly organized amphibia and reptiles shows an increase in the correlating mechanisms in comparison with the primary sensory apparatus.
FIG. 117. The relations of the cerebellum, brachium conjunctivum and gustatory tracts in a ganoid fish (Acipenser). Projection on the median plane.
The further steps in the development of the correlating mechanism in birds and mammals are not well understood for lack of comparative studies. A careful study of the structure and fiber connections of the cerebellum in a series of forms including representatives of monotremes, marsupials, bats, rodents and carnivores is much needed in order to show the course of evolution of the extremely complex cerebellum of man. No connected account of this can be given at present, but it is evident that a larger number of specialized centers come into relation with the cerebellum in man than in fishes. At the same time the structure of the cerebellum itself has become more complex. This complexity of structure and of anatomical connections indicates the number and variety of functional relationships into which the cerebellum enters. The cerebellum has become in man a great center concerned with correlated movements in response to impulses from many sources. A brief review of the human and mammalian cerebellum will illustrate this.
Fig. 118. A, the left lateral aspect of the brain of a pouch specimen of Dasyurus viverrinus; B, median sagittal section of the cerebellum of the same brain. After G. Elliot Smith. Fiss. praec., fissura praeculminis; Fiss. postn , fissura postnodularis; Fiss. sec., fissura secunda; Floe., flocculus; Para floe., paraflocculus.
In order to understand the mammalian cerebellum it is necessary first to discard the cumbersome and meaningless description of lobes and surface divisions based upon the adult appearances, which is found in text-books of anatomy. The simpler method of dividing the cerebellum based upon the developmental history is much to be preferred, but even this gains its significance only when it is combined with a consideration of the centers and tracts within. First of all must be pointed out the fallacy of the common statement that the cerebellar hemispheres of mammals are new formations, not found in sub-mammalian classes. The hemispheres are formed first. The outline of the brain of a young marsupial given in Fig. 118 shows that in these forms the cerebellar hemispheres correspond closely to those of reptiles, amphibia or even of fishes (compare Figs. 2, 3, 7). The tuberculum acusticum occupies the same position as in lower vertebrates, at the dorsal border of the medulla oblongata immediately behind the cerebellum, and in front of it the border of the cerebellar hemisphere is formed of the flocculus and paraflocculus and these are continued upward by the middle lobe as the arch of the cerebellum. In the early human embryo after the cerebellar arch has been formed by the massive walls fusing in the roof, the lateral lobes are more prominent than the middle portion and the whole cerebellum has the same form as in lower vertebrates. The vermis is formed later than the hemispheres, not earlier. So in the phylogenetic history: the lateral lobes are constant in the vertebrate series, the vermis is formed only hi birds and mammals. What does happen in the higher mammals to produce a cerebellum which seems to differ greatly from that of lower vertebrates is: (i) a great growth of the median or keystone region of the arched cerebellum to form the vermis; (2) a great growth of that part of the hemispheres which lies in front of the floccular lobe, to form what is commonly known as the hemispheres; and (3) an arrest of growth of the flocculus and paraflocculus, together with the submersion of a part of the lateral lobes of lower vertebrates to form the nucleus dentatus.
Those portions of the cerebellum which become large and important in mammals are subdivided by variously placed fissures which are merely the result of mechanical conditions of growth and most of which are of no importance except as they may serve as landmarks in practical work. Those fissures which develop earlier in all mammals may be regarded as the more primitive and important and may be taken as the boundary lines of the chief divisions of the cerebellum. Upon this basis the cerebellum may be divided into (A) the floccular lobe, including the flocculus and paraflocculus; and (B) the rest of the cerebellum; which is subdivided by two transverse fissures into (a) anterior, (b) middle and (c) posterior lobes. The transverse fissures are named the fissura prima and fissura secunda. These divisions are shown in a median sagittal section of the marsupial cerebellum in Fig. 118 and in a diagrammatic dorsal view of the higher mammalian brain in Fig. 119. The anterior and posterior lobes do not expand laterally but are formed from the region adjacent to the middorsal line. They become divided by later fissures into subsidiary lobules which are of no importance in the present connection. The anterior lobe includes at its cephalic border the lingula which is connected by means of the valve o) Vieussens with the corpora quadrigemina. This region therefore corresponds in position to the velum medullare anterius of the selachian brain and the valvula cerebelli of the brain of ganoids and teleosts. The posterior lobe includes the two transverse folds known as the uvula and nodulus, to the latter of which is attached the membranous roof of the fourth ventricle. The nodulus therefore corresponds to the meso-caudal border of the cerebellum to which the choroid plexus is attached in all vertebrates. (Compare Figs. 118 B and 120.) The middle lobe in the embryo extends laterally and connects with the extreme lateral portions which form the floccular lobe. The middle lobe is divided into a median portion or vermis and lateral lobes, each of which is further divided by subsidiary transverse fissures. The anterior lobe corresponds to the larger part of the superior vermis of anatomists, while the inferior vermis includes the posterior lobe and most of the median portion of the middle lobe.
Fig. 119. A diagram representing the more fundamental and constant fissures of the mammalian cerebellum spread out in one plane. After G Elliot Smith.
This brief survey of the surface characters of the cerebellum gains in significance when the structure and arrangement of its deeper parts are considered. Throughout the whole cerebellum except the anterior lobe it consists of the following layers from without inward, (i) Molecular layer, consisting of (a) cells, (b) non-myelinated fibers derived from the granule cells, and (c) the dendrites of Purkinje cells. (2) Layer of Purkinje cellbodies. (3) Granular layer consisting chiefly of granule cells. (4) Layer of myelinated fibers. This layer is very voluminous and its subdivision to the various lobes gives rise to the well known arbor vitae. In the region of the anterior lobe the structure is the same as elsewhere except that the fourth or fiber layer is largely occupied by a number of gray masses or nuclei. These gray masses form the roof of the fourth ventricle at its anterior end. In this region are recognized near the median line the paired nuclei tecti or jastigii; lateral to them on either side the smaller nucleus globosus and nucleus emboliformis; and farther laterally the large nucleus dentatus which in man is convoluted like the lower olive but in lower mammals has a simple compact form. In the region of these nuclei is the commissure of the deep or white layer of the cerebellum.
Fig. 1 20. The mesial surface of the right half of the brain of Squalus acanthias.
The general features of the vertebrate cerebellum may now be summarized. It consists in all classes of the dorso-lateral walls of the metencephalic segment which are arched over and connected above the ventricle, where there is a commissure of molecular layer fibers. In most vertebrates the lateral walls bulge outward and forward as the auricular lobes, the floccular lobes in man. In selachians the cerebellum contains a part of the fourth ventricle which extends up into all the folds (Fig. 120). In mammals the great increase of the white layer has obliterated the cavity in the several cerebellar folds and its position is indicated only by the branches of the arbor vitae. In median sagittal section the roof of the fourth ventricle extends up into the cerebellum in the form of an inverted letter V. The apex of the A enters the base of the arbor vitae and separates the gray masses beneath the anterior lobe from the nodulus. The position of these deep gray masses and the commissure which passes through them corresponds fully to that of the velum medullare anterius and neighboring parts in selachians. The nodulus, floccular lobe and the acustic nuclei bear the same relations in mammals as do the caudal border of the cerebellum, auricular lobe and tuberculum acusticum in selachians. The mammalian vermis is formed by a great thickening of what corresponds to the mid-dorsal region of the cerebellum of any lower vertebrate. The hemispheres correspond to the dorso-cephalic wall of the lateral lobes in lower vertebrates. At the base of the lateral walls in fishes lie the secondary gustatory nuclei whose commissure crosses through the velum. In mammals in the region corresponding to the velum there is a deep commissure accompanied by nuclei which will be described below. In all vertebrates the cerebellum has two brachia, a posterior and an anterior. The posterior brachium contains primarily fibers belonging to both somatic and visceral sensory systems. As already described, primary root fibers of the somatic sensory nerves and external arcuate (secondary sensory) fibers enter the cerebellum in all vertebrates. In lower vertebrates the secondary gustatory tract runs ventral to and parallel with the somatic sensory tracts and ends in the secondary gustatory nucleus. In mammals the direct cerebellar tract, which is the secondary visceral sensory tract from the spinal cord, runs lateral to the somatic sensory tracts and both are included in the corpus restijorme. The anterior brachium, brachium conjunctivum, in lower vertebrates (Figs. 112, 117) runs from the lateral lobe partly through and partly beneath the secondary gustatory nucleus and passes diagonally downward and forward to the base of the mesencephalon. In mammals (Fig. 126) it is enormously larger and has made room for itself by shifting other structures to one side. In all vertebrates the anterior and posterior brachia cross one another as they enter the cerebellum like the two limbs of a letter X, the anterior brachium passing mesially to the posterior. In mammals a third brachium is added, the brachium pontis, which goes vertically downward between the diverging anterior and posterior brachia.
Fig. 121. A diagrammatic transverse section of one fold of the cerebellum. From Koelliker. /, moss fibers; gl, glia cells; gr, granules; gr f , neurites of granules seen in the molecular layer; k, net-like fibers; k', endings of the same; m, small cells of the molecular layer; m', basket cell; zk, pericellular basket of the same; n, short neurite of large granule.
Structure of the Cerebellar Cortex
The most conspicuous elements in the cerebellum are the Purkinje cells. These differ from those in lower vertebrates chiefly in the mode and regularity of their arrangement. The cell-bodies are situated between the molecular and granular layers and the dendrites spread in the molecular layer. The arrangement is such that in any fold of the cerebellum the dendrites of each Purkinje cell spread like a fan across the fold (Fig. 121). The dendrites are provided with small lateral spines. From the deeper end of the cell-body arises the neurite which penetrates the white layer. In the first part of its course it gives off several collateral branches which rise toward the surface and end in the granular or molecular layer. The neurites then become myelinated and proceed as constituent fibers of the white fiber layer.
The granular layer contains both granule cells and cells of Golgi's type II. The granule cells are very numerous and closely packed together and, like those of lower vertebrates, have short dendrites which end in claw-like tufts of small branches. The neurites are exceedingly fine, rise directly into the molecular layer and divide in T T form into two branches which run parallel with the surface of the folds of the cortex. Among these fine fibers of the molecular layer are imbedded the dendrites of the Purkinje cells, the small spines of which apparently serve for connections with the fine fibers. The cells of the second type are much larger than the granule cells, their dendrites spread in both granular and molecular layers, and their neurites branch immediately and profusely in the granular layer.
The molecular layer contains two chief types of cells. In the deeper part of the layer are numerous cells the behavior of whose neurites has given them the name of basket cells. The neurites begin as slender fibers which grow thicker as they run parallel with the Purkinje cell layer. At intervals they give off collateral branches which run down and branch to form basket-like networks around the bodies of the Purkinje cells. Although such cells have been seen in rare cases in the cerebellum of fishes, they are not highly developed or numerous below the mammals. In the outer part of the layer are numerous small cortical cells whose neurites are short and either branch repeatedly close to the cell or have a longer or shorter horizontal course before ending.
Fiber Tracts of the Cerebellum
The fibers connecting the cerebellar cortex with other parts of the brain in part arise in the cerebellum, in part end in it. Since the structure of the cortex and the arrangement of the fiber endings are everywhere the same, it follows that all three peduncles of the cerebellum carry both in-coming and out-going fibers. The in-coming fibers are of two forms: moss fibers, which bear peculiar bundles of short branches in their course and at their ends, and fibers which end by complex net-like end-branches. The moss fibers occasionally bifurcate and give off frequent collaterals, so that the terminal branches are widely distributed. The peculiar small tufts of end-branches stand in relation with the similar branches of the dendrites of the granule cells. The second kind of fibers rise through the granular layer, apply themselves to the surface of the Purkinje cell dendrites and ramify upon the branches of these dendrites (Fig. 122). There is a remarkable uniformity in the character and arrangement of the nerve cells and in- coming fibers in all parts of the cerebellar cortex in mammals. The incoming fibers in mammals are very different from those in lower vertebrates. In fishes the great majority of in-coming fibers are somatic sensory fibers which end in the granular layer. In mammals somatic sensory root fibers are still received from the spinal nerves by way of the dorsal funiculi and the corpus restiforme, from the vestibular nerve and from the trigeminus, but the number of such fibers is very small in comparison with the number of fibers coming to the cerebellum from other brain centers. In spite of this great change in the character of the incoming fibers the mode of receiving impulses in the cerebellum has remained essentially unchanged. The fibers from other brain centers have conformed to the plan of structure of the cerebellum, and deliver their impulses either to the granule cells or directly to the Purkinje cells. All impulses received by the granules are transferred to the Purkinje cells through the molecular layer, either directly or by means of basket cells or other cells with short neurites as indicated in the accompanying schemes (Fig. 123).
Fig. 122. Ending of a net-like fiber in the cerebellum of man. After Cajal.
Fig. 123. Two schemes to show the course of impulses in the cerebellar cortex. After Cajal. In the upper figure: A, moss fibers; B, neurites of Purkinje cells; a, granules; 6, fibers of molecular layer; c, cell of type II; d, Purkinje cell. In the lower figure: b, basket cell; c, Purkinje cell; d, net-like fiber.
Recent experimental work on the cerebellum of the rabbit has led Van Gehuchten to the conclusion that the in-coming fibers enter the cerebellum in the corpus restiforme and brachium ponds, while the out-going fibers go by way of the brachium conjunctivum.
The fibers which enter the cerebellar cortex may be grouped under the following categories:
a. Primary somatic sensory fibers from the roots of the spinal, vestibular and trigeminal nerves.
b. Secondary somatic sensory fibers. These arise from the cells of the nuclei of the dorsal funiculi and from the superior and lateral nuclei of the vestibular nerve. The fibers from the nuclei of the funiculi run in part in the corpus restiforme of the same side, in part as external arcuate fibers in the corpus restiforme of the opposite side.
c. Fibers from the gray matter of the cord of the same and opposite side which run up in the fasciculus spino-cerebellaris ventralis (tract of Gowers) and enter the cerebellum in part by way of the brachium conjunctivum and in part by the corpus restiforme, to end in the superior vermis and hemispheres.
d. Tractus olivo-cerebellaris, from the lower olive to the cerebellar cortex.
e. Fibers from the nuclei of the pons. Some of these constitute with Purkinje cell neurites a two-miked commissure between the two hemispheres of the cerebellum. Others forward impulses which are brought to the pons from the cerebral hemispheres by way of the pyramids.
f . Fibers from the nucleus ruber to the cerebellum by way of the brachium conjunctivum (Edinger's tractus tegmento-cerebellaris). Recent researches render the existence of such fibers very doubtful.
g. Fibers from the nucleus of the lateral lemniscus and the posterior colliculus of the corpora quadrigemina. These are secondary nuclei of the auditory paths.
h. Secondary visceral sensory fibers. These are the fibers of the direct cerebellar tract from Clarke's column in the cord. Most authors state that this tract ends in the deep gray nuclei but it is sometimes described as ending in the cortex of the vermis.
FIG. 124. Two transverse sections through the cerebellum of Scyttium. A, a section at the extreme caudal end of the secondary gustatory nucleus, the position of which is marked by the cross; B, through the secondary gustatory nucleus, the decussatio veli and the origin of the tertiary gustatory tract. The secondary gustatory nucleus lies between the brachium conjunctivum and the velum.
The fibers which go out from the cerebellum are all neurites of Purkinje cells. They go to the spinal cord, the lower olive, Deiter's nucleus, nuclei of the pons and perhaps to other centers. By way of these various paths the cerebellum may exercise control over both somatic and visceral motor nerves. A part of the fibers which go through the middle peduncle to the pons go directly to the spinal cord to connect with the somatic motor column. Other fibers end in the nuclei of the pons, from which fibers go to the opposite cerebellar cortex or to the cerebral cortex. Many Purkinje cell neurites may end in the nucleus dentatus, from which the large brachium conjunctivum goes to the optic thaiami. By this path and the tractus thalamo-spinalis the cerebellum may gain a widespread connection with motor nerves.
There is no one fact more striking in the study of the human brain than the great complexity of structure and fiber connections of the cerebellar cortex and the great increase in the number of relationships of the cerebellum from lower to higher vertebrates. Much remains to be done, especially on the course of Purkinje cell neurites, in order to gain an exact knowledge of these relationships.
The nucleus dentatus is taken up separately from the other deep gray masses because of its apparent closer relation with the cortex. The peculiar form of the nucleus which gives it its name in man is not seen in lower mammals. In these it is a simple gray body which lies nearer to the surface of the brain and nearer the junction of the cerebellum with the medulla oblongata. The fibers which enter the nucleus dentatus are not fully understood but they seem to include external arcuate fibers from the nuclei of the dorsal funiculi, fibers from some of the nuclei of the vestibular nerve and fibers of Purkinje cells of the cortex. The neurites arising in the nucleus dentatus go mostly or wholly into the brachium conjunctivum which is composed almost exclusively of such fibers (Van Gehuchten). The brachium decussates in the ventral wall of the mesencephalon, passes through the nucleus ruber and ends in the optic thalami. The nucleus dentatus is concerned chiefly with somatic sensory impulses and is more closely related to the cortex than to the deep nuclei. This indicates a correspondence between this submerged and folded nucleus of the human cerebellum and the portion of the selachian cerebellum with which the brachium conjunct! vum is connected. In selachians the brachium conjunctivum enters the latero- ventral part of the cerebellum close to the junction with the tuberculum acusticum, a region which is wholly concerned with primary sensory and external arcuate fibers. It seems probable that in the evolution of the cerebellum this nucleus has lost its superficial position and been overgrown by the greatly expanded hemispheres. The course and destination of the brachium in mammals and Petromyzon (p. 117) suggests comparison with the ascending fibers of the lemniscus system (cf. p. 258).
FIG. 125. A transverse section through the cerebellum of Necturus. The figure is somewhat diagrammatic in that the whole course of the decussatio veli as drawn does not fall in the same section with the other structures. The actual section from which the outline was drawn inclined forward somewhat, so that it passed through the tectum in front of the velum. The word Cerebellum at the left is placed in the part of the fourth ventricle which extends into the auricular lobe.
Nuclei Tecti, Globosus and Emboliformis
The fiber connections of these nuclei are not well understood. The nucleus tecti is known to receive part of the fibers of the vestibular nerve and of the external arcuate fibers from the nuclei of the dorsal funiculi. A connection between the nucleus tecti and the superior olive of the pons has been described. The conflicting descriptions of the direct cerebellar tract do not allow us to decide whether it is confined to the deep gray or extends also to the overlying cortex of the anterior lobe. It seems certain, however, that the nucleus globosus and nucleus emboliformis are related to this tract and that the commissure which runs through these nuclei is formed either by the fibers of this tract or by neurites arising from these nuclei. It seems probable that these nuclei may represent the secondary gustatory nucleus of the cerebellum of lower vertebrates. Nothing is certainly known about gustatory paths in man but it is to be expected that the gustatory nucleus will be found related to the nucleus of ending of the direct cerebellar tract (secondary visceral sensory tract).
FIG. 126. A transverse section through the deep gray nuclei of the cerebellum of man. V, the radix mesancephalica V; VIII, root fibers of the vestibular nerve going to the cerebellum; B, the position of Bechterew's nucleus; D, the position of Deiter's nucleus.
In Figures 124, 125 and 126 are drawn transverse sections through corresponding regions in the brain of a selachian, an amphibian and man. With these may be compared Figures 112, 115, 116 and 117. In all these forms, although they differ widely in many respects, the relations of the brachium conjunctivum mesencephalic root of V, the primary and secondary somatic sensory tracts and the cerebellum are essentially the same. If the direct cerebellar tract has its ending in one of the deep nuclei of the cerebellum it would correspond to the gustatory tracts of lower forms. It appears that the nuclei in the roof have been pushed up from the lateral wall on account of the great size of the brachium conjunctivum. This appears the more probable if the brachium conjunctivum in mammals is related solely to the nucleus dentatus as Van Gehuchten's recent studies indicate.
The evolution of the structure and function of the cerebellum may be summarized as follows. In fishes the cerebellum consists of a large dorsal somatic sensory nucleus and of a secondary visceral (gustatory) nucleus ventral to it. The gustatory nucleus has a dorsal commissure through the cephalic border of the cerebellum. The presence of this commissure leads to extensive changes in the form of the cerebellum when the gustatory nucleus is large. Nothing further can be said regarding the visceral portion of the cerebellum until the center for the direct cerebellar tract in mammals is more fully studied and the question of its homology with the gustatory nucleus of fishes is settled.
The somatic sensory portion is not only very large in fishes but shows a higher specialization of structure than would be called for by its purely sensory function. Its great size is directly accounted for by the great development of the acustico-lateral system of sense organs. Its structure, however, differs from that of the tuberculum acusticum, which also serves as the center for the same system, in the presence of a greater number of granule cells and the higher development of the Purkinje cells. In addition, the cerebellum receives a fiber tract from the tectum opticum which serves to bring optic stimuli into relation with cutaneous. The cerebellum is no longer a purely cutaneous center, but impulses may be sent out from it in response to retinal stimuli also. The development of special functions has not gone far in the selachians, for removal of the cerebellum alone in the dogfish does not produce visible effects on locomotion (Bethe). In combined operations on the cerebellum and other parts of the brain, however, it is shown that the cerebellum plays some part in the coordination of movement.
With the disappearance of lateral line organs in land amphibia the cerebellum is greatly reduced in size. The reduction affects chiefly the primary sensory center, and from the amphibia onward the relations of the cerebellum with other brain centers increase. The fibers from other brain centers end chiefly in the dorso-median and cephalic regions of the cerebellum and in higher vertebrates the vermis and adjacent parts of the hemispheres are developed from these regions. Fibers come to these parts from the primary and secondary nuclei of the cutaneous, vestibular and cochlear nerves, and from the inferior olive, the nuclei of the pons and perhaps other sources. The fibers which go out from the cerebellum make direct or indirect connections with nearly the whole range of motor nuclei, probably both somatic and visceral.
It is reasonable to suppose that the presence of the somatic pallium in mammals has influenced the evolution of function of the cerebellum. In fishes and perhaps in amphibia and reptiles the cerebellum and the roof of the mesencephalon share between them the functions of higher centers to which somatic sensory impulses of the second and third orders are sent and from which impulses go out to control complex motor responses. In mammals these impulses are carried to the neopallium, which has taken on the direction of all voluntary movement. What then is the function of the cerebellar cortex? Disease of the cerebellum in man and its extirpation in animals always results in disturbances of voluntary muscular action. Animals from which one hemisphere or the whole cerebellum has been removed are unable to stand or walk until they have learned to make compensatory efforts. Does the cerebellum have the special function of maintaining the equilibrium, or is it necessary for the coordination of muscular contractions with reference to definite movements ? In the results of experimental investigations on mammals the function of the cerebellum which stands out most prominently is different from either of these. Dogs which have lost one cerebellar hemisphere, although they are unable to stand or walk, can swim well in water (which supports their body weight), both coordinating their movements and maintaining their equilibrium. Such animals learn after a time to compensate for the loss of the cerebellum by certain voluntary modifications of their movements; e.g., curving the spine so as to bring the center of gravity over the sound legs, spreading the feet wide apart, etc. They can then stand and walk. These and other facts show that the loss of the cerebellum does not involve loss of the power of equilibration nor of cutaneous or muscle sense on which the power of coordinated movements depends, but does result in weakness of muscular action on the injured side. It seems, therefore, that the cerebro-spinal mechanisms are sufficient to carry out all voluntary movements without the aid of the cerebellum, but the movements are lacking in strength, precision and regularity. The cerebellum is not shown to be a necessary link in the nervous mechanisms which control muscular action but it seems to add something to the voluntary movement. According to Luciani the function of the cerebellum is to maintain the tone of muscles during rest, to increase the energy of contraction when called forth by voluntary impulses and to determine the rhythm of motor impulses. In this way the imperfect actions of the dog deprived of its cerebellum would be perfected into normal movements. Whether the cerebellar cortex actually serves other functions, such as the coordination of specific movements, remains for further investigation to decide.
The fact that the cerebellum receives fibers from so many and various sources is important in this connection. It would seem that the maintenance of tone is not an abstract thing which is unrelated to present activity or sensory stimuli. If the function of the cerebellum is to maintain the tone of muscles, it is evident that it sends out the necessary impulses in response to impulses brought to it from related sensory areas or special sense organs.
Thus coordinated movements in response to sensory impulses are directed by the mesencephalic and cerebral apparatus to be described in future chapters, while the side circuit through the cerebellum furnishes the means of maintaining the requisite tone. It is evident that the cerebellum has changed its functions in the course of vertebrate history but the indications are that the broad functional relationships of its main parts have been retained. The somatic sensory part has come in mammals to play a special role in the senso-motor functions. It is still concerned chiefly with the reactions of the organism toward the external world by means of its somatic muscles. It is to be hoped that future experimental studies will be directed toward discovering the structural and functional relationships of the deep nuclei of the anterior lobe. The suggestion that some of these are homologous with the gustatory nucleus of fishes will be justified if it stimulate investigation in this direction.
Demonstration or Laboratory Work
- Review the laboratory work on the somatic sensory relations and the structure of the cerebellum in fishes given in Chapters VI and Book - The Nervous System of Vertebrates (1907) 7|VII]].
- Study the structure of the cerebellar cortex in a mammal in Golgi sections.
- Examine by dissections and by sections in various planes the relations of the several parts of the cerebellum in fishes and mammals. Verify the comparisons made above, especially the constancy of the hemispheres in the vertebrate series,
- Examine in the same way the relations of the cerebellar peduncles and of the secondary gustatory nuclei and the velum.
Bianchi, Arturo: Sulle vie di connessione del cervelletto. Archivio di Anat. e di EmbrioL, Vol. 2. 1903.
Barker, L. F.: The Nervous System.
Bethe, A.: Die Locomotion des Haifisches u.s.w. Arch. f. d. ges. Physiol., Bd. 76. 1899.
Cajal, S. R.: Textura del sistema nervioso del Hombre, etc. Madrid. 1904.
Catois, E. H.: Recherches sur Phistologie et Panatomie microscopique de 1'encephale chez les poissons. Bull. Sci. de la France. Tome 36. 1901.
Edinger u. Wallenberg: Untersuchungen iiber den Gehirn der Tauben. Anat. Anz., Bd. 15. 1899.
Van Gehuchten, A. : Le corps restiforme et les connexions bulbo-cerebeleuses. Le Nevraxe, Tome 6. 1904.
Van Gehuchten, A.: Les pedoncules cerebelleuses superieurs. Le Nevraxe, Tome 7. 1905.
Johnston, J. B.: The Brain of Acipenser. Zool. Jahrb., Bd. 15. 1901.
Johnston, J. B.: The Brain of Petromyzon. Jour. Comp. Neur., Vol. 12. 1902.
Kappers, C. U. A.: The Structure of the Teleostean and Selachian Brain. Jour. Comp. Neur. and Psych., Vol. 16. 1906.
Koelliker, A.: Gewebelehre. 6te. Aufl. Bd. 2. 1896.
Luciani, L.: Das Kleinhirn. Asher u. Spiro's Ergebnisse. III. Jahrg., II. Abth. 1904.
Ramon, P.: Investigaciones micrograficas en ed encefalo de los batracios y reptiles. Zaragossa, 1894.
Schaper, A.: Zur feineren Anatomic des Kleinhirns der Teleostier. Anat. Anz., Bd. 8. 1893.
Schaper, A.: Die morphologische und histologische Entwickelung des Kleinhirns der Teleostier. Morph. Jahrb., Bd. 21. 1894.
Smith, G. Elliot: On the Morphology of the Brain in the Mammalia. Trans. Linn. Soc. London, Ser. 2, Zool. 8. 1903.
Smith, G. Elliot: Further Observations on the Natural Mode of Subdivision of the Mammalian Cerebellum. Anat. Anz., Bd. 23. 1903.
Stroud, B. B.: The Mammalian Cerebellum. The Development of the Cerebellum in Man and the Cat. Jour. Comp. Neur., Vol. 5. 1895.
The literature of the mammalian cerebellum will be found in the general works cited above.
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