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the nucleus ambiguus from which it arises has thus been termed the laryngeal nucleus (Edinger) but it is not certain whether it is vagal or accessory.
Collaterals and fibres of the opposite lateral cerebro-spinal tract end in connexion with the cells of origin of the accessory nerve, and thus bring its nucleus into connexion with the motor area of the cerebral cortex. Fibres also from the posterior roots of the spinal nerves (afferent or sensory fibres) end in the nucleus.
Nervus Vagus, Nervus Glossopharyngeus.-The vagus and glossopharyngeal nerves present similar connexions with the brain, and they may therefore be studied together. The greater part of both nerves is composed of afferent fibres, which arise outside the brain-stem from ganglionic cells placed in relation to the nerve-trunks. Both nerves possess efferent fibres also, which spring from two special nuclei of origin situated within the medulla oblongata and termed respectively the dorsal or splanchnic nucleus and the nucleus ambiguus, which is the somatic nucleus. The afferent ganglionic fibres of the vagus and glossopharyngeal enter the brain by a series of roots which penetrate the medulla oblongata along the ventral side of the restiform body. Within the medulla oblongata they separate into two sets, viz., a series of bundles (composed chiefly of vagus fibres, i.e. afferent splanchnic), which end in the dorsal nucleus of termination of the vagus and glossopharyngeal nerves, and another series of bundles (composed chiefly of glossopharyngeal fibres, i.e. taste fibres), which join a conspicuous longitudinal tract of fibres called the tractus solitarius.
The dorsal nucleus (Figs. 488, p. 557, and 526, p. 593) of the vagus and glossopharyngeal nerves is mixed, and contains both motor cells which give origin to efferent fibres, and cells around which afferent fibres of the vagus, and possibly also of the glossopharyngeal nerve, break up into terminal arborisations. It very nearly equals in length the nucleus of the hypoglossal nerve, with which it is closely related. Above, it reaches as high as the striæ medullares, whilst, below, its inferior end falls slightly short of that of the hypoglossal nucleus. In specimens stained by the Weigert-Pal method the two nuclei offer a marked contrast. The hypoglossal nucleus presents a dark hue, owing to the enormous numbers of fine fibres which twine in and out amidst its cells; the vago-glossopharyngeal dorsal nucleus is pale, from the scarcity of such fibres within it. Its cells, like those of all splanchnic efferent nuclei, are much smaller than the somatic cells of the nucleus ambiguus. In the closed part of the medulla oblongata the dorsal vago-glossopharyngeal nucleus lies in the central gray matter immediately behind the hypoglossal nucleus, and upon the lateral aspect of the central canal; in the open part of the medulla oblongata it lies in the gray matter of the floor of the fourth ventricle, immediately to the lateral side of the hypoglossal nucleus and subjacent to the surface area termed the trigonum vagi or ala cinerea.
All the fibres which arise from this dorsal or splanchnic efferent nucleus are very fine, and in sections of the vagus nerve can readily be distinguished from the much coarser somatic fibres, which come from the nucleus ambiguus, and also from the medium-sized sensory fibres, which spring from the ganglia placed upon the nerves. The fine fibres from the dorsal nucleus are distributed (probably indirectly, i.e. after being interrupted in a peripheral ganglion), to the involuntary striped muscle of the oesophagus and heart, and the unstriped muscle of the oesophagus, stomach and respiratory system (van Gehuchten and Molhant, La Névraxe, June 15, 1912, p. 55).
The nucleus ambiguus (Figs. 488, 530, 526) gives origin to the somatic motor fibres of the glossopharyngeal and vagus nerves. All the fibres from this nucleus which pass into the glossopharyngeal nerve end in the stylo-pharyngeus muscle; the vagal branches are distributed to the striated muscles of the pharynx and larynx. The cells of the nucleus ambiguus are large, multipolar, and similar in every respect to the large cells in the anterior column of the spinal medulla. These cells are arranged in a slender column which is best developed in the open part of the medulla oblongata. Here the nucleus can easily be detected, in transverse sections, as a small area of compact gray matter which lies in the substantia reticularis grisea, midway between the dorsal accessory olive and the nucleus tractus spinalis nervi trigemini. It therefore lies more deeply in the substance of the medulla oblongata than the dorsal vago-glossopharyngeal nucleus. Kölliker states that it can be traced downwards as low as the level of the decussation of
the medial lemniscus, and upwards as high as the place of entrance of the cochlear root of the acoustic nerve. From its dorsal aspect the axons of the cells proceed, and in the first instance they pass backwards towards the floor of the fourth ventricle; then, bending suddenly laterally and forwards, they join the afferent roots of the vagus and the glossopharyngeal nerves, and emerge from the brain in company with these.
Sensory or Terminal Nuclei of the Glossopharyngeal and Vagus. Splanchnic and Gustatory Components.-The cells in the portion of the dorsal nucleus which acts as a nucleus of termination are spindle-shaped in form and
FIG. 530.-DIAGRAM, showing the brain connexions of the vagus, glossopharyngeal, acoustic,
similar to those found in the posterior column of gray matter in the spinal medulla. In connexion with these cells, the greater number of the afferent fibres of the vagus nerve, and a small proportion of the afferent fibres of the glossopharyngeal nerve, end in fine terminal arborisations. A small part of the superior portion of the nucleus may be said to belong to the glossopharyngeal nerve and the remainder of the nucleus to the vagus nerve.
The tractus solitarius (Figs. 494, p. 561; 495, p. 561; and 530) is a round bundle of longitudinal fibres which forms a very conspicuous object in transverse sections through the medulla oblongata. It begins at the superior limit of the medulla oblongata, and can be traced downwards through its whole length. Its precise point of termination is not known, but some authorities believe
that it is carried for some distance downwards into the superior part of the spinal medulla, and, according to Kölliker, to the level of the fourth cervical nerve. Most modern writers, however, limit it to the medulla oblongata. The relations of the tractus solitarius are not the same in all parts of its course. It lies immediately to the lateral side of the dorsal vago-glossopharyngeal nucleus; but, whereas in the superior part of the medulla oblongata it is situated somewhat on the ventral side of that nucleus, in the inferior, closed part of the medulla oblongata it is placed on its dorsal aspect. Throughout its entire length it is intimately associated with a column of gelatinous gray substance called the nucleus tractus solitarii, which constitutes the nucleus of termination in which its fibres end. When traced from above downwards, the tractus solitarius is observed to become gradually smaller owing to the loss of fibres which it thus sustains. The great bulk of the tractus solitarius is formed of fibres derived from the glossopharyngeal nerve; only a few of the afferent fibres of the vagus enter it, but fibres of the sensory root (nervus intermedius) of the facial also enter it. As the fibres of the three nerves join the fasciculus they immediately turn downwards, and at different levels come to an end in the associated nucleus tractus solitarii.
As the afferent root-bundles of the vagus and the glossopharyngeal nerves traverse the substance of the medulla oblongata in a backward and medial direction to reach the tractus solitarius and the dorsal nucleus of termination, they pass through the tractus spinalis of the trigeminal nerve and the nucleus of that tract. As the afferent root of the vagus passes through the trigeminal tractus spinalis and its nucleus, which is somatic sensory in nature, it gives off to this nucleus its own somatic sensory branches, the peripheral ends of which constitute the auricular branch, distributed to the skin on the back of the auricle. The other afferent fibres in the glossopharyngeal and vagus nerves include taste fibres, sensory fibres from the pharynx, larynx, and other parts of the respiratory and alimentary systems, and other splanchnic afferent fibres. Although there is no sharp demarcation between the terminal nuclei of these various components, it is probable that the taste fibres proceed to the nucleus tractus solitarii, the splanchnic afferent fibres to the dorsal nucleus, and the somatic afferent fibres to the nucleus of the spinal trigeminal tract.
Nervus Acusticus. As this is a nerve of special sense it will be left for consideration after the rest of this series.
Nervus Facialis (Figs. 530 and 531). The facial nerve is composed of two distinct parts, viz., a large efferent (mainly motor) portion, the facial nerve proper, and a small afferent sensory portion termed the nervus intermedius.
The facial nerve proper emerges from the brain at the inferior border of the pons, to the medial side of the acoustic nerve, whilst the nervus intermedius sinks into the superior part of the medulla oblongata between the facial and acoustic nerves, but alongside the latter, rather than the former, from which it is separated by the fasciculus obliquus pontis (Fig. 527). The three nerves, therefore, lie in intimate relation with each other, where they are attached to the surface of the brain, and they pass in company into the internal acoustic meatus.
The fibres of the nervus intermedius arise from the cells of the ganglion geniculi of the facial nerve. These, like the cells of a spinal ganglion, are unipolar, the single process in each case dividing into a peripheral and a central branch. The group of peripheral fibres represent parts of the greater superficial petrosal nerve and chorda tympani branch of the facial nerve, whilst the central fibres form the nervus intermedius. The central fibres penetrate the brain, and, passing either through or on the dorsal side of the tractus spinalis of the trigeminal nerve, they finally reach the superior part of the column of gray matter in connexion with the tractus solitarius, and in this they end. The nervus intermedius presents, therefore, the same terminal connexions within the brain as the glossopharyngeal nerve.
The motor nucleus of the facial nerve contains elements serially homologous with both the somatic (nucleus ambiguus) and splanchnic (nucleus dorsalis) efferent nuclei of the glossopharyngeal and vagus. It is composed partly of the larger cells characteristic of the former and the smaller cells distinctive of the latter. The axons of the somatic cells innervate the striated muscles of the face, whereas the splanchnic efferent fibres pass to the spheno-palatine, otic
and submaxillary ganglia (as their white rami communicantes), and are largely concerned with the regulation of the secretory activity of the large salivary glands and other glands around the mouth.
The facial nucleus is situated close to the place where the nerve emerges from the brain, but the nerve does not at once pass to this point of exit. It pursues a long and devious path within the pons before it finally reaches the surface. This intrapontine part of the nerve may be divided into three parts, viz.: (1) a radicular part, (2) an ascending portion, and (3) an emergent part.
The radicular part of the facial nerve (Fig. 531) is composed of a large number of fine, loosely arranged bundles of fibres, which issue from the lateral and dorsal aspect of the nucleus and proceed backwards and slightly medially through the pons. Reaching the floor of the fourth ventricle they curve medially, and the bundles which lie highest up sweep over the lateral and dorsal aspect of the inferior part of the nucleus of the sixth nerve. Close to the median plane they turn sharply upwards and are collected into a single solid nerve-bundle, which constitutes the ascending part of the facial nerve (Figs. 530 and 531). This proceeds upwards immediately beneath the ependyma of the ventricular floor on the dorsal aspect of the medial longitudinal bundle, and along the medial side of the abducent nucleus for a distance of about five millimetres. Then the nerve bends laterally at a right angle, and curves a second time over the dorsal aspect of the abducent nucleus. This gives rise to a prominent hemispheral projection in the floor of the fourth. ventricle, the colliculus facialis (Fig. 531 and Fig. 482, p. 550). The nerve now passes straight to the place of exit from the brain, and this part of the intrapontine trunk may be termed the emergent portion (Figs. 498 and 531). The facial nerve thus forms a curved loop over the dorsal aspect of the abducent nucleus. The emergent part of the nerve takes an oblique course through the pons to reach the surface. It inclines laterally and downwards as it proceeds towards the ventral aspect of the pons, and on its way it passes between its own nucleus and the tractus spinalis of the trigeminal nerve.
ERGENT PART V
RADICULAR PART VII
STALK OF SUPA OLIVE
FIG. 531.-DIAGRAM OF THE INTRAPONTINE COURSE OF
Entering the facial nucleus, and ending in fine terminal arborisations around its cells, are many fibres from the opposite pyramidal Sub. gel. rol. refers to the nucleus of the spinal trigeminal tract; fibres from the spinal tract of the fifth nerve; fibres from the corpus trapezoideum, etc. The nucleus is thus brought into connexion with the motor area of the cerebral cortex, with the trigeminal nerve or sensory nerve of the face, and with the acoustic nerve.
The peculiar course of the efferent fibres of the facial nerve within the pons is to be explained in accordance with the general principle regulating migrations of nerve-cells, to which reference has already been made (p. 554). In the embryo the nucleus facialis develops alongside the nucleus abducens. The latter, controlling one of the eye-muscles, receives most of its afferent impulses from the medial longitudinal bundle (descending from the optic centres in the superior colliculus), and therefore it remains alongside the medial longitudinal bundle
and perhaps moves slightly upwards, i.e. towards the mesencephalon. The facial nucleus, however, receives most of its stimuli from the nucleus tractus spinalis nervi trigemini, and therefore, as the walls of the metencephalon thicken during their growth, this nucleus retains its proximity to the trigeminal nucleus (Fig. 531), and so migrates along a course which remains mapped out by its emerging fibres. Streeter, working with human embryos, and Arïens-Kappers, on comparative and therefore broader lines, have elucidated the meaning of this peculiar intracentral course of the facial nerve.
Nervus Abducens (Figs. 498 and 531). The abducens nerve is a small motor nerve which emerges from the brain at the inferior border of the pons above the lateral side of the pyramid of the medulla oblongata. It is the nerve of supply to the lateral rectus muscle of the eyeball. Its nucleus of origin is a small spherical mass of gray matter, containing large multipolar cells, which lies in the dorsal part of the tegmental portion of the pons, close to the median plane and immediately subjacent to the gray matter of the floor of the fourth ventricle. Its position can be easily indicated on the ventricular floor, seeing that it is placed subjacent to the colliculus facialis and immediately above the level of the striæ medullares. Its peculiar and intimate relation to the intrapontine portion of the facial nerve has already been indicated. It lies on the ventral aspect of, and within the concavity formed by, the two limbs of the loop of that nerve.
The axons of the multipolar cells of this nucleus emerge from the medial aspect of the nucleus in the form of several bundles, which proceed through the whole dorso-ventral thickness of the pons towards the place of exit. As they pass forwards they incline downwards and slightly laterally. In the dorsal part of the pons they proceed forwards on the medial side of the nucleus olivaris superior, whilst in the basilar part of the pons they keep for the most part to the lateral side of the pyramidal bundles, although several of the nerve fila pierce these on their way to the surface.
It would appear probable that certain of the axons of the cells of the abducens nucleus enter the medial longitudinal fasciculus and proceed upwards in it to end in the oculomotor nucleus of the opposite side. Fibres and collaterals from the basis pedunculi of the opposite side enter the nucleus, and, ending around the cells, bring the nucleus into connexion with the motor area of the cerebral cortex. The pedicle of the nucleus olivaris superior ends partly within the nucleus of the abducent nerve (Fig. 530).
Nervus Trigeminus.-The trigeminal nerve strikes its roots deeply into the brain and establishes a connexion with it which extends from the upper part of the mesencephalon above to the level of the second cervical nerve below. No other cerebral nerve presents so extensive a connexion (Fig. 530, p. 597). It is composed of two roots- -a large afferent or sensory root and a small efferent or motor root. Both roots appear close together on the surface of the pons, rather nearer its superior than its inferior border, and in the same line as the facial, and glossopharyngeal and vagus nerves (Fig. 527, p. 594).
The sensory root of the trigeminal nerve is composed of fibres which arise outside the brain from the cells of the semilunar ganglion. They end within the brain in a somewhat tadpole-shaped terminal nucleus, the swollen body of which is situated in the pons and is termed the main sensory nucleus of the trigeminal nerve : the tail is a long column of gray matter which is directly continuous below with the substantia gelatinosa of the spinal medulla.
The main sensory nucleus (Fig. 532) is an oval mass of gray matter placed half-way up the pons in the lateral part of its dorsal or tegmental portion. It lies close to the lateral surface of the pons and immediately subjacent to the ventral submerged margin of the brachium conjunctivum. It is directly continuous with the substantia gelatinosa, and may be regarded as being merely the enlarged superior end of that column of gray matter.
The fibres of the sensory root of the trigeminal nerve, on reaching the sensory nucleus, divide, in the same way as the fibres of the entering posterior roots of the spinal nerves, into a system of ascending and descending branches (Fig. 530, p. 597). The ascending fibres are short, and almost immediately enter the sensory nucleus and end within it; the descending fibres turn sharply downwards and form the