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information regarding the constitution of the restiform body. Thus, the posterior spino-cerebellar tract (Fig. 481), from the lateral column of the spinal medulla, can be traced into it; and large numbers of fibres which take a curved course on the surface of the medulla oblongata may likewise be followed into it; these are the external arcuate fibres. Numerous other fibres enter the restiform body on its
deep aspect, but these will be studied at a later stage.
Fibræ Arcuatæ Externæ.-The external arcuate fibres enter into the constitution of the restiform body, after pursuing a longer or shorter course on the surface of the medulla (Fig. 481).
They are more particularly seen in the neighbourhood of the olive, over the surface of which they may be observed coursing in the form of a number of fine curved bundles or as a continuous sheet of fibres. They vary greatly in number and in distinctness, and they are sometimes so numerous that they cover the olive almost entirely. An attentive examination will show that the majority of them come to the surface in the median fissure between the pyramids, and also not infrequently, in the groove between the pyramid and olive, or through the substance of the pyramid itself. The anterior median fissure in its upper part is often almost completely blocked up by these emerging fibres. The external arcuate fibres, reaching the surface of the medulla in this manner, turn backwards, and the great majority enter the restiform body and form a considerable part of its outer portion.
Other arcuate fibres arise in the cuneate and gracile nuclei, and enter the restiform body of the same side. Van Gehuchten, however, denies this.
There is frequently present, especially upon the left side, a bundle of fibres that is usually mistaken for a group of arcuate fibres. It is the fasciculus circumolivaris pyramidis (Fig. 481). It consists of a bundle of varying size which emerges from the pyramid, bends backwards, curving round the inferior border of the olive, and then passes obliquely upward and backwards to end in a fusiform ridge of gray matter, the corpus ponto-bulbare (Essick), which crosses the restiform body very obliquely (Fig. 481, the ridge immediately posterior to the fila of the vagus nerve). These structures are of great morphological interest, and will be referred to again in the succeeding pages (see Fig. 499, p. 566).
The pons (O.T. pons Varolii) is a marked white prominence on the basal aspect of the brain which is interposed between the medulla oblongata and the pedunculi cerebri, and lies in front of the cerebellum. It is convex from side to side, and from above downwards, and transverse streaks on its surface show that, superficially at least, it is composed of bundles of nerve-fibres, most of which course transversely. On each side these transverse fibres are collected together in the form of a large compact strand, which sinks in a backward and lateral direction into the white matter of the corresponding hemisphere of the cerebellum. This strand is termed the brachium pontis, and the term "pons," applied to the entire structure, expresses in an admirable way the arch-like manner in which this portion of the brain bridges across the interval between the two cerebellar hemispheres.
The ventral surface of the pons is in relation to the basilar part of the occipital bone and the dorsum selle of the sphenoid bone. It presents a median groove (sulcus basilaris), which gradually widens as it is traced upwards: it lodges the basilar artery. This median depression is produced by the prominence which is caused on each side by the passage of the cerebro-spinal fasciculus downwards through the pons. The trigeminal nerve, with its large entering sensory root and its small emerging motor root, is attached to the side of the anterior aspect of the pons, nearer its superior than its inferior border (Fig. 481). It is usual to restrict the term "pons to that portion of the structure which lies between the two trigeminal nerves, and to apply the designation of brachium pontis to the part which extends beyond the nerve into the hemisphere of the cerebellum. The abducens nerve, the facial nerve, and the acoustic nerve are attached to the brain at the inferior border of the pons. The abducens nerve emerges at the inferior border of the pons
opposite the lateral border of the pyramid; the facial and acoustic are also attached to the inferior edge of the pons, but far away from the median plane. The acoustic nerve is in contact with the cerebellum and the facial is on its medial side, with its sensory root (the nervus intermedius) between them (Figs. 478, 481). A large bundle of fibres upon the front of the pons departs from the transverse course pursued by most of the pontine fibres, and starting at the medial side of the trigeminal nerve, passes almost vertically downwards between the facial and acoustic nerves (Fig. 526, p. 593) and reaches the lateral aspect of the medulla oblongata, where it passes into the corpus ponto-bulbare (Fig. 481). This bundle is known as the fasciculus obliquus [pontis]. It is interesting to observe that while the facial nerve lies upon the medial side of this oblique bundle, its sensory root (the nervus intermedius) is placed on its lateral aspect, alongside the acoustic nerve (Fig. 527, p. 594).
Immediately below the insertion of the acoustic nerve at the inferior margin of the pons a little calyx-like appendage of the epithelial roof of the fourth ventricle (recessus lateralis) projects laterally, partly behind the glossopharyngeal nerve. Through an elliptical aperture in this epithelial process (apertura lateralis ventriculi quarti) a little cauliflower-like mass of chorioid plexus becomes extruded between the acoustic and the glossopharyngeal nerves (Fig. 527, p. 594).
The posterior surface of the pons looks backwards towards the cerebellum, and presents a triangular area covered with gray matter, which forms the superior part of the anterior wall or floor of the fourth ventricle. This area is directly continuous inferiorly with the medullary part of the floor of the fourth ventricle, and is bounded on each side by a band of white matter termed the brachium conjunctivum (Fig. 482). Brachia Conjunctiva (O.T. Superior Peduncles of the Cerebellum).—The brachia conjunctiva are hidden from view by the superior part of the cerebellum, under cover of which they lie. They emerge from the hemispheres of the cerebellum, and, as they proceed upwards on the dorsal aspect of the pons, they converge towards each other until, at the inferior level of the corpora quadrigemina, the medial margins of the two brachia become almost contiguous (Fig. 482, p. 550). At first they form the lateral boundaries of the superior part of the fourth ventricle; but, as they ascend and approach closer to each other, they gradually come to overhang that cavity, and thus enter into the formation of its roof. They disappear from the surface of the brain by dipping under cover of the quadrigeminal bodies and entering the substance of the mesencephalon.
Velum Medullare Anterius.-Filling up the triangular interval between the two brachia conjunctiva, and stretching across from the medial and free margin of the one to the corresponding margin of the other, is a thin layer of white matter which completes the roof or dorsal wall of the upper part of the fourth ventricle, and receives the name of the anterior medullary velum. When traced downwards, the velum is seen to be carried, with the brachia conjunctiva, into the white matter of the cerebellum. Spread out on its posterior or superior surface is a small, thin, tongue-shaped prolongation of gray matter from the cortex of the cerebellum, which is termed the lingula, whilst issuing from its substance close to the inferior quadrigeminal bodies are the two trochlear nerves.
Fourth Ventricle. The fourth ventricle of the brain is somewhat rhomboidal in form. Below, it tapers to a point and becomes continuous with the central canal of the lower half of the medulla oblongata; above, it narrows in a similar manner and is continued into the aquæductus cerebri, which tunnels the mesencephalon. The posterior wall is termed the tegmen or roof and is concealed by the cerebellum. The anterior wall is called the floor and is formed by the dorsal surface of the pons and the corresponding surface of the medulla oblongata. On each side a long, curved and narrow prolongation of the ventricular cavity is carried laterally from its widest part and curves round the upper part of the corresponding restiform body. This is termed the recessus lateralis. The roof of the cavity is very thin and is intimately connected with the cerebellum. It is better, therefore, to defer its description until that part of the brain has been studied.
Fossa Rhomboidea (floor of the fourth ventricle).-In its inferior part the floor of the fourth ventricle is formed by the dorsal surface of the ventral part of the medulla oblongata, whilst in its superior part it is formed by the dorsal surface of the pons (Fig. 482). The area thus constituted is lozenge-shaped, its widest part being opposite the superior ends of the restiform bodies or inferior peduncles of the cerebellum. A thick layer of gray matter, continuous with that which surrounds the central canal, is spread out like a carpet over the ventricular floor, and covering this there is the usual ependymal layer which lines all the ventricles of the brain. The area is circumscribed by definite lateral boundaries. Thus, below, it is bounded on each side by (1) the clava, (2) the expanded upper end of the funiculus cuneatus, and (3) the restiform body; whilst above, the lateral limits are formed by the brachia conjunctiva.
The floor of the fourth ventricle is divided into two symmetrical portions by a median groove. Its lower narrow pointed portion between the two clave receives the name of the pars inferior, or, from its resemblance to the point of a pen,
FIG. 482.-FLOOR OF THE FOURTH VENTRICLE. On the right side the right half of the cerebellum has been removed by cutting through its three peduncles and dividing it in the median plane. On the left side the left half of the cerebellum is drawn over to the left so as to expose the floor of the ventricle fully. the calamus scriptorius. Crossing each half of the floor, at its widest part, are several more or less conspicuous bundles of fibres termed the striæ medullares. They begin upon the lateral and posterior aspects of the restiform body, where they spring from the cochlear nuclei, pass transversely medially, and disappear from view in the median furrow. The striæ medullares exhibit a large amount of variation in different specimens, both in their degree of prominence and also in the direction which they pursue. It is not uncommon to find that no trace of them is visible upon the surface.
On the inferior (bulbar) district of the ventricular floor a small triangular depression, placed immediately below the striæ medullares, catches the eye. This is termed the fovea inferior. It is shaped somewhat like an arrow-head. The apex or point looks towards the striæ, whilst the lateral angles of the base are prolonged downwards in the form of diverging grooves (Fig. 482). Of these, the medial groove runs towards the opening of the central canal at the calamus scriptorius, whilst the lateral groove runs towards the lateral boundary of the floor. In this
manner the portion of the floor which lies below the striæ medullares is mapped out t into three triangular areas. The medial subdivision is slightly elevated and is termed the trigonum nervi hypoglossi, because subjacent to the medial part of this area is the nucleus of origin of the hypoglossal nerve. The intermediate area, between the two diverging grooves which proceed from the base of the fovea inferior, is the ala cinerea. It is sometimes called the trigonum n. vagi because the nucleus of the vagus and the glossopharyngeal nerves lies subjacent to it. Near the lateral angle is the area acustica. The base of this area is directed upwards and runs directly into an eminence over which the striæ medullares pass. Subjacent to this district of the floor of the ventricle lies the large terminal chief nucleus of the vestibular division of the acoustic nerve. A more accurate name for the area acustica would be area vestibularis.
A close inspection of the pars inferior fossa rhomboidea will show that the base of the trigonum vagi is separated from the medial margin of the clava by a narrow lanceolate strip of the ventricular floor, to which Retzius has given the name of area postrema. Beneath this area is some vascular tissue (Streeter), and marking it off on its superior and medial aspect from the base of the trigonum vagi there is a translucent cord-like ridge called the funiculus separans. When the floor of the ventricle is examined under water with a magnifying glass, the trigonum hypoglossi is seen to consist of a narrow medial strip which corresponds to the hypoglossal nucleus, and a wider lateral part which has been shown to be the surface representation of another nucleus termed the nucleus intercalatus (Streeter).
On the part of the floor of the ventricle which lies above the striæ medullares, and corresponds to the dorsal surface of the pons, there is also a slight depression, termed the fovea superior. Between it and the median groove is a marked prominence called the eminentia medialis. Inferiorly this elevation passes downwards and becomes continuous with the trigonum hypoglossi, whilst superiorly it is carried upwards towards the opening of the aquæductus cerebri. În both directions it gradually becomes less prominent, but still it forms a distinct elongated elevation, which stretches alongside the whole length of the median groove. As already stated, the area acustica extends upwards into the pontine part of the ventricular floor and forms an elevated region in the most lateral part of its widest portion, below and to the lateral side of the fovea superior. Proceeding upwards from the fovea superior to the opening of the aquæductus cerebri there is a shallow depression termed the locus coeruleus, seeing that it usually presents a faint slate-blue colour. When the ependyma is scraped away from the surface of this part of the floor, the colour is seen to be due to the substantia ferruginea, a name applied to a linear group of strongly pigmented cells, which lies in the lateral part of the gray matter covering this portion of the ventricular floor. When transverse sections are made through the superior part of the pons, the substantia ferruginea appears on the cut surface as a small black spot or dot.
INTERNAL STRUCTURE OF MEDULLA OBLONGATA AND PONS. The structure of the medulla oblongata and pons differs in a marked degree from that of the spinal medulla: indeed, in its superior part, it presents very little in common with the latter. Some of the largest fasciculi which come up from the spinal medulla (such as the funiculus posterior) end in the lower part of the medulla oblongata; others leave the medulla oblongata and pass into the cerebellum; and of the bundles of fibres which pass upwards or downwards, from or to the spinal medulla respectively, most of them come to occupy very different positions in the medulla oblongata and pons.
The gray matter instead of being moulded into one compact column, as is the case in the spinal medulla, becomes broken up into a series of discrete nuclei. Thus, there are developed from the basal lamina of the rhombencephalon not one compact mass like the spinal anterior column, but three distinct broken columns of efferent nuclei (Fig. 526, p. 593): (1) a medial somatic column, which in turn is broken up into two parts, a bulbar nucleus (the hypoglossal) which supplies the motor fibres to the tongue muscles, and a pontine nucleus (the abducens) which supplies the lateral rectus muscle of the eye; (2) a lateral somatic column, broken up into separate nuclei, viz., accessorius, ambiguus, facial, and trigeminal, supplying
the sterno-mastoid and trapezius muscles and the striated muscles of the larynx, pharynx and face and those concerned with mastication; and (3) a splanchnic column of nuclei, giving efferent fibres which pass out in the vagus, glossopharyngeal and facial nerves, to be widely distributed to unstriped muscle, glands and other tissues in the head, neck, thorax and abdomen.
Further, the terminal nuclei of the sensory nerves which are developed in the alar lamina of the rhombencephalon do not unite to form a definite posterior column, as happens in the spinal medulla, but form discrete masses; and as these act as receptive organs for a much greater variety of sensory nerves than are represented in the spinal nerves there is a much greater number of nuclei than would be formed if the various components of the posterior column in the spinal medulla were dissociated. Thus, there are terminal nuclei in the medulla oblongata not only for the ordinary cutaneous nerves, but also for nerves coming from the mucous membranes of the alimentary and respiratory organs, as well as from other visceral structures; and there are also special nerves of taste (nervus intermedius and glossopharyngeal), of hearing (cochlear part of the acoustic) and of equilibration (vestibular part of the acoustic). But this does not exhaust the peculiar features of the terminal sensory nuclei of the rhombencephalon. In the description of the spinal medulla attention was called to the fact that certain of the fibres of the posterior nerve-root did not end in the gray matter of the spinal medulla, but passed upwards throughout the whole length (above their points of entry) of the funiculus posterior to reach the medulla oblongata. Special terminal nuclei are developed from the alar lamina to receive these fibres. They are the nucleus gracilis and nucleus cuneatus.
In addition, part of the terminal vestibular nucleus receives accessions of fibres from these (gracile and cuneate) nuclei as well as from other sensory terminal nuclei in the spinal medulla and develops into that great mass of tissue, the cerebellum, to which vast numbers of other fibres come and go, adding considerably to the complexity of the region of the pons and medulla oblongata. Moreover, there is developed from the alar lamina a whole series of other masses of gray matter the nucleus olivaris inferior, nuclei arcuati, nucleus pontobulbaris and nuclei pontis-as links in the complex chains that bind all parts of the central nervous system to the great co-ordinating mechanism of the cerebellum.
Thus it comes about that, instead of having, as in the spinal medulla, a definite column of gray matter ensheathed in a thick mass of white substance, the rhombencephalon is composed of many scattered masses of gray matter; and its white substance is represented partly by great longitudinal strands, but also by many great systems of fibres passing transversely through its substance, or upon its surface, e.g., the superficial fibres of the pons and many of the arcuate fibres.
From what has already been said concerning the external form of the medulla oblongata and pons it will be apparent that the distortion of the neural tube which occurred as the result of the pontine flexure has also been largely responsible for the distinctive features of this region of the brain.
As the pontine flexure develops, a strain is thrown upon the thin roof-plate, which yields and becomes stretched so as to permit the thick lateral walls of the neural tube to fall laterally (Figs. 483 and 484). One result of this process is the great lateral expansion of the cavity of the hind-brain, which assumes the characteristic rhomboid form. If the thin and greatly attenuated epithelial roof is torn away from the rhombencephalon of an embryo of the third month the fourth ventricle will present the appearance (viewed from behind) shown in Fig. 483. The ventricle is seen to be prolonged laterally, on each side, to form a little recess upon the lateral aspect of the rhombencephalon. This is called the recessus lateralis.
This thin epithelial roof becomes invaginated towards the cavity of the fourth ventricle, on each side of the median plane, in the whole length of the epithelial roof, i.e. from the cerebellar attachment, above, almost as far as the closed part of the medulla oblongata below (Fig. 519). The upper end of this invaginated fold becomes prolonged laterally as far as the extremity of the recessus lateralis (Fig. 527). Pia mater and blood-vessels extend into these folds, which are then known the chorioid plexuses of the fourth ventricle. At the extremities of these two