Book - An Atlas of the Medulla and Midbrain 5
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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 V. The Cerebral Nerves And Their Nuclei
The grouping of the cerebral nerves An indication has already been given of the grouping of the cerebral nerves, suggested by the model, namely, into a median and a lateral group. This grouping has been used before by several authors, notably by His, but it is rather more common to consider the nerves in three groups according to the region they occupy, namely, four in the medulla, four in the pons and two in the midbrain. Dr. L. F. Barker, in his recent book, has divided the nerves into a sensory and a motor group on the basis of the direction followed by the fibres. The division herein used, on the basis of the position of the nuclei, it seems to me has decided advantage, not only for the purpose of describing the model, but in illustrating the relation of these nuclei to the gray matter of the cord. 1
The two groups possess characters in contrast. Each contains four motor nuclei, while the lateral group has sensory nuclei as well. The median group consists of the nuclei of the ~N. hypoglossus, N". abducens, N. trochlearis and !N". oculomotorius. The four motor nuclei of the lateral group are the nuclei of the N. accessorius, IN", vagus, "N. glossopharyngeus, !N". facialis and !N". trigeminus. Its sensory nuclei are nuclei of termination associated with the ~N. vestibuli, 2sT. cochleae and JST. trigeminus. The nuclei of the median group all lie near the middle line just ventral to the central canal and closely related to the fasciculus longitudinalis medialis (Plate v). The nuclei of the "N. hypoglossus and IT. abducens are very near the dorsal surface, while the nuclei of the N. trochlearis and !N". oculomotorius are farther ventral inasmuch as they lie on the midbrain curve. The nerve-fibres from all the nuclei of the groups, except those of the root of the ET. trochlearis, pass directly ventralward to the surface near the middle line. The root of the N. trochlearis, on the contrary, takes an anomalous course, since it passes dorsalward, decussates in the velum and has its superficial origin on the dorsal surface of the isthmus of the rhombencephalon.
1 This division is in accord with the embryological work of His. Cf. His, W., Zur Geschichte des Gehirns sowie der centralen und peripherischen Nervenbahnen beim menschlichen Embryo. Abhandl. d. math, phys. Cl. d. K. sacfis. Gesellsch. d. Wissensch., Leipz., Bd. xiv (1887-88), S. 339-392.
The motor nuclei of the lateral group lie in the f ormatlo reticularis at a considerable distance from the middle line, and at a level ventral from the central canal. With the exception of the 1ST. trigeminus the root-fibres from these nuclei do not take a direct course toward the surface of the central nervous system. The root-fibres of all four of the nerves have their superficial origin in the lateral sulcus. In the model the nucleus of the "N. accessorius does not appear, for it could not be outlined with sufficient definiteness for reconstruction. This is an interesting form relation, for the nucleus of the "N. accessorius lies just at the point where the ventral horn of the spinal cord divides into two parts to make the median and lateral motor groups of the region modeled. The nucleus of the N. accessorius then marks the transition from the indefinitely outlined nuclei of the cord to the definitely circumscribed nuclei of the cerebral motor-nerves.
The sensory nuclei of the entire region belong to the lateral group. The sensory nuclei are associated with the IsT. vagus, !N". glossopharyngeus, 1ST. vestibuli, N". cochlese, E". intermedius and N. trigeminus, and the superficial entrance of their root-fibres is without exception on the lateral surface (Plates n and iv). These nuclei tend to occupy a position farther dorsal than the motor nuclei of the lateral group, which is, as one might expect, inasmuch as these nuclei correspond more or less closely to the gray matter of the dorsal horn of the cord. In making this general statement, I do not intend to enter into the discussion in regard to the development of the optic and cochlear nuclei. These sensory nuclei cover a wide area, thus contrasting with the compact motor nuclei; in some cases they spread even to the midline. In general, the sensory nuclei lie nearer the surface and so show on Plates n and iv, while the motor nuclei are deeper and show on Plate v.
The Median Group
Nucleus, N. in. (a) N. Hypoglossus (Fig. 31). The first nerve of the median motor group is the N. hypoglossus (Plate v). In two instances, namely, in regard to the nucleus of the hypoglossus, and the nucleus ambiguus, the model represents the position but not the exact form. Inasmuch as the nucleus !N". hypoglossi lies on the medulla curve, it will be seen that only a small portion of it would be cut in any one longitudinal section. Moreover, the nucleus lies in the central gray matter, and the scarcity of fibres tends toward the rapid washing out of the stain from the cells. However, the nucleus itself has a few fibres which have a characteristic arrangement. They are fine fibres that come up into the centre of the nucleus and spread out like a fountain. 1 In longitudinal section these fibres are cut across, and make a row of fine dots that represents the nucleus. The position of the nucleus was determined (1) by the presence of these fibres, (2) by well-known relations to neighboring structures, (3) by measurements from the transverse series and (4) by the position of the nuclear end of the root-bundle. Of the form of the nucleus, I shall make but general statements, namely, that it is long, that its dorsal border conforms with the cervical curve, and that its breadth, judging from the transverse series, is not wholly uniform.
The nucleus extends more than one-half the length of the medulla and corresponds in length, generally speaking, to the nucleus olivaris inferior. The distal end is slightly farther spinalward than the olive, being opposite the distal end of the median accessory olive. It lies lateral to the central canal before it opens out into the fourth ventricle. Farther toward the cerebrum, the nucleus is just beneath the floor of the fourth ventricle. Mention has been made of the slight curve in the fasciculus longitudinalis medialis which corresponds to this nucleus (Plate vi). The accessory nucleus of Roller lies within the fasciculus longitudinalis medialis (Fig. 10) opposite the proximal end of the nucleus of the hypoglossal nerve.
The root-fibres of the nerve leave the nucleus from its ventral Root-fibres, border (Plate v). When modelled, the root-bundle makes a N ' ra " sheet which passes ventralward and slightly lateralward to a superficial origin between the olive and the pyramidal tract. As seen from the side, this sheet has three borders a distal, a proximal and a ventral. The distal border is shorter than the proximal, owing to its position on the cervical curve. It will be noticed that near the ventral surface of the medulla some of the root-fibres leave the surface of the sheet and turn lateralward. They enter the hilus of the nucleus olivaris inferior and there the bundle ends abruptly, for the sections were too decolorized to permit one to trace it to a superficial origin (Fig. 21).
1 They have been well pictured by Koch and termed Fibrae afferentes mi. xii. Cf. Edinger, L., Vorlesungen iiber den Bau der nervosen Centralorgane, Leipz. (1893), S. 180.
The ventral border, namely, the line of the superficial origin of the fibres, has a slightly greater length than the nucleus. It lies in the groove between the pyramid and the olive. Its inferior limit is just distal to the olive, while its proximal end is at the junction of the medulla and pons. The surface of the sheet is smooth, except for the bundle of fibres that projects into the hilus of the olive. The direction of the fibres is worthy of attention. Starting from the ventral border of the nucleus the fibres pass ventralward and slightly lateralward to the level of the dorsal border of the olive. In passing the olive the fibres turn slightly more lateral, while farther ventral still the lateral curve is marked. The reason for these two curves will be evident. In the dorsal part only the medulla sheet intervenes between the middle line and the hypoglossal nerve. In the middle part, the medial accessory olive, as well as the medulla sheet, lies between the raphe and tne nerve, while in the ventral region the pyramidal tract intervenes.
N. in in sections. These points are shown in Fig. 31. The nerve can be traced through the following sections (Figs. 7-23) : The first section shows a few cells of the nucleus, while Fig. 9 shows the fine fibres that aided in its determination. In all of the succeeding sections it will be noted that the root-fibres of the nerve pass out in small, definite bundles rather than as single fibres. By tracing the series the course of the fibres outlined above can be seen. In the dorsal part the fibres lie against the medulla sheet; in the middle part they lie between the medial accessory olive and in the inferior olivary nucleus, while in the extreme ventral portion the fibres are bent markedly from the middle line, owing to the volume of the pyramidal tract. The last two sections of the series give a good idea of the surface-origin.
Nucleus, N. vi. (&) N. Abduceus (Figs. 37 and 38). The N. abducens is the second nerve of the median group. Its nucleus makes a landmark in all of the views of the model, but shows best in Plates v and vi. In Plate v it will be seen that a considerable distance (2.5 mm. in the medulla) intervenes between the nucleus N. hypoglossi and the nucleus N. abducentis.
The nucleus of the !N". abducens lies in the distal part of the g9^pons, just beneath the fourth ventricle and lateral to the fasciculus longitudinalis medialis. In this relation to the fasciculus longitudinalis medialis it differs from the other nuclei of the group, all of which lie dorsal to the tract and make a groove on its surface. The model will show that the fibres of the N". f acialis in curving around the nucleus of the "N. abducens appear to draw it from the middle line. The root-fibres leave the nucleus at its ventromedial portion, and pass ventralward and slightly spinalward to emerge between the pons and medulla near the median line.
The nucleus itself is nearly round, with a diameter of 1.7 mm. It projects above, that is to say, dorsal to the fasciculus longitudinalis medialis (Plate vi). The relations of the root of the facial nerve to the nucleus of the 1ST. abducens will be considered in detail in connection with the former nerve. The median boundary of the nucleus is made by the root-fibres of the facial nerve and by the fasciculus longitudinalis medialis (Figs. 6 and 7). The proximal boundary is made in part by the root of the facial nerve. The root-fibres of the ~N. abducens leave the medio-ventral part of the nucleus in small but dense bundles (Fig. 37). In passing to the superficial origin at the junction of the pons and medulla, these small bundles make a fairly compact bundle which passes ventralward and spinalward. In Plate vi it will be seen, however, that the fibres describe a curve in their course, for in starting from the nucleus they pass first toward the cerebrum, as well as ventralward, before turning toward the cord. The bundle passes through the trapezoid body, and leaves the central nervous system just proximal to the groove between the medulla and pons.
The nucleus is prominent in both series: in the transverse series N. vi in sections, because it projects so far into the central gray matter; in the longitudinal series because it is so definitely outlined by the root of the facial nerve.
Fig-. 36 and Fig-. 37. In tracing 1 the nerve in the transverse series, the curve, seen in Plate vi, is slightly exaggerated by the obliquity of the sections (Fig. 52). Three sections will illustrate this point; Fig. 36, which shows simply the nucleus; Fig. 38, which shows the root-fibres just as they are leaving the nucleus and again at the superficial origin, and Fig. 39, which shows simply the middle of the curve with neither the nucleus nor the superficial origin.
The shape of the bundle in the dorsal portion is fairly rounded, as seen in Figs. 16 and 19, but farther ventral, near the superficial origin, the bundle becomes oval in shape, with the long axis perpendicular to the long axis of the ports. The fibres emerge from the lower part of the pons just proximal to the groove between the pons and medulla.
Nuclei, N. iv. (c) N. Trochlearis (Fig. 41). In the case of the remaining two nerves of the group, the N. trochlearis and the N. oculomotorius, the nuclei of both sides have been modelled (Plates n, in and iv). These nuclei lie in the trough of the fasciculus longitudinalis inedialis on its midbrain curve. The walls of the trough, as has been seen, are hollowed out on either side of the deep central groove to receive them (Plate vni). The nucleus of the trochlear nerve lies farther distal, i. e., nearer the cord, and hence farther dorsal on the curve. It lies in the proximal or cerebral part of the colliculus inferior (Plate in).
The nuclei of the two sides are not symmetrical, being single on the left side and double on the right. However, the volumes of the cell-masses of the two sides are about equal. In the case of the double nucleus, the proximal part is evidently the main nucleus, that is to say, it corresponds to the nucleus of the other side. The left or single nucleus is approximately cubical in shape, with a projection at the distal medial angle. It measures about 1 mm. in diameter. On the right side it is as if the small projecting portion had been pulled dorsalward until it just separated from the main mass.
Nuclei of N. iv This lack of symmetry of the nuclei of the two sides is brought out in sections. j n the sections. The accessory part of the double nucleus, which lies farther dorsal and farther distal, shows on Fig. 10, while sections in Figs. 11 and 12 show the main nuclei of both sides. The transverse series does not show just the same irregularity, for in it the nuclei of both sides are double. Moreover, the distal nucleus of either side lies at least 1 mm. from the proximal. The section in Fig. 44 shows this distal part, while Fig. 46 shows the main proximal part. In both sections the root-fibres of the nerve can be seen.
Root-fibrea, N. IT. The nuclei of the nerve are compact and definitely outlined; nevertheless small groups of cells may be traced all along the rootfibres on either side. When the root-fibres leave the lateral and dorsal border they first pass laterally in scattered bundles and then turn dorsalward and slightly spinalward. In their dorsal course the fibres are collected into from two to four compact bundles. The fibres lie in the central gray matter just within the stratum profundum album. Plate in shows the decussation dorsal to the central canal. The superficial origin of the fibres is asymmetrical. On the side of the single nucleus the fibres have a larger and more ventral superficial origin than those of the other side.
The course of the fibres can be traced in the longitudinal series in the N. iv in sections, following sections: Figs. 12-10 show the nucleus and its relation to the fasciculus longitudinalis medialis. It will be noticed that the section in Fig. 12 shows no root-fibres, since it passes through the ventral portion of the nucleus. The root-fibres can be followed through the rest of the series from Fig. 10 to Fig. 3. They are to be distinguished from the fibres of the fasciculus longitudinalis medialis by a difference in direction. The last section, Fig. 3, shows the decussation.
By looking at the nerve in the model from the side and from the dorsal aspect, the appearances in the transverse series can be readily predicted (Plates n and in). The most distal section w 7 ould show the decussation, while each succeeding section would show two or three small bundles cut across or slightly obliquely, and occurring a littlefarther ventral in each section until the level of the nucleus is reached, and here the fibres would turn directly medialward. This course can be followed in the following series, Figs. 41 to 46. 1
(d) N. oculomotorius (Fig. 48). The E". oculomotorius is the Nuclei, N. m. last of the median motor group to be considered. The position of its nucleus is best seen from the dorsal surface (Plate m), but the course of the root of the nerve and the relations must be followed in a view from the side (Plate iv). The nucleus as seen in Plate in is a long mass of cells lying in the midbrain trough of the fasciculus longitudinalis medialis. The root-bundle passes directly ventralward near the middle line and emerges in the fossa interpedunculare. The position of the nucleus in the trough of the fasciculus longitudinalis medialis determines two facts: (1) that the nucleus as a whole lies farther ventral than the nucleus N. trochlearis; (2) that the nucleus itself is placed obliquely to a horizontal plane, so that the distal end is farther dorsal than the proximal.
1 The root-fibres have been described as making a double bend, passing at first dorsalward, then spinalward, and again dorsalward at a right angle. Cf. von Kolliker, A., Handbuch der Gewebelehre, Bd. n, Leipz. (1896); also Van Gehuchten, A., Anatomic du systeme nerveux de 1'homme, 2 ed., Louvain (1897); and Barker, L. F., The Nervous System and its Constituent Neurones, N. Y. (1899), p. 938, et seq. This course does not appear on the model, which shows a gradual dorsal curve from the very start. Such an angle was not suggested in either of my series but there is no doubt that it might be missed in building the model. The matter might fee settled by referring to sagittal sections. On the other hand Forel (Arch. f. Psychiat., etc., Berl., Bd. vn , S. 439) describes the course as it is shown in the model.
The oculomotor nerve is the only nerve in the central nervous system of which the nuclei of the two sides lie near enough together to be modelled as one. The nucleus as a whole consists of two lateral parts which are fused together in the ventral portion so as to make a gutter 3.1 mm. long for the median nucleus. The entire nucleus is 5.3 mm. long.
The shape of the combined mass is that of a triangular prism with the apex pointing ventralward and the base dorsalward. The dorsal surface is a triangle with the apex cut off. The distal end is the base of the triangle and the walls slant medialward, so that the proximal edge is about one-half the length of the distal. These two walls make the chief nucleus Hauptkern of the Germans.
The distal fourth of the nucleus is bounded definitely by the fasciculus longitudinalis medialis. In the next fourth of the nucleus, however, the cells scatter out into the fasciculus longitudinalis medialis, and these cells have been sacrificed to the fibres in the model (Fig. 48). These cells make the lateral part of the main nucleus as described by v. Bechterew. 1
The median nucleus is clearly shown in Plate in. The small rounded proximal nuclei of v. Bechterew are shown as the proximal end of the main nucleus, but the small paired accessory nuclei did not come out clearly in the longitudinal series, though they were seen in the transverse series.
N. in in The following- series taken from the transverse sections will show sections. mO re details of the nucleus than have been introduced into the model (Figs. 47 to 50) :
In the first place there are variations in the shape of the lateral nuclei: for example, Figs. 47 and 48 show an interesting contrast. The former shows cells spreading out to the side, making the outlines of the nucleus rounded, while the latter shows straight clean-cut sides narrowing to a sharp apex. Secondly, the sections show contrasts in the arrangement of the fibres within the nuclei. For example, compare the first two sections with the last two. Thirdly, the sections show that the median nucleus is not uniform. (Compare Figs. 48 and 49.) The longitudinal series shows practically the same points (Figs. 12 to 19). The last section, however, gives a different view of the anterior fused mass. All these details need a higher magnification.
Root-fibres, Turning now to the course of the root-fibres after leaving the nucleus, it will be found that the best view is given in Plate iv.
- v. Bechterew, W., Ueber die Kerne des Oculomotorius, Abducens u.
Trochlearis. Arch. f. Anat. u. Phys., Anat. Abth., Leipz. (1897), S. 308.
This view shows the brachium conjunctivum, the stratum profundum album of the superior colliculus and the posterior commissure. The shell of deep white layer is connected with the fasciculus longitudinalis medialis. Through a space left in this shell a portion of the nucleus of the oculomotor nerve can be seen, while just ventral to this space the nerve-fibres emerge. The root-bundle runs ventralward between the brachium conjunctivum and the red nucleus to the superficial origin in the fossa interpedunculare.
The bundle representing the fibres appears to be an irregular one, but that is merely because it is encroached upon by other structures. Were it not for these, the bundle would be nearly regular in outline. The root-bundle can be considered as divided into two parts: (1) the part extending from the nucleus of the nerve to the ventral surface of the red nucleus including about three-quarters of the length of the bundle, and (2) the portion ventral to the red nucleus. The reason for this division can be made plain. In passing from the nucleus the fibres spread out to the side and make a bundle which fits around the distal surface of the red nucleus (Fig. 48). This makes the greatest breadth of the bundle in a transverse diameter. On the other hand, the second portion covers the exit zone of the nerve-fibres where the entire bundle rotates so that its broad side faces the surface of the fossa interpedunculare (Fig. 51). This is to be seen from the mesial aspect, but the idea can be obtained from Plate iv. It is important to note that this relation of the shape of the bundle does not involve any rotation of the fibres. The first portion of the bundle has two curves: (1) a curve in a dorsoventral diameter which is concave toward the spinal cord and enables the bundle to fit in between the decussation of the brachium conjunctivum and the nucleus ruber; (2) a curve in the transverse diameter with the convexity toward the cerebrum by which the bundle accommodates itself to the ventral surface of the nucleus ruber. It must be stated that some of the nerve-fibres pass directly through the nucleus ruber, so this curve is only an arbitrary one. Opposite the extreme ventral part- of the nucleus ruber the bundle is separated by a considerable space from the nucleus inasmuch as the surface of the nucleus curves away from the plane of the nerve.
Of the irregularities on the surface of the bundle only one is due to the nerve-fibres themselves, namely, the projection on the lateral border near the dorsal surface. This is due to the fact that just ventral to the lemniscus medialis the nerve-fibres spread out laterally. The other irregularities are due to spaces left for other structures: (1) a nucleus in the lateral capsule of the red nucleus, (2) a little mass of cells I have referred to as nucleus columnaris, and (3.) the fasciculus retroflexus (Meynerti) (Plates iv and vn).
The shape of the bundle can best be seen in the longitudinal sections (Figs. 16 to 24).
In the transverse series the nerve-fibres show in the same sections as the nucleus. The obliquity of the section must be taken into account in comparing it with the model. The transverse sections show well how some of the fibres pass through the nucleus ruber and the substantia nigra (Figs. 47 to 51).
Summary of This completes the description of the median group of nerves, "erebrafnerves! and the group characteristics will now be plain. First, all the nerves are purely motor. Second, all the nuclei lie near the midline ventral from the central canal. Third, all are embedded in the fasciculus longitudinalis medialis except the nucleus of the !N". abducens, which lies farther lateral than the others, possibly due to the knee of the facial nerve. Fourth, the fibres of each nerve are collected into small definite bundles soon after leaving the nucleus. Fifth, three of the nerves pass directly ventralward, while the trochlear nerve passes dorsalward and decussates dorsal to the central canal.
The relation of this group of nuclei to the medial part of the ventral horn of the cord is clearly illustrated by the model, and motor cells can be traced in the sections from the ventral horn all along the border of the medulla sheet to the nucleus of the hypoglossal nerve (red in Plates v and vi). On the other hand, no such scattered cells exist between the hypoglossal nucleus and the nucleus of the N. abducens, for it is characteristic of the cerebral nerves, in contrast with the spinal, that their nuclei form definite and isolated groups of cells rather than a part of a column of cells. This distinction is being broken down by the recent work of Kaiser, Sano, van Gehuchten and others on the groups of cells in the spinal cord.
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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
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