Book - An Atlas of the Medulla and Midbrain 8
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Sabin FR. and Knower H. An atlas of the medulla and midbrain, a laboratory manual (1901) Baltimore: Friedenwald.
|Florence Rena Sabin (1871 - 1953) and her collaborator presents one of the very earliest atlases of the human central nervous system, describing the midbrain and brainstem. This atlas was extremely useful for later researchers attempting to both understand the development and mapping of the midbrain and medulla. Florence Sabin later work was as a key historic researcher in early 1900's establishing our early understanding of both vascular and lymphatic development in the embryo.
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Chapter VIII. The Midbrain
1. The Relation of its Structures to the Central Fibre Mass
The midbrain has been difficult to model, inasmuch as it is not easy to give definite outlines to all of its nuclei and fibre-bundles. The key to the form relations of the region was given in the description of the medial lemniscus. The central fibre mass, including the medial lemniscus, the superior lemniscus and the lateral part of the capsule of the red nucleus, is placed obliquely in the midbrain and divides it into two areas. The medial area contains the red nucleus, a formatio reticularis area, the nuclei of the oculomotor and trochlear nerves and the fasciculus longitudinalis medialis (Plates ni, vn, vni), while the lateral area contains the substantia nigra, and, later in course of development, the pyramidal tract (Plate n). In the view from the lateral surface given in Plate n, the midbrain sheet is shown, with the substantia nigra in place. In Plate v the substantia nigra has been removed and the complete fibre-sheet is thus revealed. In a third view (Plate iv) the fibre-sheet itself has been removed and the whole midbrain medial to it is visible. In the ventral portion is seen the nucleus ruber and its capsule; dorsal to it is the space for the formatio reticularis, while toward the median line can be seen the fasciculus longitudinalis medialis, the stratum profundum album of the superior colliculus, together with the nucleus and root-fibres of the oculomotor nerve. These three views give a general idea of the whole area. Plate vni shows the midbrain seen from above. The form relations of the region are (1) the great size of its nuclei, namely, the nucleus ruber and the substantia nigra and (2) the deflection of the medial lemniscus, apparently due to the development of the nucleus ruber. In describing the midbrain, then, the relations of the fibre-sheet will first be considered; secondly, the structures that lie medial to it; and finally, those that lie lateral to it.
The fibre-sheet as a whole has been described in connection with the medial lemniseus, and reference will be made here only to those parts of it that belong essentially to the midbrain, namely, the lemniscus superior and the lateral capsule of the red nucleus. The latter will be considered in connection with that nucleus.
The lemniscus superior is best seen in a lateral view (Plates n superior and v). Its shape has already been described; it is a triangular sheet of fibres placed upon the lemniscus medial is. In the model the only means of distinguishing it from the medial lemniscus lies (1) in the thinness of the sheet and (2) in the fact that, while the medial lemniscus passes on beyond the region of the model, and is therefore shown with a square-cut edge, the lemniscus superior ends within the limits of the model and has a rounded edge.
The lemniscus superior appears to begin opposite the proximal part of the pons, where a few of the fibres of the medial lemniscus seem to radiate from the main bundle, making a fairly thick sheet at the start, which, however, grows thinner as it becomes wider. A further point of interest is that the dorsal border of the superior lemniscus comes to lie adjacent to the capsule of the nucleus colliculi inferioris.
The transverse series shows many interesting points in regard to the g uper i or lemniscus superior. In Fig. 40 there is no distinction to be made be- lemniscus tween the medial and the lateral lemnisci; but by passing farther cere- insectlons bralward to Fig. 42, it becomes evident that the two separate; between them is an area of fine fibres, those of the lemniscus superior. In Fig. 43 the fibres of the lemniscus superior are passing farther dorsalward. This shows clearly on the model. The lemniscus superior lies even farther lateral than the lemniscus lateralis itself. The next section (Fig. 44) shows interesting relations. The nucleus colliculi inferioris has almost disappeared. The lemniscus medialis is gradually curving into its characteristic midbrain position and the superior lemniscus is making its way to the region just lateral from the capsule of the nucleus of the inferior colliculus. Figs. 46, 47 and 48 take the superior lemniscus well into the region of the superior colliculus, and here the fibres are few, fine, and cut in cross-section. The transverse series is not complete enough to show the proximal limit of the lemniscus superior. In the longitudinal series, on the other hand, the lemniscus superior is apparently complete and the sections show that it does not pass beyond the midbrain (Fig. 12). Three sections will show the relative widths of the lemniscus medialis and the lemniscus superior: (1) Fig. 16, which shows the lemniscus medialis; (2) Fig. 13, a transition, and (3) Fig. 12, which shows the lemniscus superior. In this last section, as well as in Fig. 9, the superior lemniscus borders the large area of gray matter in the superior colliculus and, indeed, appears to have some relation with its cells. Indeed, there is a closely packed group of cells, almost a definite nucleus, opposite the end of the tract. 1 In passing still farther dorsalward it becomes impossible to distinguish the superior lemniscus from the capsule of the nucleus colliculi inf erioris (Fig. 7) ; but in the next section (Fig. 6) is seen the area of the nuclear capsule without question.
The model bears out closely the description of the relations of the lemniscus superior given by Forel. 2 If any of its fibres pass on, it must be those in the ventral part, where the bundle lies adjacent to the lemniscus medialis. On the other hand, the view of Elechsig, that the superior lemniscus ends in the superior colliculus, is well sustained.
The end of the tract, as seen in the model, is about opposite the point at which the fasciculus retroflexus of Meynert plunges into the nucleus ruber.
2. The Nucleus Ruber and its Capsule (Plate IV)
Red nucleus. The nucleus ruber has a capsule of cells and fibres on its dorsal, lateral and superior surfaces. The spinal surface of the nucleus, on the other hand, is related (1) to the fibres of the !N". oculomotorius, (2) to the brachium conjunctivum, (3) possibly to some fibres of the lemniscus medialis. The ventral surface at this stage lies in a mass of cells which underlies both the nucleus ruber and the substantia nigra (Plates iv, v and vm). This mass I have called the lectus or bed of the two nuclei.
The red nucleus will be described first, inasmuch as it makes so prominent a feature of the midbrain. It is seen from the side in Plate iv, and from a mesial aspect, in Plate vn. In the latter view a portion of the superior capsule of the red nucleus has been removed, as can be seen by comparing with Plates v and vm. The connection of the brachium conjunctivum with the red nucleus is seen in part on Plate iv, but far better in Plate vin. where other Red nucleus, structures have been sacrificed to show this relation on the right side of the view. The position of the nucleus ruber in this view is judged by the shape of its capsule.
1 This is in accord with v. Monakow, C., Experimentelle und pathologisch-anatomische Untersuchungen ueber die Haubenregion, den Sehhuegel und die Regio subthalmica, nebst Beitragen zur Kenntniss friih erworbener Gross- und Kleinhirn defecte. Arch. f. Psychiat., Berl., Bd. 27 (1895), S. 1-128. On S. 452 in the same volume, he refers to what I have termed the proximal limit of the lemniscus superior, as the place where the superior lemniscus and the chief part of the lemniscus fuse.
2 Forel, A., Untersuchungen iiber die Haubenregion un ihre oberen Verkniipfungen im Gehirne des Menschen und einiger Sangethiere mit Beitragen zu den Methoden der Gehirnuntersuchung. Arch. f. Psychiat., Berl., Bd. VII (1877), S. 393-495.
The nucleus is roughly oval but not regular in shape. It is placed in the ventral portion of the midbrain, surrounded on its dorsal, lateral and superior surfaces by a capsule of cells and fibres. The lemniscus medialis, in passing toward the thalamus, lies adjacent to the dorsolateral angle of the capsule, where the two fibre masses are practically indistinguishable. Just distal to the red nucleus, or at least to its dorsal portion, is situated the decussation of the brachium conjunctivum. In order to study the relations of the brachium conjunctivum to the red nucleus, it will be necessary to recall the various decussations of the tegmeiitum. In the description of the brachium conjunctivum, note was made of three decussations : (1) a commissure between Bechterew's nuclei, (2) the dorsal bundle of the brachium conjunctivum and (3) the ventral or main part of the brachium conjunctivum (Plate vin).
In regard to the relations of the brachium conjunctivum to the nucleus ruber, the model makes three points clear: first, that some of the fibres of the brachium conjunctivum pass into the dorsal capsule of the nucleus ; second, that some of the fibres pass through the nucleus ; and third, that others end in the nucleus. The fibres entering the dorsal capsule are distinctly visible on the left side of Plate vin, where they appear to spread out over the nucleus. The relation is evident in section (Fig. 16) which is taken just dorsal to the level of the nucleus ruber. The fibres that pass through the nucleus appear as a bundle cut in cross-section at the lateral, proximal angle of the dorsal capsule (Plate vin, left side). In regard to this bundle, a comparison of the two sides of the model will show three points: (1) that the bundle passes obliquely through the dorsal portion of the nucleus and leaves its dorsolateral portion ; (2) that it enters the lateral region of the capsule, which is Forel's x Feld BATh; (3) that it comes to lie immediately adjacent to the lemniscus medialis. The fibres that end in the red nucleus enter the dorsomedian portion and the space has been left vacant in
1 Forel says that BATh. consists mainly of an upward continuation of the brachium conjunctivum. Forel, op. cit., S. 426.
Plate vin to show their position. A single section will make these relations plain (Fig. 19). In passing ventralward through the nucleus, the fibres become much fewer and more scattered (Figs. 20 to 23). In fact, there are no medullated fibres in the ventral portion.
Capsule of the
The capsule of the nucleus ruber is peculiar in being a complex
- of cells as well as fibres. The dorsal capsule is almost wholly made
up of fibres; in the proximal capsule, however, cells predominate, while the ventral capsule at this stage of development is made up of cells with no medullated fibres.
The spinal surface of the nucleus is related to three groups of fibres; first, to the brachium conjunctivum ; second, to the fibres of the jSL oculomotorius, and third, to a few fibres that enter the midbrain from the lemniscus medialis (Plate iv).
The medial wall of the capsule is incomplete and consists of a few fine fibres adjacent to the dorsal capsule. Besides these, the fasciculus retroflexus of Meynert bounds a part of this surface (Plate vn). The dorsal capsule consists mainly, as has been said, of fibres from the brachium conjunctivum (Plates iv and vm). It covers the dorsal surface of the nucleus, and its lateral border lies adjacent to the medial lemniscus. Dorsal to this capsule is the formatio reticularis region of the midbrain, and medial to it is the fasciculus longitudinalis medialis. According to Forel, 1 the capsule receives fibres from each of these structures.
The dorsal capsule passes immediately into the superior capsule. Plate vm shows well the superior capsule with its relations to the fasciculus longitudinalis medialis, the fasciculus retroflexus Meynerti and the lemniscus medialis.
The area adjacent to the lemniscus medialis, including a portion of the dorsal and superior walls of the capsule, corresponds to Forel's 2 Feld BATh.
1 Forel, op. cit., S. 424.
2 Forel describes the area BATh as being dorsolateral to the nucleus ruber. Op. cit., S. 415.
The proximal capsule of the model will serve to illustrate in part ForeFs description of the area between the nucleus ruber and the thalamus, though the model includes but the lower border of the region. The dorsal part of the proximal capsule is rich in fibres, which it receives from the dorsal capsule. The middle part consists of mixed fibres and cells, while the ventral part has more cells than fibres; indeed, toward the lateral border there is a fairly definite nucleus. 1
In the centre of the dorsal edge of the proximal capsule the fasciculus retroflexus of Meynert plunges through the capsule and into the nucleus ruber (Plates iv and vm). The capsule is thickest at its lateral border. The relation of the fasciculus longitudinalis medialis to the capsule is an interesting one. In Plate vm will be seen the trough of the fasciculus longitudinalis medialis ' and the groove for the nucleus of Darkschewitsch. This groove opens out onto the surface of the superior capsule. "While the main bulk of the fibres of the fasciculus longitudinalis medialis do not pass beyond the nucleus, yet a small bundle of fine fibres passes onward into the medial border of the superior capsule and is gradually lost among its cells and fibres. This relation has been demonstrated already by Forel. 2
Reference has already been made to the lateral capsule as a part of the midbrain sheet. The lemniscus medialis itself forms a part of the lateral wall of the nucleus. The part of the capsule adjacent to the lemniscus medialis is by far its densest portion and its fibres enter Forel's B Feld BATh, and Flechsig's Haubenstralilung. The fibres of the ventral part are few and scattered.
The origin of the fibres of the lateral capsule deserves consideration. (1) The lemniscus medialis, as has been said, forms a part of the capsule; (2) fibres of the medial and ventral portion of the pontal sheet enter the midbrain and spread out over the lateral surface of the nucleus. In the sections it is hard to separate these fibres from those of the brachium conjunctivum (Fig. 20). (3) Fibres of the brachium conjunctivum appear in some sections to enter the lateral capsule. (4) In Plate v of the model can be seen a small nucleus lying in a lateral capsule and in the corresponding sections, certain fibres appear to be definitely related to this nucleus (Fig. 21). According to Fore], the capsule receives fibres from the nucleus ruber.
1 This agrees exactly with Forel, who has divided the area into three zones: a dorsal or Forel's Feld H, a middle or the zona incerta, and a ventral, or Liiy's body. Forel, op. cit., S. 415.
2 Forel, op. cit., S. 420.
3 Forel, op. cit., S. 425; v. Monakow, op. cit., S. 28. 7
The description of the lateral capsule as given in For el's article appears to me to apply more to its dorsal portion. Their origin is hard to trace. In passing farther ventralward (Fig. 20), there is a great thinning out of the capsular fibres. In the first place, the lateral wall has become reduced to a few scattered fibres. By following carefully between the last two sections, it seems clear that some of the fibres of the lateral wall come from the pontal sheet and the brachium conjunctivum. At the level of Fig. 21, however, its fibres appear to be directly related to a small mass of cells lying distal to the nucleus ruber and surrounded by the fibres of the root of the 1ST. oculomotorius (K\i. x. of 1. c. of N"u. r., Plate iv). Forel's Feld H shows clearly in Fig. 21. The fasciculus retroflexus on the medial border will be described later. In passing through Figs. 22 and 23, it is evident that the fibres around the nucleus are becoming fewer, while the cells become more and more numerous, especially in the proximal and adjacent lateral capsules. In Fig. 24 are the cell masses that underlie both the nucleus ruber and the substantia nigra.
The longitudinal series is better than the transverse for obtaining a clear idea of the relations described above. Starting from, the dorsal aspect, in Fig. 13, is seen the area of the formatio reticularis which lies dorsal to the nucleus ruber; from this, one passes into the area of the definite dorsal capsule in Fig. 16. The relation of the fasciculus longitudinalis medialis to the nucleus of Darkschewitsch and the superior capsule of the nucleus ruber may be followed at the same time as far as Fig. 21. In Fig. 19 are seen, (1) the lemniscus medialis as a part of the lateral capsule; (2) the brachium conjunctivum just distal to the nucleus; (3) the relations of the N. oculomotorius; (4) Feld BATh of Forel; and (5) the fibres of the fasciculus longitudinalis medialis passing into the nucleus of Darkschewitsch (Nu. f. 1. m.). In this section it will be noted that there are a few fibres making a medial capsule.
3. Fasciculus Retroflexus of Meynert
Fasciculus The fasciculus retroflexus of Meynert is easy to follow in sections t 8 (Fig. 19). Its position and relation to the nucleus ruber are clear in the illustrations (Plates iv, vn and vm). Its nucleus of origin is outside of the limits of the model. The bundle enters on the proximal aspect of the model as a band 1 mm. wide, and passes obliquely medialward and ventralward, and plunges through the nucleus ruber and its capsule. It both enters and leaves the medial surface of the nucleus entering near the proximal border just dorsal to the middle of the nucleus and leaving near the distal border at about the middle of the dorsoventral diameter. On emerging from the nucleus the bundle immediately spreads out into a sheet more than twice as broad as the entering bundle (Plate vn). The fibres end in the borders of the fossa interpedunculare just distal to the nucleus ruber. Between the fibres are to be seen a few scattered cells, the remains of the ganglion interpedunculare. The root-fibres of the !N". oculomotorius pass through the area of the bundle as it emerges from the nucleus ruber.
In longitudinal sections the bundle is easily traced. In Fig. 16 are seen a few fibres just entering* the edge of the section. It will be noted that only the peripheral fibres of the bundle are medullated. Figs. 19 and 20 carry the bundle to the edge of the nucleus ruber, while Figs. 21 and 22 take it through the nucleus. The last trace of the bundle is seen in Fig. 23, and here the cells of the ganglion are especially clear. The transverse series does not go entirely through the nucleus ruber, so the entrance of the bundle can not be seen; however, Fig. 50 shows its fibres near the region of the ganglion interpedunculare.
4. DECUSSATIO TEGMENTI DORSALIS MEYNEKTI.
The decussatio tegmenti dorsalis Meynerti shows in Plate vm Dorsal tegmentai (Fig. 47). It lies proximal to the ventral part of the brachium conjunctivum. The course of the fibres to the decussatio is indicated in Plate iv. If the bundle in Plate iv be imagined as continued to the level of the radix descendens (m.) 1ST. trigemini, the tract can be well traced. The fibres are difficult to follow in the longitudinal series in this part of their course, but easy to trace in transverse sections. The bundle then starts just lateral to the radix descendens (m.) E". trigemini, 1 and curves across the midbrain between the brachium conjunctivum and the stratum album profundum.
1 Meynert believed that the bundle has a definite relation to this nerve. Cf. Forel, op. cit., S. 442.
The decussation is just ventral to the fasciculus longitudinalis medialis. In Plate vm it can be made out that the decussation curves so that the fibres on leaving it turn toward the spinal cord. In the longitudinal sections these fibres can be seen to pass through the brachium conjunctivum and to enter the longitudinal bundles of the formatio reticularis alba of the pars dorsalis pontis. These fibres have not been differentiated in the model from the other longitudinal bundles of the middle region of the tegmentum. 1
In tracing the fibres of this bundle in the longitudinal series, it will be easiest to begin with the decussation (Fig 1 . 19). By passing dorsalward, one can trace at the same time the fibres coming- to the decussation from the level of the radix descendens (m.) N. trigemini, and the fibres leaving the decussation to form a longitudinal path in the pons. In Figs. 16 and 18 are seen the fibres turning toward the pons. The bundles passing to the decussation do not come out in these drawings, since the fibres are delicate and are mixed with the bundles of the root of the N. oculomotorius. In Fig. 13 (D. t.), however, these fine fibres are plain.
In the transverse series the decussation is seen just ventral to the nucleus of the N. oculomotorius (Fig. 48), while the fibres passing to it are plain on Fig. 47. In this section the fibres show a broad curve and there is a space between them and the stratum profunduin album.
5. Decussatio Tegmenti Ventralis of Forel
ventral tegmentai The decussatio tegmenti ventralis of Forel is represented at this VonL stage by a few delicate fibres ventral to the decussatio tegmenti dorsalis Meynerti. They show in Plate vm, and in Fig. 48 and in Fig. 20 (D. t).
6. Stratum Album Profundum
Deep white layer The stratum album profundum of the superior colliculus is conn ' spicuous in the model of the midbrain (Plates in, iv, v and vm). The deep white fibre layer is, in reality, a composite of fibre systems, but its shape as a whole will be considered first. It is a hollow shell that outlines the central gray matter of the midbrain (Plate m). Through the centre of the cavity of the shell passes the central canal describing the midbrain curve; the sides of the shell rest upon the fasciculus longitudinalis medialis as a base (Plate iv). As seen in Plate m, the shell is open on its distal and dorsal aspects, while at the proximal end it is completed across the midline by an arch. The arch is placed vertically (Plate vm). The side view is of value in studying its relations (Plate iv). This view shows the curve of the fasciculus longitudinalis medialis through which passes the radix N. oculomotorii. Dorsal to the
1 The fibres from Meynert's decussation have been called the descending bundle of the tegmentum, and are pictured by Eamon y Cajal. Ramon y Cajal, op. cit., S. 114.
fasciculus longitudinalis medialis stretches the lateral wall of the deep white shell. In this wall is an oval space, the distal end of which is just dorsal to the opening for the root of the oculomotor nerve. The wall is divided into two parts by a curved ridge that runs in a dorsoventral direction. This ridge lies at the junction of the middle and proximal thirds of the wall and extends dorsalward from the proximal end of the space just mentioned. Distal to the ridge is a depression, which is due simply to the prominence of the ridge. This ridge appears to mark a difference in structure between the two parts, for distal to it the wall consists of a thin flat sheet in which the fibres run longitudinally, while proximal to it is the arch of the shell in which the fibres curve across the midline. This arch, in contrast to the thin sheet below the ridge, is thick and densely packed, at least in its ventral portion. The details of the direction of the fibres are to be considered later. The distal margin of the flat sheet is marked on the external surface by a dorsoventral groove, below which pass the fibres of the radix descendens (m.) !N". trigemini. This root of the trigeminal nerve passes spinalward in about the same plane as that occupied by the deep white sheet.
The internal wall of the shell is practically the converse of the external (Plate m), for opposite the external ridge is a concavity, while just distal to it is an internal ridge. Passing spinalward from this internal ridge is the flat sheet, which runs down to a second ridge on the internal surface. This ridge lies in the lower part of the midbrain and practically limits the deep white layer. Distal to it runs the radix descendens 1ST. trigemini. The exact point at which the deep white ends and the root of the ~N. trigeminus begins has never been determined, but the model at least suggests that this ridge is the junction of two different fibre systems. The arch or commissure is to be seen in Figs. 5 and 12. In the latter the inner wall of the base of the arch comes into sight. Just at the junction of the pillars of the commissure with the fasciculus longitudinalis medialis and the proximal capsule of the nucleus ruber is the groove for the nucleus of Darkschewitsch. This groove has already been noted as opening out upon the superior capsule. The relative thickness of parts of the arch varies markedly. The ventral part contains most of the fibres. Dorsal to it is an area which shows no medullated fibres at all. The sections in this area, however, are decolorized more than the rest of the series. Just dorsal to this space is a narrow band of a few, fine, decussating fibres.
Deep white layer These form relations, namely, the external and internal ridges and the in sections. i ower limiting ridge, together with the arch over the canal, will make clear the direction of the fibres, as seen in sections. For example, between the proximal, internal ridge and the lower limiting ridge, the fibres run in a longitudinal direction and form a definitely circumscribed, though narrow bundle (Figs. 5 and 6). At the limiting groove these fibres appear to break, and distal to the groove is situated the radix descendens (m) N. trigemini. This can be traced in Fig. 5 directly to the level of the main root of the N. trigeminus, a point which is seen clearly in the model (Plate v). In section (Fig. 6) the descending root of the nerve is more broken, inasmuch as this is the level of its nucleus, of origin, which is situated in the locus cseruleus.
To return to the midbrain shell, the same two sections (Figs. 5 and 6) will show that the external ridge is made up of fibres running in a wholly different direction. Indeed, they slant into the deep white from the formatio reticularis, so that while the internal border of the deep white is definite, the external border is extremely indefinite. These fibres from (or to) the formatio reticularis slant into the ridge and decussate in the commissure.
Passing Tentralwa'rd we come to a level in which no decussating fibres can be seen, but the external ridge continues to have the same structure (Fig. 9). The next few sections, however, enter into the area of the decussation and the pillars of the arch (Figs. 11, 12 and 13). The section in Fig. 16 passes through the nucleus of Darkschewitsch at the base of the pillars of the arch. In the transverse series the stratum album profundum can be traced through Figs. 43 to 51. Certain points are brought out more clearly here than in the longitudinal series: (I) the fibres that slant into the ridge (Figs. 47 and 48), (2) the posterior commissure and its relation to the nucleus of Darkschewitsch (Figs. 50 and 51) (Nu. c. p.).
In regard to the course of these fibres the model confirms the findings of Held rather than those of Ramon y Cajal, the former stating that the fibres slant into the posterior commissure from the superior colliculus. These fibres decussate in the arch and pass down in the pillars to the nucleus of Darkschewitsch.
The fibre relations of the deep white layer have now been completed, except the fact that the space left in the lateral wall really contains fibres (Plate iv). A single section will show, however, why this space was left in the model (Fig. 12). The contrast between this section and Fig. 6 is marked, for instead of having the narrow band between the medial ridge and the N. trigeminus, as in Fig. 6, the corresponding area in Fig. 12 is evidently a place where fibres of the fonnatio reticularis alba end (or begin) in the central gray matter of the midbrain. This stratum profundum album forms the boundary of the central gray matter, and the area just described shows that at least some of its fibres are connected with the cells within. The contents of the midbrain shell are: (1) the nuclei of the oculomotor and trochlear nerves and the roots of these nerves, (2) the central gray matter, and (3) the central canal.
7. Substantia Centralis Grisea
The shape of the central gray matter, substantia centralis grisea, central gray is worthy of mention. Its outline is given in the model only in the m region of the midbrain, where the stratum album profundum forms a definite border for it (Fig. 6). In the medulla oblongata, however, it can be readily constructed from the shape of the floor of the fourth ventricle. Certain structures in the central gray matter have already been considered, namely, the various nuclei of the cerebral nerves. These can be divided into two groups, first, the median motor groups of nuclei, that is, of the Nn. hypoglossi, abducentes, trochleares et oculomotorii, which lie embedded in the fasciculus longitudinalis medialis on the border of the central gray matter (Figs. 6 and 12); second, certain of the sensory nuclei, namely, the superior and medial, vestibular nuclei, the nucleus tractus solitarii and the nucleus alse cinerese (Figs. 6 and 7).
Besides these nuclei connected with the cerebral nerves, there is a definite and clearly defined nucleus in the pons region. It corresponds to the descriptions of the substantia ferruginea and lies opposite the decussation of the brachium conjunctivum (Plate m, Fig. 7). In both series, and in the model as well, this nucleus is situated in the central gray matter, exactly dorsal to the fasciculus longitudinalis medialis, whereas, in ForeFs picture, it is placed slightly dorsolateral (Figs. 7 and 41).
The central gray matter of the midbrain is better developed than that elsewhere. Its shape is outlined by the stratum album profundum and the fasciculus longitudinalis medialis; it contains the nuclei of the oculomotor and trochlear nerves, as well as the nucleus of Darkschewitsch. It has been mentioned that certain of the fibres of the formatio reticularis appear to be connected with it.
To complete the description of the area of the midbrain medial to the midbrain fibre-sheet, there is left the large area which lies between the stratum album profundum and the lemniscus superior and dorsal to the nucleus ruber (Plate vm). This area contains, first, the nucleus colliculi inferioris, which has already been described, and second, the formatio reticularis of the inferior and superior colliculi. The formatio reticularis of the entire model, however, is to be considered later.
These structures, lateral from the midbrain sheet, are the substantia nigra (together with a small nucleus possibly derived from it) and the pyramidal tract.
8. The Pyramidal Tract
The pyramidal The pyramidal tract is non-medullated at birth, but its position ' can be related as follows: In Plate n it lies just external to the substantia nigra. (This is best seen in Quain's Anatomy, op. cit., Vol. in, Pt. i, page 42, Fig. 33; also see other text-books). It plunges through the pons (Fig. 22) and lies on the ventral surface of the medulla oblongata. Its decussation at the junction of the cord and medulla is on Fig. 21.
9. Substantia Nigra
substantia nigra. The substantia nigra lies in the ventral part of the midbrain (Plate n). It is larger than the nucleus ruber. It is oblong in shape and is placed against the lateral surface of the lemniscus medialis and lateral capsule of the nucleus ruber. It lies at an angle to the long axis of the model, so that its proximal edge is farther from the midline than the distal. Just lateral to it is the area in which the pyramidal tract develops at a later stage, and though the model does not show the tract, it can readily be imagined plunging into the pons in this region.
The shape of the nucleus is fairly regular. The ventral border a straight line in the model rests upon the cellular area which the substantia nigra and the nucleus ruber have in common. The nucleus ruber can be easily outlined from this cell-mass by its color, but it is practically impossible to make a definite ventral limit in these sections for the substantia nigra. The dorsal border of the substantia nigra is curved, and, at the distal dorsal angle, the nucleus is notched to receive a bundle of fibres from the lemniscus medialis (Plate n). This bundle is shown in Plate v.
The substantia nigra is familiar in the sections ; it is to be seen in all Substantia nigrs the cross-sections of the midbrain and in all the longitudinal sections in sections. of the ventral part of the same. Its relations to the lemniscus medialis and the nucleus ruber are brought out well in both series (Figs. 19 to 24). The first two sections of the series show the bundle of fibres of the lemniscus medialis, that enters the substantia nigra. The last section shows the cell-area common to the two large nuclei of the midbrain. (Lectus substantia nigrse et nuclei rubri.) The absence of fibres except the small bundle from the lemniscus medialis is to be noted in the sections. The transverse series (Fig. 49) may be taken as a type of the nucleus. The angle, at which this section is cut, readily explains itself on the model. A cross-section taken at the extreme spinal end of the nucleus shows the bundle entering from the lemniscus medialis (Fig. 46). The connection appears to be much more intimate from this aspect than the longitudinal series showed.
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An Atlas of the Medulla and Midbrain (1901): Chapter I. Introductory | Chapter II. The Long Tracts | Chapter III. The Columns Of The Spinal Cord | Chapter IV. Cerebellar Peduncles | Chapter V. The Cerebral Nerves And Their Nuclei | Chapter VI. The Cerebral Nerves And Their Nuclei (Continued). Lateral Group | Chapter VII. The Inferior And Accessory Olives | Chapter VIII. The Midbrain | Chapter IX. The Formatio Reticularis Alba And Grisea | General Summary of what Is shown In Reconstruction | References To Literature | Abbreviations | Description of Figures and Plates
Cite this page: Hill, M.A. (2021, April 10) Embryology Book - An Atlas of the Medulla and Midbrain 8. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_An_Atlas_of_the_Medulla_and_Midbrain_8
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