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and the nucleus of the inferior colliculus is very significant. The lateral lemniscus, to a large extent, comes from the nuclei of termination of the cochlear nerve of the opposite side. We must associate, therefore, the inferior colliculus, and also the corpus geniculatum mediale, which likewise receives fibres from the lateral lemniscus, with the organ of hearing.
This view regarding the inferior colliculi is supported both by experimental and by morphological evidence. Speaking broadly, it may be stated that the inferior colliculi become prominent only in mammals, and then they are invariably correlated with a spirally wound and welldeveloped cochlea. That they have nothing to do with sight is shown by the fact that, when the eye-balls are extirpated in a young animal, the inferior colliculi remain unaffected, whilst the superior colliculi after a time atrophy (Gudden). When, on the other hand, the cochlear terminal nuclei are destroyed, fibres which have undergone atrophy may be followed to the inferior colliculi of both sides, but particularly to that of the opposite side (Baginski, Bumm, and Ferrier and Turner). A very considerable tract of ascending fibres takes origin within the inferior colliculus and passes upwards, in the inferior brachium, into the tegmentum subjacent to the medial geniculate body. Within the tegmentum they proceed up to the thalamus (Ferrier and Turner).
Colliculi Superiores (or superior quadrigeminal bodies). The superior colliculus presents a more complicated structure (Fig. 521). Superficially, it is coated with a very thin layer of white matter, which is termed the stratum zonale. Underneath this there is a gray nucleus, called the stratum griseum, which, in transverse section, exhibits a crescentic outline and rests in a cap-like manner upon the subjacent part of the eminence. The succeeding two strata, which respectively receive the names of stratum opticum and the stratum lemnisci, present this feature in common, that they are composed of gray matter, traversed by numerous fibres. The source from which the fibres are derived is different, however, in each case.
Nerve-fibres reach the superior colliculus through—(1) the lemnisci and (2) the superior brachium.
The fibres of the lemnisci constitute the stratum lemnisci. The superior brachium contains fibres of two different kinds, viz., fibres from the optic tract and fibres from the cortex of the occipital lobe of the cerebrum. By the former it is connected with both retina, and by the latter with the visual centre in the occipital region of the cerebral cortex. The great majority of these fibres pass into the margin of the colliculus superior and form a layer-stratum opticum-underneath the stratum griseum, in which they ultimately terminate.
Tegmentum. The tegmentum of the pedunculus cerebri may be regarded as the continuation upwards of the formatio reticularis of the medulla oblongata and the dorsal or tegmental portion of the pons into the mesencephalon. consists, therefore, of fine bundles of longitudinal fibres intersected by arching fibres, which take a transverse and curved course. The interstices between these nervebundles is occupied by gray matter containing irregularly scattered nerve-cells. On its dorsal aspect the tegmentum is continuous, at the side of the central gray matter, with the bases of the corpora quadrigemina, whilst, ventrally, it is separated from the basis pedunculi by the substantia nigra. The tegmenta of opposite sides are, to some extent, marked off from each other in the median plane by a prolongation upwards of the median raphe of the pons and medulla oblongata, although, in the inferior part of the mesencephalon, this is much obscured by the decussation of the brachia conjunctiva. The two longitudinal strands, termed the medial longitudinal bundle and the medial lemniscus, are prolonged upwards throughout the entire length of the mesencephalon; and they present the same relations to the tegmentum as in the inferior parts of the brain. The medial longitudinal fasciculus is placed in relation to its dorsal aspect, whilst the lemniscus is carried up in its ventral part.
The tegmentum of the mesencephalon may be considered as presenting two parts: viz., (1) an inferior part, which is placed subjacent to the inferior colliculi and is largely occupied by the decussation of the brachia conjunctiva (Fig. 520); | and (2) a superior part, subjacent to the superior colliculi which is traversed by the emerging bundles of the oculomotor nerve. The superior part contains a large and striking nuclear mass, termed the nucleus ruber or the red tegmental nucleus (Fig. 521). In the inferior part of the central gray matter of the mesencephalon is the nucleus of the trochlear nerve; in the superior part, the nucleus of the oculomotor nerve is situated.
Brachia Conjunctiva.-As the brachia conjunctiva leave the pons and sink into the tegmentum of the mesencephalon, they undergo a complete decussation, subjacent to Central gray the inferior colliculi and the central gray matter (Figs. 520, 521, p. 587; and 522, p. 588). In this manner each brachium is transferred from one side, across the median plane, to the opposite side. The decussation is completed at the level of the superior borders of the inferior colliculi, and then each brachium proceeds upwards into the superior part of the tegmentum, where it encounters the red nucleus. Into this a large proportion of its fibres plunge, and come to an end in connexion with the nuclear cells. Many of the fibres, however, are carried around the nucleus so as to form for it a
FIG. 520.--TRANSVERSE SECTION THROUGH THE HUMAN MESENCEPHALON
Central gray matter
Nucleus of oculomotor
Fibres of brachium conjunctivum
FIG. 521.-TRANSVERSE SECTION THROUGH THE HUMAN MESENCEPHALON AT THE LEVEL OF THE
capsule, which is thicker on the medial than on the lateral side (Fig. 521). These are prolonged into the thalamus, and end ultimately in connexion with the ventral thalamic cells. The brachium conjunctivum is, therefore, a great efferent tract which issues from the nucleus dentatus of the cerebellum, crosses the median plane in the inferior part of the mesencephalon, and ends in the red nucleus and the ventral part of the thalamus.
Nucleus Ruber.-The red nucleus is a rounded nuclear mass, of a reddish tint in the fresh brain, which lies in the superior part of the tegmentum, and in the path of the brachium conjunctivum. In transverse section it presents a circular outline. It begins at the level of the inferior border of the superior colliculus and it extends upwards into the hypothalamus. At first it is small and is placed at a little distance from the median plane; but, as it proceeds upwards, it increases in bulk and approaches more nearly to the median raphe, and to its
fellow of the opposite side. The curved emerging bundles of the oculomotor nerve pass through it on their way to the surface. The relation which the fibres of the opposite brachium conjunctivum present to it has been described. These fibres traverse its inferior part in such numbers that in Weigert-Pal specimens it presents a very dark colour; but higher up, as the fibres gradually end in nuclear mass, they become less numerous in its midst, and the nucleus assumes a paler tint.
Numerous fibres which descend from the cerebral cortex, and others from the corpus striatum, enter the red nucleus. It also sends out fibres which proceed in two directions: (1) upwards into the thalamus; (2) downwards to the spinal
THE LEVEL OF THE NUCLEUS OF THE TROCHLEAR NERVE IN THE ORANG. (The decussation of the brachia conjunctiva and the course of the trochlear medulla. The thalamic fibres may nerve in the central gray matter are seen.)
be regarded as carrying on the continuity of the path of the brachium conjunctivum after its nodal interruption in the red nucleus. The fibres to the spinal medulla, called the rubro-spinal tract and first described by Monakow, cross to the opposite side and then descend in the tegmentum to reach the lateral funiculus of the spinal medulla (Fig. 473, p. 534).
root of trigeminal nerve Trochlear
FIG. 522.-SECTION THROUGH THE INFERIOR COLLICULUS
AND THE TEGMENTUM OF THE MESENCEPHALON BELOW
Fasciculus Longitudinalis Medialis.-The medial longitudinal fasciculus is a very conspicuous tract of longitudinal fibres which extends throughout the whole length of the medulla oblongata, pons, and mesencephalon, in the formatio reticularis or tegmental part of each. Below, at the level of the decussation of the pyramids, it becomes continuous with the fasciculus anterior proprius of the spinal medulla (p. 562), whilst, by its opposite or superior end, it establishes intricate connexions in the region immediately above the mesencephalon. Throughout its whole length it lies close to the median plane and its fellow of the opposite side. In the mesencephalon it is applied to the ventral aspect of the central gray matter, whilst in the pons and medulla oblongata it is situated immediately subjacent to the gray matter of the floor of the fourth ventricle. One of its most salient features is the intimate association which it presents with the three motor nuclei from which the nerves for the supply of the muscles of the eyeball take origin, viz., the oculomotor nucleus, the trochlear nucleus, and the abducent nucleus. The first two of these are closely applied to its medial and dorsal aspect, whilst the abducent nucleus is placed on its lateral side. Into each of these nuclei it sends many collaterals, and probably also some of its constituent fibres, and these end around the nuclear cells. It would appear, therefore, that one of the most
important functions of this strand is to bind together these nuclei, and thus enable them to act in harmony one with the other. Fibres also enter the medial longitudinal fasciculus from the vestibular nucleus of the acoustic nerve system. The results obtained by degeneration would seem to indicate that, to a large extent, it is formed of fibres which run a short course within it.
It is evident that it' is a brain tract of high importance, from the fact that it is present in all vertebrates, and, further, that its fibres assume their medullary sheaths at an extremely early period. In fishes, amphibians, and reptiles, it is one of the largest bundles of the medulla oblongata. In man, its fibres medullate between the sixth and seventh months of foetal life, and at the same time as the fibres of the fasciculus anterior proprius of the spinal medulla, with which it stands in connexion.
According to van Gehuchten and Edinger, it extends upwards beyond the level of the oculomotor nucleus, and in the thalamic region its fibres take origin from a special nucleus of its own in the gray matter of the third ventricle, immediately behind the level of the corpora mamillaria. Fibres also enter the medial longitudinal bundle from a nucleus common to it and the posterior commissure of the brain. This nucleus is placed in the anterior part of the central gray matter of the mid-brain. Held asserts that numerous fibres, arising from cells in the superior colliculus, curve in an arcuate manner in the tegmentum outside the central gray matter, to take part on the ventral aspect of this in what
is called the fountain decussation. FIG. 523.-SECTION THROUGH THE INFERIOR COLLICULUS
Reaching the opposite side, these
AND THE TEGMENTUM OF THE MESENCEPHALON, AT A
Mesencephalic -root of trigeminal nerve Trochlear
fibres turn downwards and join the medial longitudinal bundle. The same authority considers that fibres from the ventral part of the posterior commissure can also be traced downwards into the medial longitudinal bundle. Edinger, on the other hand, places these fibres as a distinct tract on the ventral and lateral aspect of the medial longitudinal bundle, although in apposition with it.
Mendel believed that fibres from the oculomotor nucleus are carried down in the medial longitudinal bundle, and, from this, into the facial nerve for the supply of the orbicularis oculi and the corrugator supercilii, bringing these muscles, therefore, under the control of the same nucleus as the levator palpebræ superioris muscle. This view was adopted by many clinicians because this upper group of facial muscles is often spared in cases of facial paralysis; but Harman has adduced reasons in support of the view that there is a superior prolongation of the facial nucleus which innervates these muscles. It has been suggested further that fibres from the hypoglossal nucleus may, by the medial longitudinal bundle, reach the facial nerve, and through it the orbicularis oris. In this manner the same nucleus would hold sway over the tongue and the sphincter muscle of the lips. The close relation which exists between the ascending part of the intrapontine portion of the facial nerve and the medial longitudinal bundle would render the passage of fibres from one to the other a circumstance which could easily be understood. But the balance of evidence now available inclines us to regard the facial nucleus as the origin of the fibres innervating all the facial muscles. Another interchange of fibres through the medial longitudinal bundle has been described by Duval and Laborde. According to these authorities, fibres from the abducens nucleus ascend in the medial longitudinal bundle into the mesencephalon, and establish connexions
with that part of the oculomotor nucleus from which the nerve for the medial rectus of the opposite side derives its fibres. If this view is correct, it affords a ready and simple anatomical explanation of the harmonious action of the lateral and medial recti muscles in producing movements of the two eyeballs simultaneously to the right and to the left. From the investigations of E. H. Fraser it would appear that no fibres from the abducens nucleus go directly into the oculomotor nerve. The same observer has shown that many fibres from Deiters' nucleus, a part of the vestibular nucleus of the acoustic nerve to be described later in this account, enter the oculomotor and the trochlear nuclei through the path afforded by the medial longitudinal bundle.
Lemniscus Lateralis.-The lateral lemniscus is a definite tract of longitudinal fibres, which extends upwards through the lateral part of the tegmental substance of the superior portion of the pons and the mesencephalon. It is formed by the fibres of the corpus trapezoideum and striæ medullares in the inferior part of the pons, turning abruptly upwards and taking a course towards the quadrigeminal region. But the details of the arrangement and connexions of this important fasciculus must be left for fuller consideration when we are discussing the central connexions of the acoustic nerve.
Lemniscus Medialis.-The medial lemniscus has already been followed through the medulla oblongata and pons, and its position in each of these portions of the brain-stem has been defined (pp. 561 and 562). In the tegmentum of the inferior part of the mesencephalon it is carried up in the form of a more or less flattened