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present the form of two compact bundles. Superiorly, however, they are broken up into smaller bundles by the transverse fibres of the pons, and are spread out over a wider area. At the upper border of the pons they again come together and form two solid strands, each of which is continuous with the central part of the corresponding basis of the cerebral peduncle. Added to these there are twice as many other fibres entering the pons from the basis pedunculi to terminate in the nuclei pontis.
The fibræ pontis at the inferior border of the pons are placed on the superficial or ventral aspect of the pyramidal bundles. As we proceed upwards they increase in number, and many are seen breaking through the pyramids and even passing across upon their dorsal aspect. Laterally, these transverse fibres are collected together into one compact mass, which enters the white central core of the cerebellum and constitutes the brachium pontis (O.T. middle cerebellar peduncle). At the median plane the transverse fibres of the two sides of the basilar portion of the pons intercross and form a coarse decussation.
The nuclei pontis form a considerable part of the bulk of the basilar portion of the pons. The gray matter is packed into the intervals between the intersecting transverse and longitudinal bundles.
There is some analogy between the pyramidal portions of the medulla oblongata and the ventral part of the pons. In the medulla oblongata fine arcuate fibres, on their way to the surface, pass through the pyramids. Other external arcuate fibres sweep over the surface of the pyramids. These present a strong resemblance to the transverse fibres of
the pons. They likewise reach the cerebellum, although by a different route, viz., the restiform body. The nuclei pontis are represented also in the pyramidal part of the medulla oblongata by the arcuate nuclei, which are covered over by the external arcuate fibres, and even tend to penetrate, to a slight extent, into the pyramidal tracts. These arcuate nuclei, as already pointed out, are continuous with the nuclei pontis.
Connexions of the Longitudinal and Transverse Fibres. When transverse section through the superior part of the pons is compared with one close to its inferior border, it becomes at
once apparent that the numerous scattered bundles of longitudinal fibres which enter the ventral part of the pons from above, if brought together into one tract, would form a strand very much larger than the two pyramids which leave its lower aspect and enter the medulla oblongata. It is clear, therefore, that many of the longitudinal fibres which pass into the pons from above do not pass out from it below into the medulla oblongata. What becomes of these fibres that are thus absorbed in the pons? It is known that the pyramidal bundles suffer a small loss by the fibres which they send to the nuclei of origin of the efferent nerves which arise within the pons (viz., the motor root of the trigeminal, abducens, and facial nerve nuclei); but this loss is, comparatively speaking, trifling. It is clear, therefore, that other longitudinal bundles enter the pons from above apart from those which form the pyramidal tracts. These bundles occupy a lateral and dorsal position in the ventral part of the pons, and may be termed the cerebro-pontine fibres, seeing that they come from the cerebral cortex and end in fine ramifications around the cells of the nuclei pontis (Fig. 498).
Corpus ponto - bulbare.
FIG. 499.-DIAGRAM OF THE LEFT LATERAL ASPECT OF THE FETAL
The transverse fibres take origin as axons of the cells of the nuclei pontis. Crossing the median plane, they enter the brachium pontis of the opposite side, and thus reach the cerebellar cortex, where they end in ramifications round certain of the cortical cells. Some authorities believe that there are also fibres passing in the opposite direction, i.e.
from the cerebellum to the nuclei pontis; but there is some doubt concerning the existence of any such fibres. The brachium pontis thus may contain both efferent and afferent cerebellar fibres; but no fibres pass continuously through the pons from one brachium pontis into the other.
Certain of the transverse fibres of the pons turn backwards and enter the dorsal or tegmental part of the pons, but the precise connexions of these are doubtful.
Corpus Trapezoideum.-This name is applied to a group of transverse fibres which traverse the lower part of the pons (Fig. 498). They are quite distinct from those which have been just described as entering the brachium pontis, and they lie in the boundary between the dorsal and basilar parts of the pons, but encroaching considerably into the ground of the former. They arise from the cells of the terminal nucleus of the cochlear division of the acoustic nerve, and constitute a tract which establishes certain central connexions for that nerve. They will be more fully described when we treat of the cerebral connexions of the acoustic nerve.
Pars Dorsalis Pontis (Dorsal or Tegmental Part of the Pons).—On the dorsal surface of the tegmental part of the pons there is spread a thick layer of gray matter, covered with ependyma, which forms the floor of the upper or pontine part of the fourth ventricle. Beneath this the median raphe of the medulla oblongata is continued up into the pons, so as to divide its tegmental part into two symmetrical halves.
In the inferior part of the pons, immediately beyond the medulla oblongata, the restiform body is placed on the lateral side of the dorsal part (Fig. 498). In transverse sections through the pons it appears as a large, massive oval strand of fibres which inclines backwards into the cerebellum, and thus leaves the pons. Between the restiform body and the median raphe the tegmental part of the pons is composed of substantia reticularis, continuous with the same material in the medulla oblongata. Thus, arcuate or transverse fibres, curving in towards the raphe, and also longitudinal fibres, are seen breaking through a mass of gray matter which occupies the interstices of the intersecting fibres. To the naked eye the formatio reticularis presents a uniform gray appearance, but its constituent parts are revealed by low powers of the microscope in properly stained and prepared specimens. Embedded in this substantia reticularis are various clumps of compact gray matter and certain definite strands of fibres. These we shall describe as we pass from the restiform body medially towards the median raphe.
(1) Spinal Root of the Trigeminal Nerve and its Nucleus.-Close to the medial side of the restiform body, but separated from it by the vestibular root of the acoustic nerve as it proceeds backwards through the pons, is seen a large crescentic group of coarse transversely divided bundles of fibres. This is the tractus spinalis (O.T. spinal root) of the trigeminal nerve; and applied to its medial concave side is a small mass of gray matter, which is the direct continuation upwards of the substantia gelatinosa.
(2) The nucleus of the facial nerve comes next. It is sunk deeply in the dorsal part of the pons and lies close to the transverse fibres of the corpus trapezoideum. It is a conspicuous, obliquely placed, ovoid clump of gray matter. From its lateral and dorsal aspect the root-fibres of the facial nerve stream backwards and medially towards the gray matter on the floor of the fourth ventricle. Passing forwards between this nucleus and the trigeminal sensory nucleus a solid nerve-bundle may be observed. This is the facial nerve, traversing the pons towards its place of emergence from the brain.
(3) Immediately medial to the facial nucleus, but placed more deeply in the tegmental part of the pons, is the superior olivary nucleus. It lies in a bay formed for it by the transverse fibres of the corpus trapezoideum. These fibres curve round its ventral aspect, and many of them may be observed penetrating into its substance. In man, it is a very small mass of gray matter, and presents little resemblance to the inferior olivary nucleus, except in the size and shape of its constituent cells. In sections through the part of the pons where it attains its greatest size, it appears in the form of two, or it may be three, small isolated masses of gray matter. It is intimately connected with the acoustic fibres, and establishes manifold connexions between them and the nuclei of other nerves.
Upon the medial and dorsal aspect of the superior olive there is a dense group of longitudinal fibres. These constitute the fasciculus thalamo-olivaris or central tegmental tract, to which we have already referred in discussing the inferior olivary nucleus (Fig. 498). It is uncertain whether this tract arises in the thalamus.
(4) The medial longitudinal bundle and the lemniscus medialis come next. As they proceed upwards through the tegmental part of the pons, these longitudinal tracts occupy the same relative position as in the medulla oblongata. They are placed close to the median raphe; but they have drawn further apart from each other, and their fibres are more distinctly concentrated into separate strands, with an interval of some little width between them, which is occupied by the tecto-spinal tract. The medial longitudinal bundle lies immediately under cover of the gray
Anterior medullary velum
Transverse fibres of pons
FIG. 500.-TRANSVERSE SECTION THROUGH THE PONS AT THE LEVEL OF THE NUCLEI OF THE
matter of the floor of the fourth ventricle. The lemniscus medialis is placed close to the trapezial fibres, many of which traverse it as they pass towards the median plane. (5) The nucleus of the abducens nerve also forms a conspicuous object in sections through the lower part of the pons. It is a round mass of gray matter, which is situated close to the lateral side of the medial longitudinal bundle, and immediately under cover of the gray matter of the floor of the fourth ventricle. From its medial side numerous root-bundles of the abducens nerve pass out and proceed forwards between the lemniscus medialis and the superior olivary nucleus. They occupy in the pons, therefore, a position similar to that occupied by the hypoglossal rootfibres in the medulla oblongata.
Up to the present only the inferior part of the dorsal portion of the pons has been described, i.e. the portion immediately adjoining the medulla oblongata. As we proceed upwards and gain a point above the level of the trapezial fibres, many of the structures which have attracted attention lower down gradually disappear from the formatio reticularis. The lemniscus medialis becomes markedly
increased in size by the addition of the fibres of the spino-thalamic tract. Further, the floor of the fourth ventricle becomes narrower, and other objects appear in the tegmental substance.
The brachium conjunctivum (O.T. superior cerebellar peduncle) is a very conspicuous object in sections through the middle and upper parts of the pons. In transverse section it presents a semilunar outline, and as it emerges from the cerebellum it lies immediately on the lateral side of the fourth ventricle, towards which its concave aspect is turned (Fig. 500). Its dorsal border is joined with the corresponding brachium of the opposite side by the thin lamina of white matter, termed the anterior medullary velum, whilst its ventral border is sunk to a small extent in the dorsal part of the pons. As it is traced upwards it sinks deeper and deeper into the pons until it becomes completely submerged, with the exception of the dorsal border to which the anterior medullary velum is attached. It now lies on the lateral side of the tegmental or reticular substance of the pons, and this position it maintains until the mesencephalon is reached (Fig. 501).
About half-way up the pons the nuclei of the trigeminal nerve mark a very
FIG. 501. SECTION THROUGH THE SUPERIOR PART OF THE PONS OF THE ORANG, ABOVE THE LEVEL OF THE TRIGEMINAL NUCLEI.
important stage in its tegmental portion. These nuclei are two in number on each side, viz., a large oval terminal nucleus for certain of the sensory fibres of the nerve and a nucleus of origin, equally conspicuous, for certain of the motor fibres (Fig. 500). The sensory nucleus lies close to the lateral surface of the pons, deeply sunk in its tegmental part, and in the interval between the submerged anterior border of the brachium conjunctivum and the ventral part of the pons. The motor nucleus is placed on the medial side of the sensory nucleus, but somewhat nearer the dorsal surface of the pons. At this level the tractus spinalis of the trigeminal nerve begins by the bending downwards of the fibres of the sensory portion. The sensory and motor roots of the fifth nerve traverse the ventral part of the pons on their way to and from the region of the nuclei.
Above the level of the nuclei of the trigeminal nerve a new tract of fibres comes into view. This is the mesencephalic root of the trigeminal nerve, as it descends towards the rest of the nerve. It is a small bundle of nerve-fibres, semilunar in cross section, which lies close to the medial side of the brachium conjunctivum and on the lateral and deep aspect of the gray matter on the floor of the fourth ventricle (Figs. 500 and 501).
On a slightly deeper plane than the mesencephalic root of the fifth nerve, between it and the medial longitudinal bundle, and in close relation to the gray
matter of the floor of the ventricle, is the collection of pigmented cells which constitutes the substantia ferruginea.
The medial longitudinal bundle, as it is traced upwards through the tegmental part of the pons, maintains the same position throughout, and as it ascends it becomes more clearly mapped out as a definite and distinct tract. It lies close to the median raphe, and immediately subjacent to the gray matter of the floor of the fourth ventricle.
The lemniscus medialis, as it ascends through the tegmental part of the pons, undergoes striking changes in shape. In the lower portion of the pons its fibres, which in the medulla oblongata are spread out along the side of the median raphe, are collected together in the form of a loose bundle, which occupies a wide field, somewhat triangular in shape, on either side of the median raphe and immediately behind the basilar portion of the pons. As it proceeds up, the fibres spread out laterally until a compact ribbon-like layer is formed in the interval between the tegmental and basilar portions of the pons (Figs. 501 and 502).
Above the level of the trigeminal nuclei another flattened layer of fibres comes
FIG. 502.-Two SECTIONS THROUGH THE DORSAL PORTION OF THE PONS AT ITS SUPERIOR PART,
into view to the lateral side of the lemniscus medialis. To this the name of lemniscus
In the foregoing account it has been seen that the cerebellum is formed from two distinct rudiments, each derived from the posterior edge of the alar lamina immediately above the pontine flexure and the insertion of the vestibular nerve. As development proceeds during the second month there is a rapid proliferation of cells in the mantle layer of the cerebellar rudiments, and they become considerably thickened. But at first this thickening manifests itself not so much as a swelling of the superficial aspect of the cerebellum but as a bulging inwards into the cavity of the fourth ventricle (Fig. 503).