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band on the ventral aspect of the decussating brachia conjunctiva. To its lateral side, and forming an angle with it (as seen in transverse section), is the lateral lemniscus (Figs. 522 and 523), and at this level there is no clear demarcation between these two tracts. In the superior part of the mesencephalon the appearance of the red nucleus in the tegmentum causes the medial lemniscus to take up a more lateral and dorsal position, so that it now comes to lie subjacent to the corpus geniculatum mediale (Fig. 521, p. 587). At this level it exhibits a crescentic outline in transverse section, and the lateral lemniscus has to a large extent disappeared from its lateral side.
A part of the medial lemniscus, which is called the fasciculus bulbothalamicus, takes origin in the inferior part of the medulla oblongata from the gracile and cuneate nuclei of the opposite side (p. 560). Seeing that the posterior funiculus of the spinal medulla ends in these nuclei, the medial lemniscus may be considered to continue that funiculus upwards into the brain. Other fibres arise from the terminal nuclei of the various sensory cerebral nerves of the opposite side. The rest of the tract consists of the superior part of the fasciculus spinothalamicus from the spinal medulla. In the mesencephalon a considerable contribution of fibres is given by the medial lemniscus to the superior colliculus, and then the remainder of the tract proceeds into the lateral (ventro-lateral) nucleus of the thalamus. Here its fibres end amidst the thalamic cells.
Ganglion Interpedunculare and Fasciculus Retroflexus.-Immediately above the pons a small collection of nerve-cells is found in the median plane, wedged in between the two cerebral peduncles. It is all that is found in the human brain to represent a large nucleus projecting into the interpeduncular fossa in most other animals, especially those with a highly developed sense of smell. In this interpeduncular ganglion ends the fasciculus retroflexus, a tract of fibres which comes from the nucleus habenula of the epithalamus. We shall return to the consideration of this tract later.
Fountain Decussation.-If the region ventral to the medial longitudinal bundles is examined in the superior part of the mesencephalon a very close decussation of fibres in the median plane will be observed in the interval between the two red nuclei. This is the "fountain decussation." According to Held, the fibres which take part in the dorsal portion of the fountain decussation (decussation of Meynert) come from the superior colliculi, and, after they have gained the opposite side, they turn downwards in the medial longitudinal fasciculus.
Many of the fibres that cross in this decussation enter a descending tract (fasciculus tecto-bulbaris et spinalis) which connects the corpora quadrigemina with the motor nuclei on the other side of the medulla oblongata and spinal medulla.
Basis Pedunculi.-The basis pedunculi presents a somewhat crescentic outline when seen in transverse section, and it stands quite apart from its fellow of the opposite side. It is composed of a compact mass of longitudinally directed fibres, all of which, as Déjerine has shown, arise in the cortex of the cerebrum and pursue an unbroken corticifugal course into and through the pedunculus cerebri. These fibres may be classified into two distinct sets, viz., cerebro-pontine and pyramidal or cerebro-spinal.
The cerebro-pontine fibres possess this leading character: in their course downwards they are all arrested in the ventral part of the pons, and end amidst the cells of the nuclei pontis. These tracts would appear to hold a very definite position within the crus. Thus, it has been satisfactorily established that the fibres coming from the temporal area of the cerebral cortex (temporo-pontine strand) form the lateral fifth of the basis pedunculi, whilst those coming from the frontal area (fronto-pontine strand) hold a similar position in the medial part of the basis pedunculi.
The pyramidal fibres constitute the great motor tract from the cerebral cortex. They occupy a position corresponding to the middle three-fifths of the basis. The pyramidal tract differs from the cerebro-pontine strands in being carried downwards through the ventral part of the pons and on the ventral aspect of the medulla oblongata into the spinal medulla, which it enters in the form of the fasciculi cerebrospinales lateralis and anterior. On its way through the pons and
medulla oblongata it sends fibres across the median plane to the various motor nuclei on the opposite side of those sections of the brain-stem.
DEVELOPMENT OF THE MESENCEPHALON.
Even in the early embryo the mesencephalon constitutes the smallest section of the brain-tube, although the disproportion in size between it and the other primitive subdivisions of the brain is not nearly so marked as in the adult. Owing to the cephalic flexure, the mid-brain for a time occupies the summit of the head. completely covered over by the expanding cerebral hemispheres. Later it becomes
The corpora quadrigemina are derived from the alar laminæ of the side walls of the brain-tube, whilst the basal lamina thicken and ultimately form the tegmenta. The original cavity of the mid-brain is retained as the aqueduct.
For a considerable time the cavity of the mesencephalon remains relatively large, and the lower part of its dorsal wall is carried downwards in the form of a diverticulum or recess, which overlaps the cerebellar plate. About this time, also, the dorsal wall shows a median fold or ridge. Both of these conditions are transitory. As the corpora quadri gemina take shape, the median ridge disappears and is replaced by the median longitudinal groove, which separates the quadrigeminal bodies. Only its inferior part is retained, and this is represented by the frenulum veli of the adult brain. The diverticulum of the cavity gradually becomes reduced, and finally disappears as the aqueduct assumes form.
The precise mode of origin of the red nucleus is not known.
Later in this account reasons will be given for the belief that the representatives of the neural crests in the region of the mesencephalon become absorbed and assimilated in the walls of the neural tube as it closes in.
THE DEEP CONNEXIONS OF THE CEREBRAL NERVES ATTACHED TO
There are twelve pairs of cerebral nerves, of which the inferior eight are attached to the medulla oblongata and pons. From above downwards these are named the trigeminal (fifth), the abducens (sixth), the facial (seventh), the acoustic (eighth), the glossopharyngeal (ninth), the vagus (tenth), the accessory (eleventh), and the hypoglossal (twelfth). Two others, the trochlear (fourth) and oculomotor (third) spring from the mesencephalon. The hypoglossal, the accessory, the greater part of the facial, the abducens, the motor root of the trigeminal, the trochlear and the oculomotor are efferent nerves; the acoustic, the nervus intermedius (sensory root of the facial) and the sensory root of the trigeminal are purely afferent nerves; whilst the vagus and the glossopharyngeal are composed of both efferent and afferent fibres. In all these cases (with a possible reservation in the case of part of the trigeminal) afferent fibres arise from ganglionic cells placed outside the brain and penetrate the brain-stem, to end in connexion with the cells of certain nuclei of termination. Efferent fibres, on the other hand, take origin within the brain as the axons of cells which are grouped together in certain places in the form of nuclei of origin.
Nuclei of Origin, or Motor Nuclei.-In the spinal medulla the nuclei of origin are represented by elongated columns of cells which run more or less continuously in the anterior column of gray matter of successive spinal segments, and from these the series of efferent anterior nerve-roots take origin. In the medulla oblongata, pons, and mesencephalon the nuclei of origin, or, in other words, the motor nuclei of the individual nerves, become, for the most part, discontinuous, and are represented by certain isolated clumps of compact gray matter, in which are placed the clusters of cells from which the fibres of the efferent nerves arise. The nucleus ambiguus, however, which consists of a column of cells from which root-fibres of the bulbar part of the accessory, of the vagus, and also of the glossopharyngeal are derived, is an exception to this rule. At the decussation of the pyramids, the anterior column of gray matter of the spinal medulla is broken up by the intercrossing bundles into a detached head and a basal part which remains in relation with the ventrolateral aspect of the central canal. Certain of the efferent nuclei of the medulla oblongata, pons, and mesencephalon lie in the line of the basal portion of the anterior column of gray matter of the spinal medulla, and, thus, close to the median plane. These are termed medial somatic nuclei, and are met with at different levels in the
brain-stem. This group comprises the hypoglossal nucleus, the abducens nucleus and, in the mesencephalon, the trochlear nucleus and part of the oculomotor nucleus. Other motor nuclei of origin are present in the form of isolated clumps or columns of gray matter, which lie at different levels in the medulla oblongata and pons in a more lateral and deeper situation. They are the nucleus ambiguus of the accessory, vagus and glossopharyngeal, the facial nucleus, and the nucleus of the motor root of the trigeminal nerve. From their position in the substantia reticularis of the medulla oblongata and pons they constitute a group to which the name of lateral somatic nuclei is applied.
In addition to these two columns of motor nuclei there is a third efferent column of splanchnic nuclei represented by the dorsal nucleus of the vagus and glossopharyngeal nerves, and similar nuclei emitting sympathetic fibres into the
FIG. 526. DIAGRAM REPRESENTING THE DIFFERENT KINDS OF COMPONENTS FOUND IN THE CEREBRAL NERVES AND OF THEIR NUCLEI OF ORIGIN OR TERMINATION.
facial and oculomotor nerves.
pass into the trigeminal nerve.
It is possible some splanchnic efferent fibres may
The different nuclei of origin of the efferent fibres which belong to the various cerebral nerves, both medial and lateral, are connected with the motor area of the cerebral cortex by fibres of the cerebro-spinal tract of the other side, which enter the nuclei and end in association with their cells.
Nuclei Terminales.-The general scheme of arrangement of the terminal nuclei has already been explained (Fig. 526); its details will be further elucidated as the various nerves are considered seriatim.
The axons of many of the cells of the nuclei of termination enter the substantia reticularis as arcuate fibres, and, crossing the median plane, are carried upwards in the substantia reticularis of the opposite side, to establish direct connexions with the thalamus and, indirectly through it, with the cerebral cortex (Fig. 525). Others pass to the nuclei of motor nerves, to the cerebellum or other nerve-cells, to form connexions necessary for the performance of reflex actions.
Nervus Hypoglossus.-The nucleus of origin of the hypoglossal nerve, the motor nerve of the tongue, lies in the substance of the medulla oblongata. It is composed of several groups of large multipolar cells, which closely resemble the cells in the anterior column of gray matter in the spinal medulla, and is pervaded by an intricate network of fine fibrils. In form it is elongated and rod-like, and in length it is about 18 mm. It extends from a point immediately above the decussation of the pyramids up to the level of the striæ medullares. The inferior portion of the nucleus is thus placed in the closed part of the medulla oblongata (Fig. 494, p. 561), whilst its superior part is situated in the open part (Fig. 495, p. 561). The former lies in that part of the central gray matter which is continuous with the basal part of the anterior column of gray matter of the
FIG. 527. THE VENTRAL ASPECT OF THE MEDULLA OBLONGATA, PONS, AND MESENCEPHALON,
showing the nerve roots.
spinal medulla. It is thus placed on the anterior and lateral aspect of the central canal, close to the median plane and the corresponding nucleus of the opposite side. The superior part of the nucleus occupies a position in the gray matter on the floor of the fourth ventricle, subjacent to the medial part of the surface area, which has been described under the name of the trigonum hypoglossi. Within the nucleus the axons of the cells arrange themselves in converging bundles of fine fibres, which come together and leave the ventral aspect of the nucleus as the fila radicularia of the nerve. The nerve bundles thus formed traverse the entire antero-posterior thickness of the medulla oblongata, between the substantia reticularis grisea and the substantia reticularis alba, and emerge on the surface, in linear order, at the bottom of the furrow between the olive and the pyramid. After they emerge these fibres collect to form three definite bundles like the anterior nerve-roots of three spinal nerves (Fig. 527). In the substance of the medulla oblongata the fila radicularia of the
Er hypoglossal pass between the main inferior olivary nucleus and the medial accessory gata olivary nucleus, and many of them on their way to the surface pierce the ventral selamina of the main olivary nucleus.
No decussation between the nerves of opposite sides takes place in the medulla oblongata, but commissural fibres pass between the two nuclei (Kölliker). Further, numerous fibres from the opposite pyramidal tract enter the nucleus and end in connexion with its cells. The nucleus is thus brought into connexion with the motor area of the opposite side of the cerebral cortex.
Nervus Accessorius.-The accessory nerve also is a motor nerve, and it is generally described as consisting of a spinal and a cerebral or accessory part.
The spinal part of the nerve emerges by a series of roots which issue from the surface of the lateral column of the superior part of the spinal medulla as low down as the fifth cervical nerve. These take origin in a column of cells situated in the anterior column of gray matter of the spinal medulla, close to its lateral margin, and
FIG. 528.-DIAGRAM OF THE SPINAL ORIGIN OF THE ACCESSORY NERVE (after Bruce).
FIG. 529.-SECTION THROUGH THE SUPERIOR PART OF THE
immediately behind the nerve-cells which give rise to the fibres of the anterior roots of the upper five cervical nerves. The cells of the accessory nucleus are large, multipolar, and in every respect similar to the motor cells of the spinal nerves. The axons from these cells leave the dorsal aspect of the nucleus in converging groups to form the fila radicularia or root-bundles of the nerve. These, in the first place, proceed straight backwards in the anterior column of gray matter. Reaching the bay between the two columns of gray matter, they turn sharply laterally into the white matter and traverse the lateral funiculus to gain their points of exit from the spinal medulla. At the decussation of the pyramids, fila, which join the accessory nerve, are seen to proceed from the detached head of the anterior column of gray matter.
The cerebral part of the accessory nerve has its nucleus of origin in the medulla oblongata; and its fila, as they proceed laterally from this, can be distinguished by the fact that they pursue a course on the ventral side of the tractus spinalis of the trigeminal nerve, whereas the vagus roots, with which they are apt to be confused, pass through or lie on the dorsal aspect of the trigeminal root (Kölliker). The nucleus of origin of the cerebral part of the accessory nerve is formed by the same column of cells which constitutes the nucleus ambiguus, and which, at a higher level, gives motor fibres to the vagus and glossopharyngeal nerves.
The part of the accessory nerve which takes origin in the spinal medulla supplies the sterno-mastoid and trapezius muscles. The cerebral portion joins the vagus, and through the external laryngeal and recurrent nerves it supplies the muscles of the larynx. The portion of