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branches, which enclose the cells of Purkinje, as well as the short non-medullated portions of their axons, in a close basket-work of fine filaments.
The granular layer is, for the most part, composed of large numbers of small granule-like bodies closely
packed together. Each of these possesses a somewhat large nucleus, with a very small amount of surrounding protoplasm. From the cell body three or four, or perhaps five, dendrites and one axon proceed. The dendrites are short and radiate out from different aspects of the cell body. They end in tufts of clawlike twigs, which either embrace or are otherwise in contact with neighbouring granule cells. The whole multitude of granule cells, therefore, are brought into intimate connexion with each other. The axon passes into the molecular layer, in which it ends, at a varying distance from the surface, by dividing into two branches. These diverge so sharply from each other that they form almost aright angle with the parent stem, and they run parallel to the long axis of the P.
folium, threading their way F
among the branches of the K and K1. Fibres from white core of folium ending in molecular layer in connexion with the dendrites of the cells of Purkinje. Small cell of the molecular layer.
various dendritic planes of M.
is borne in mind that the GL. Neuroglial cell.
Basket-work around the cells of Purkinje.
is very great, and that
each sends an axon into the molecular layer, the important part which these fibres, with their longitudinal branches, take in building up the molecular layer will be understood. They are found pervading its entire thickness--from the surface down to the bodies of the cells of Purkinje.
The mesencephalon or mid-brain is the short, narrow part of the brain-stem which occupies the aperture of the tentorium cerebelli (incisura tentorii), and connects the cerebrum, which lies above, with the parts which occupy the posterior cranial fossa. It is about three-quarters of an inch in length, and it consists of a dorsal part, composed of the corpora quadrigemina, and a much larger ventral part, which is formed by the two pedunculi cerebri.
The pedunculi cerebri can be seen to some extent on the base of the brain, where they bound the posterior part of the interpeduncular fossa. Encircling the upper end of each cerebral peduncle, where it emerges from the cerebrum, is the optic tract (Fig. 527, p. 594).
The mesencephalon is tunnelled from below upwards by a narrow passage, called the aquæductus cerebri, which connects the fourth ventricle with the third
ventricle (Fig. 519, p. 585). This channel lies much nearer the dorsal aspect than the ventral aspect of the mesencephalon.
Corpora Quadrigemina.-This name is applied to four rounded eminences or colliculi on the dorsal aspect of the mesencephalon (Figs. 516 and 517). The superior pair are larger and broader than the inferior pair, but they are not so well defined nor are they so prominent. A longitudinal and a transverse groove separate the colliculi from each other. The longitudinal groove occupies the median plane and extends upwards to the posterior commissure of the brain. The superior end of this groove widens out into a shallow depression, in which the pineal body, a small conical structure which belongs to the diencephalon, rests. From the lower end of the same groove a short but well-defined and projecting band, the frenulum
veli, passes to the anterior medullary velum, which lies immediately below the inferior colliculi. The transverse groove curves round below each of the superior pair of colliculi and
Stalk of pineal body mesencephalon.
The quadrigeminal bodies
are not marked off laterally from the sides of the mesencephalon, but each has in connexion with it, on this aspect,
prominent strand, which is prolonged superiorly and ventrally towards the thalamic region. These strands are called the brachia of the corpora quadrigemina, and they are separated from each other by a continuation, on the side of the mesencephalon, of the transverse groove which intervenes between the two pairs of colliculi.
The corpus geniculatum mediale is closely associated with the brachia, although it does not form part of the mesencephalon, but belongs to the prosencephalon. It is a small, sharply defined oval eminence, which lies on the lateral side of the superior part of the mesencephalon under shelter of the posterior end of the thalamus. The brachium quadrigeminum inferius, proceeding upwards from the colliculus inferior, advances towards the corpus geniculatum mediale and disappears from view under cover of this prominence.
The brachium quadrigeminum superius is carried upwards and ventrally between the overhanging thalamus and the corpus geniculatum mediale. A superficial examination of the mesencephalon is sufficient to show that, while a large part of this strand enters the corpus geniculatum laterale, a considerable portion is a continuation of the lateral root of the optic tract.
Pedunculi Cerebri.-The cerebral peduncles (Figs. 517 and 527) appear upon the ventral or basal aspect of the mesencephalon as two large rope-like strands which emerge from the cerebral hemispheres and disappear below by plunging into the pars basilaris of the pons. At the place where each peduncle emerges from the corresponding side of the cerebrum it is encircled by the optic tract.
Each pedunculus cerebri is composed of two parts, viz., a dorsal tegmental part (tegmentum), which is prolonged upwards into the region below the thalamus (hypothalamus), and a ventral portion (basis pedunculi), which, when traced upwards into the cerebrum, is seen to take up a position on the lateral side of
FIG. 517. THE LEFT LATERAL ASPECT OF THE BRAIN-STEM AFTER THE CEREBRAL HEMISPHERE (EXCEPT THE CORPUS STRIATUM) AND THE CEREBELLUM (EXCEPT THE NUCLEUS DENTATUS) HAVE BEEN REMOVED.
the thalamus and to be continuous with the internal capsule of the brain; and an intermediate part, the substantia nigra. When the base of the brain is examined, it is the basis pedunculi which is seen, and it is observed to be white in colour and streaked in the longitudinal direction. In the tegmentum the longitudinallyarranged fibres are, in large part, corticipetal, or, in other words, fibres which are ascending towards the cortex of the cerebrum; the basis pedunculi, on the other hand, is composed entirely of longitudinal strands of fibres which are corticifugal, or fibres which descend from the cerebral hemisphere.
On the surface of the mesencephalon the separation between the tegmental and basal portions of the pedunculus cerebri is clearly indicated by a medial and a lateral groove.
The medial furrow is the more distinct of the two.
into the interpeduncular fossa, and from it emerge the fila of the oculo-motor nerve. It is termed, therefore, the sulcus n. oculomotorii. The lateral groove, which is placed on the lateral aspect of the mesencephalon, is called the sulcus lateralis [mesencephali]. Its lower end becomes continuous with the furrow between the brachium pontis and brachium conjunctivum of the cerebellum.
A close inspection of the lateral surface of the tegmental part of the pedunculi cerebri, below the level of the brachia, will reveal some faintly-marked bundles of fibres curving obliquely upwards and backwards to reach the inferior colliculus (Fig. 517, p. 583). These are fibres of the lateral lemniscus, coming to the surface at the sulcus lateralis and sweeping over the subjacent brachium conjunctivum to gain the inferior colliculus, inferior brachium, and medial geniculate body.
INTERNAL STRUCTURE OF THE MESENCEPHALON.
When transverse sections are made through the mesencephalon the aquæductus cerebri is seen to be surrounded by a thick layer of gray matter, which receives the name of the stratum griseum centrale or the central gray matter of the aqueduct. On the dorsal aspect of this gray matter the corpora quadrigemina form a layer which
CORPORA QUADRIGEMINA STRATUM GRISEUM
SULCUS N. OCULOMOTORI
FIG. 518.-DIAGRAMMATIC VIEW OF THE CUT SURFACE OF
OF THE MESENCEPHALON.
separates it from the surface, and to which the term lamina quadrigemina is applied. On the anterior. and lateral aspects of the central gray matter are the tegmental portions of the cerebral peduncles; whilst, intervening between each of the tegmenta and the corresponding basis pedunculi, there is a conspicuous mass of dark pigmented matter, termed the substantia nigra.
Aquæductus Cerebri and Stratum Griseum Centrale.-The aqueduct is the canal which leads from the fourth ventricle below, upwards through the mesencephalon, to the third ventricle above. It is not quite three-quarters of an inch in length, and it lies much nearer the dorsal than the ventral surface of the mesencephalon. When examined in transverse section, it presents a triangular outline as it into the fourth ventricle and a T-shaped outline close to the third ventricle. In the intermediate part of its course it assumes different outlines, and not always the same form at the same level in different specimens.
The aqueduct is lined with ciliated epithelium, and outside this is the thick layer of central gray matter, which is directly continuous below with the gray matter spread out on the floor of the fourth ventricle, and above with gray matter on the floor and sides of the third ventricle. Scattered more or less irregularly throughout the central gray matter are numerous nerve-cells of varying forms and sizes, whilst in addition to these there are three definite collections or clusters of cells, which constitute the nuclei of origin of the trochlear nerve, the oculomotor nerve, and the mesencephalic root of the trigeminal nerve. The position and relations of these will be given at a later stage.
Substantia Nigra.-When seen in transverse section, the substantia nigra presents a semilunar outline. It consists of a mass of gray matter, in the midst of which are large numbers of deeply pigmented nerve-cells. It is only when this substance is examined in bulk that it appears dark; in thin sections it does not differ much in colour from ordinary gray matter, although, under the microscope, the brown-coloured cells stand out very conspicuously, even under low powers. The substantia nigra is disposed in the form of a thick layer, interposed between the tegmental and basal portions of the cerebral peduncle. It begins below at the superior border of the pons and extends upwards into the hypothalamus. The margins of this layer of dark-coloured substance come to the surface at the
oculomotor and the lateral sulci of the mesencephalon, and its medial part is traversed by the emerging fila of the oculomotor nerve. It is not equally thick throughout. Towards the lateral sulcus it becomes thin, whilst it thickens considerably near the medial aspect of the pedunculus cerebri. The surface of the substantia nigra, which is turned towards the tegmentum, is concave and uniform; the opposite surface is convex and rendered irregular by the presence of numerous slender prolongations of the substance into the basis pedunculi.
The morphological and physiological significance of the substantia nigra is not fully understood, and the connexions established by its cells are imperfectly known. Bechterew, however, is of the opinion that fibres arising in the motor area of the cerebral cortex end in relationship with the cells of the substantia nigra, the axons of which proceed to the motor trigeminal nucleus for the purpose of coordinating the muscles of mastication.
Colliculi Inferiores (or inferior quadrigeminal bodies).- Each inferior colliculus
FIG. 519. THE MEDIAL ASPECT OF THE RIGHT HALF OF THE BRAIN EXPOSED BY A MEDIAN SAGITTAL SECTION.
is composed largely of a mass of gray matter which, in transverse section, presents an oval outline (Fig. 520, p. 587). This central nucleus is, to a large extent, encapsulated by white matter. Numerous cells of various sizes are scattered throughout it, and the whole mass is pervaded by an intricate interlacement of fine fibres, which are derived, to a large extent, from the lateral lemniscus.
In transverse sections through this region, the lateral lemniscus is seen to abut against the lateral margin of the central nucleus. Many of the fibres of this tract enter it at once and become dispersed amongst its cells; others sweep over its dorsal surface, so as to give it a superficial covering; whilst a third group is carried medially, in the form of a thin layer, on its ventral aspect, so as to mark it off from the subjacent central gray matter of the aqueduct (Fig. 520, p. 587). In this manner, therefore, the inferior colliculus becomes partially circumscribed by the fibres of the lateral lemniscus. Several of the lateral lemniscus fibres, which proceed over the superficial or dorsal aspect of the nucleus, reach the median plane and form a loose decussation with the corresponding fibres of the opposite side.
The intimate connexion which is thus exhibited between the fibres of the lateral lemniscus