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plexuses, which are situated at the three corners of the epithelial roof of the ventricle, oval or elliptical perforations develop in the roof at about the fifth month of foetal life. These are known as the apertura medialis ventriculi quarti (O.T. foramen of Magendie), which opens between the clava on the posterior surface and the aperturæ laterales upon the anterior faces of the lateral recesses (Fig. 527), behind the insertion of the glossopharyngeal nerve on each side. Through each of these lateral openings the great swollen cauliflower-like extremity of the chorioid plexus becomes extruded from the ventricle. The inferior extremities of the two plexuses lying side by side present an analogous relationship to the apertura medialis, but they are. exceedingly attenuated and the epithelial lamella from which they spring becomes dragged backwards into contact with the cerebellum (Fig. 477), so that, when seen from below, the apertura medialis is a great funnel-shaped tube leading into the fourth ventricle and the chorioid plexuses look like two delicate vascular fringes on the cerebellum.
FIG. 484. THE BRAIN OF AN EMBRYO OF ELEVEN WEEKS, viewed from behind. The epithelial roof of the fourth ventricle has been removed. At this stage the cerebellum is in the form of a simple band or plate
which arches over the posterior aspect of the anterior part of the cavity of the hind-brain. (From His.)
one with the other and also with the floor-plate. At the time this process is in operation (see Fig. 483) the alar and basal laminæ are particularly well defined, and the limiting sulci are accentuated by the bending fourth ventricle of the side wall; but
this sharp distinction is soon lost as the result of the great expansion of the basal lamina (Fig. 485). This is due not only to growth of its intrinsic elements, but even more to its inVasion by large numbers of neuroblasts which migrate from the alar into the basal lamina.
Later still, the development of the great sensory and motor tracts contributes largely to the dimensions of the basal lamina.
As the two basal laminæ (one on each side of the median plane) increase in thickness the epithelial cells in the intervening floor-plate become stretched and lengthened (Fig. 483), so that a definite septum or raphe is formed between the two halves of the rhombencephalon.
The fate of the extreme posterior edge of the alar lamina is very interesting. The nervus acusticus is inserted into this edge in the region of the recessus lateralis, and from it masses of neuroblasts develop to form receptive nuclei for the two parts of this nerve, these being the cochlear and vestibular. These are the nucleus cochlearis and nucleus vestibularis respectively. Sensory fasciculi, bringing impulses from muscles, skin and related structures in all parts of the body, make their way into the superior part of the vestibular nucleus, and it grows and forms a large thickening of the posterior edge above the recessus lateralis. Eventually, as it extends medially (Fig. 484), it reaches and invades the roof-plate and fuses with the corresponding rudiment of the other side. Thus a semilunar band, the primitive cerebellum, is formed, arching across the posterior aspect of the metencephalon. The part of the dorsal edge which lies below the vestibular nucleus becomes bent over (forwards) to form what is known as the rhombic lip (Fig. 483). It is destined to be transformed into a series of masses of gray matter, the chief function of which is to emit fibres to carry impulses into the cerebellum. But most of these fibres pass not so much to the part of the cerebellum derived from their own side as to that of the opposite side. Thus, from above downwards, the thickened margin of the fossa rhomboidea on each side develops into the following structures:-cerebellum, the rest of the vestibular nucleus, the cochlear nucleus, the nuclei pontis (and arcuate nuclei), the olivary nucleus, the nucleus gracilis and the nucleus cuneatus.
FLOOR OF VENTRICLE IV.
FIG. 485.-TRANSVERSE SECTION OF THE HUMAN
SHOWN IN FIG. 483. (After His.)
EMBRYO AT A LATER STAGE THAN THOSE At an early stage of development most of the neuroblasts that form the rudiments of the nuclei pontis, nuclei arcuati, and nucleus olivaris inferior begin a process of migration, the course of which is determined by the source and direction of the afferent tracts passing into each nucleus. Such migrations are of common occurrence throughout the brain, and attempts to explain them have given rise to much discussion. The attractive force which appears to lead certain nerve-cells away from the place where they originally developed has been called neurobiotaxis by Ariens-Kappers. But the solution of the problems of these migrations is quite a simple one. If we take the case of a nerve-cell (A), at an early stage of development, which collects afferent impulses through its dendrites from the cell B, and emits an efferent impulse through its axon to the cell C: as the whole nervous system is very small at the stage under consideration, the three cells necessarily will be comparatively close the one to the other a fact which may be represented by the positions of the letters thus:
In the course of subsequent growth it must inevitably happen that the points B and C will become removed further and further apart. If we suppose that the cell B remains constant, the cell A will be faced with two alternatives if it is to continue to link together the elements B and C: either its dendrites or its axon must elongate. Now the axon is specially modified in structure for conducting impulses for long distances, and the dendrites are not so specialised. Therefore it invariably happens that it is the axon that becomes lengthened. In other words the cell-body A, considered in its relations to C, appears to migrate towards the direction B from which its chief supply of afferent impulses comes. This may be represented thus:
In the specific case we are considering the vestibular nucleus and the cerebellum receive their chief supply of afferent fibres from the incoming vestibular nerve: hence there is no reason for migration. Similarly the nucleus gracilis and nucleus cuneatus receive the fibres which come up through the funiculus posterior and remain where they are. But the nuclei pontis, the olivary nucleus, and the arcuate nuclei are "fed" with impulses passing downwards (and some perhaps upwards) in the basal lamina, close to the median plane, and they "migrate" towards the direction from which their afferent paths are approaching; the nuclei pontis towards the peduncles of the cerebrum bringing cerebro-pontine fibres from the cerebral cortex, and the olivary nucleus to the neighbourhood of certain descending tegmental tracts and ascending spinal sensory tracts that seem to supply the attractive force, which leads them to forsake the rhombic lip of the alar lamina and migrate into the basal lamina.
The majority of the cells destined to form the nuclei pontis wander obliquely upwards and forwards between the facial and acoustic nerves to reach the basal
Nucleus tractus spinalis
[nerve Tractus spinalis of trigeminal
Fasciculus rubrospinalis Olivo-cerebellar fibres
-External arcuate fibres
FIG. 486.-TRANSVERSE SECTION THROUGH THE MIDDLE OF THE OLIVARY REGION OF THE HUMAN MEDULLA
The floor of the fourth ventricle is seen, and it will be noticed that the restiform body, on each side, has now taken definite shape. Some of the descending tracts in red; ascending tracts in blue.
lamina of the metencephalon. But strewn along this pathway from the edge of the fossa rhomboidea to the front of the pons are scattered nerve-cells which have, so to speak, fallen by the way, and remain to indicate in the adult brain the path taken by the majority of their sister cells. This remnant forms the corpus pontobulbare: the pontine fibres that spring from its cells and are making their way upwards to fall in line (Fig. 499, p. 566) with the other transverse fibres of the pons form the fasciculus obliquus [pontis], and the cerebro-pontine fibres that pass below the pons in order to reach this outlying part (corpus ponto-bulbare) of the nuclei pontis constitute the fasciculus circumolivaris pyramidis (Fig. 517, p. 583).
But not all of the elements of the nuclei pontis that migrate pass into the metencephalon; a certain proportion of them invariably pass into the myelencephalon. These collect upon the anterior surface of the pyramids to form small irregular patches of gray matter which have received the name nuclei arcuati. Their afferent fibres (probably cerebro-pontine) come from the pyramids; and their efferent fibres (which proceed to the cerebellum) form some of the fibræ arcuata externæ, which are visible upon the surface of the medulla oblongata (Fig. 486).
Olivary Nuclei.-The most conspicuous of the isolated clumps of gray matter
in the medulla are the inferior olivary nucleus and the two accessory olivary nuclei. The nucleus olivaris inferior is the mass of gray substance which produces the swelling known as the olive, and constitutes a very striking object in transverse sections through this region. It presents the appearance of a thick wavy or undulating line of gray matter, folded on itself, so as to enclose a space filled with white matter. It is in reality a crumpled lamina arranged in a purse-like manner, with an open mouth or slit, which is called the hilum (hilus nuclei olivaris), directed towards the median plane. The hilum does not reach either extremity, so that in transverse sections through either end of the nucleus the gray lamina is seen in the form of a completely closed capsule. Into and out of the open mouth of the olivary capsule streams a dense crowd of fibres. These constitute what is called the olivary peduncle.
The accessory olivary nuclei are two band-like laminæ of gray matter, which are respectively placed on the dorsal and medial aspects of the main nucleus. In transverse section each of these nuclei presents a rod-like appearance (Fig. 486).
The medial accessory olivary nucleus extends lower down in the medulla oblongata than the main nucleus, and it is much larger in its lower than in its upper part. It begins immediately above the decussation of the pyramids, where it is seen lying on the lateral side of the cerebro-spinal fasciculus and the lemniscus medialis (Fig. 486). Higher up it lies across the mouth of the main nucleus and on the lateral side of the medial lemniscus. The dorsal accessory olivary nucleus is placed close to the dorsal aspect of the main nucleus. The two accessory nuclei fuse together before they finally disappear.
The nerve-cells of the inferior olivary nucleus are small and round, and emit a large series of short radiating, complexly branched dendrites, so that the cell-body seems to lie
in the centre of a spherical mass formed by its own dendrites and an almost equally complex mass of intertwined end branches of the axons which bring impulses into these cells. There is no definite information as to the place of origin of these afferent fibres. Flechsig and Bechterew, using different methods of investigation, have demonstrated the presence of a large descending tract in the mesencephalon and rhombencephalon, which ends amidst the cells of the lateral
FIG. 487.-THE INFERIOR OLIVARY NUCLEUS, as reconstructed and pole of the olivary nucleus. figured by Florence R. Sabin.
View of the dorso-lateral and lateral surfaces.
This has been called the fasciculus thalamo-olivaris, but it is not quite certain that it arises
in the thalamus, although its origin must be somewhere in the neighbourhood of it. Flechsig denies that any fibres reach the olivary nucleus from the spinal medulla, but the proximity of the spino-thalamic and bulbo-thalamic (lemniscus medialis) fibres and the demonstration of Ramon y Cajal that fibres enter the nucleus olivaris from adjoining fasciculi in these regions suggest that there may be a spinal afferent path.
There seems to be a direct relationship between the size of the inferior olivary nucleus and the extent of the cortical area that presides over highly skilled movements.
The axons emitted by the cells of the olivary nucleus cross the median raphe and pass through the opposite side of the medulla oblongata as internal arcuate fibres, which enter the restiform body and pass into the cerebellum.1
These fibres are seen only in the superior part of the medulla oblongata. They form the deep part of the restiform body and constitute its chief bulk. Streaming out from the hilum of the inferior olivary nucleus, they cross the median plane, and in the opposite side of the medulla oblongata they either pass through the inferior
1 These fibres should be called the fasciculus olivocerebellaris, by which designation they will be referred to in this account, but in the recognised nomenclature (which most writers do not follow in this instance) the *ract is called "cerebello-olivaris."
olivary nucleus of that side or sweep round it. Ultimately, on the dorsal aspect of the olivary nucleus, they are gathered together in the form of a conspicuous group of arcuate fibres, which curve backwards to take up a position in the deep part of the restiform body. In passing back, they traverse the tractus spinalis of the trigeminal nerve and break it up into several separate bundles. The olivo-cerebellar fibres thus connect the inferior olivary nucleus of one side with the opposite side of the cerebellum. Each part of the inferior olivary nucleus is connected with a definite part of the cerebellum.
Decussation of the Pyramids and the Changes produced thereby.-As we examine, under the microscope, a series of successive transverse sections through the inferior end of the medulla oblongata and the upper end of the spinal medulla, the most striking change which meets the eye is the decussation of the lateral cerebrospinal tracts. From their position alongside the anterior median fissure of the medulla oblongata most of the fibres of the pyramid cross the median plane and, after passing through the anterior column of gray matter, bend downwards in the lateral funiculus of the opposite side of the spinal medulla. Strands from the right lateral cerebro-spinal tract alternate with corresponding strands from the left side, and the interval between the bottom of the anterior median furrow and the gray matter surrounding the central canal becomes filled up with a great mass of intercrossing bundles of fibres.
As a rule the medial three-fourths of the pyramid are composed of fibres which, lower down in
the opposite lateral funiculus of the spinal medulla, form the fasciculus cerebrospinalis, whilst the lateral fourth of the pyramid proceeds downwards in the anterior funiculus of the spinal medulla of the same side, as the fasciculus cerebrospinalis anterior. A considerable amount of variation, however, occurs in the proportion of fibres which is allotted to the formation of these two tracts. Sometimes the lateral cerebrospinal tract is much larger than usual, and then the anterior cerebro-spinal tract suffers a corresponding diminution in size. Cases, indeed, occur in which the entire pyramid enters into the decussation, and in these there is no anterior cerebro-spinal tract. Further, it is not uncommon to meet with variations of an opposite kind which lead to an increase of the anterior cerebro-spinal tract at the expense of the lateral cerebrospinal tract. Sometimes the decussation is asymmetrical, and the corresponding cerebrospinal tracts on opposite sides of the spinal medulla are then unequal in size. One factor that often comes into play and causes asymmetry is the prolongation downwards into the pyramid on one side (usually the left) of some of the cerebro-pontine fibres. In such cases these fibres soon leave the pyramid and form the fasciculus circumolivaris.
The variations indicated above receive an additional interest when viewed in the light of comparative anatomy. It would appear that only in man and the anthropoid apes is the decussation of the pyramids in the inferior part of the medulla oblongata incomplete.
FIG. 488.-DIAGRAM OF THE FASCICULUS OLIVOCEREBELLARIS (CEREBELLO-
(This diagram has been constructed from the specimen figured on p. 555.) N.X., Vago-glossopharyngeal nucleus. N.XII., Hypoglossal nucleus.