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The fasciculus cerebrospinalis anterior (O.T. direct pyramidal tract) is usually a nerve-strand of small size which lies near the anterior median fissure. As a rule it cannot be traced lower than the middle of the thoracic region of the spinal medulla. It is a descending tract and must be associated with the lateral cerebro-spinal fasciculus of the opposite side, seeing that both of these strands arise from the motor area of the cortex of the same cerebral hemisphere. From this it must be clear that the anterior cerebro-spinal fasciculus does not cross the median plane as it enters the spinal medulla, but descends on the side of the spinal medulla corresponding to the cerebral hemisphere in which it arises. Nevertheless, its fibres do not end in the same side of the spinal medulla, but at every step along the path of the strand they make use of the anterior commissure, and cross to the opposite side of the spinal medulla, to terminate in relation to the opposite ventral motor cells in the same manner as the lateral cerebro-spinal fibres.
From this crossing of the cerebro-spinal fasciculi, it follows that the destruction of the fibres which compose them as they descend in one side of the brain must result in paralysis of the muscles supplied by the efferent nerves of the opposite side of the spinal medulla."
In cases of old brain lesion it is sometimes possible to detect some degenerated fibres in the lateral cerebro-spinal fasciculus of the sound side of the spinal medulla, and from this it is supposed that this tract contains a few uncrossed fibres. If this is the case, each side of the spinal medulla stands in connexion with the motor area of both cerebral hemispheres.
It is well to note that the fibres of both lateral cerebro-spinal fasciculi are not medullated until the time of birth. They are the latest of all the fasciculi of the spinal medulla to myelinate.
Commissura Anterior Alba.-The anterior white commissure is composed of medullated nerve-fibres passing from one side of the spinal medulla to the other and entering the anterior column of gray matter, and also the anterior funiculus of white matter. It is to be regarded more as a decussation than as a commissure, and its width, which varies somewhat in different regions, fluctuates in correspondence with the diameter of the spinal medulla.
Amongst the fibres which cross in the anterior commissure may be mentioned: (1) The fibres of the fasciculus cerebrospinalis anterior; (2) collaterals from both the anterior and lateral funiculi; (3) axons of many of the cells of the gray matter; (4) the dendritic processes of some of the medial anterior cells.
Commissura Grisea. Although this is composed of gray matter with a large admixture of neuroglia, numerous nerve-fibres pass transversely through it, so as to establish relations between the cells in the gray matter on the two sides of the spinal medulla.
THE ENCEPHALON OR BRAIN.
The brain is the enlarged and greatly modified upper part of the cerebro-spinal nervous axis. It is surrounded by the same membranes that envelop the medulla spinalis (viz., the dura mater, the arachnoid, and the pia mater), and it almost completely fills up the cavity of the cranium. So closely, indeed, is the skull capsule moulded upon the brain that the impress of the latter is almost everywhere evident upon the inner surface of the cranial wall. The relations, therefore, of cranium to brain are totally different from those presented by the vertebral canal to the spinal medulla. As we have noted, the medulla spinalis occupies only a part of its bony case; and there is not only a wide and roomy space between the arachnoid and the pia mater, but also an interval of some width between the dura mater and the walls of the vertebral canal.
General Appearance of the Brain. When viewed from above the brain presents an ovoid figure, the broad end of which is directed backwards. Its greatest transverse diameter is usually found in the neighbourhood of that part which lies between the two parietal tuberosities of the cranium. The only parts which are visible when the brain is inspected from this point of view are the two convoluted cerebral hemispheres. These present an extensive convex surface, which
is closely applied to the internal aspect of the cranial vault, and are separated from each other by a deep median cleft, termed the fissura longitudinalis cerebri, which extends from the front to the back of the brain.
The inferior aspect of the brain is usually termed the basis cerebri. It presents an uneven and irregular surface, which is more or less accurately adapted to the inequalities on the floor of the cranial cavity. Upon this aspect of the brain some of its main subdivisions may be recognised. Thus, posteriorly, is seen the short cylindrical portion, called the medulla oblongata, through which, at the foramen magnum, the brain becomes continuous with the medulla spinalis. The medulla oblongata lies on the ventral aspect of the cerebellum, and occupies the vallecula or hollow which intervenes between the two cerebellar hemispheres. The cerebellum
FIG. 476.-THE BASE OF THE BRAIN WITH THE CEREBRAL NERVES ATTACHED.
is a mass of considerable size which is placed below the posterior portions of the two cerebral hemispheres. It is easily recognised on account of the closely set, curved, and parallel fissures which traverse its surface and give it a foliated appearance. Above the medulla oblongata, and in close connexion with it, is a prominent white elevation called the pons. Immediately in front of the pons is a deep hollow or recess. This is bounded behind by the pons, on each side by the projecting temporal lobe of the cerebral hemisphere, and in front by the orbital portions of the frontal lobes of the cerebral hemispheres. Passing out from each side of the anterior part of this recess is the deep lateral fissure of the brain which intervenes between the pointed and projecting extremity of the temporal lobe and the frontal lobe of the cerebrum, whilst, in the median plane in front, the longitudinal fissure, which separates the frontal portions of the cerebral hemispheres, opens into it.
Within the limits of this deep hollow, on the base of the brain, two large rope
like strands, the pedunculi cerebri, may be seen issuing from the inferior surface of the cerebral hemispheres. As they pass downwards these peduncles are inclined obliquely towards the median plane, so that when they plunge into the pons they are situated in close apposition the one to the other (Fig. 478). Turning round the lateral side of each peduncle, where it emerges from the cerebrum, a flattened band termed the optic tract may be observed. These bands come from the anterior part of the hollow, where they are joined together by a short connecting piece termed the optic chiasma. The optic nerve is inserted, on each side, into the antero-lateral angle of the chiasma.
The pedunculi cerebri, the optic tracts, and the optic chiasma enclose a deep rhomboidal or lozenge-shaped interval on the base of the brain, which is termed the fossa interpeduncularis. Within the limits of this area the following parts may be seen as we pass from behind forwards: (1) the substantia perforata posterior; (2) the corpora mamillaria: (3) the tuber cinereum and the stalk of the hypophysis cerebri (O.T. pituitary body).
At its posterior angle, immediately in front of the pons, the interpeduncular fossa is very deep and is floored by a layer of gray matter, in which are numerous small apertures. This is the substantia perforata posterior. Through the apertures which are dotted over its surface the small postero-medial basal branches of the posterior cerebral artery enter the brain.
The corpora mamillaria are two small white pea-like eminences placed side by side in front of the substantia perforata posterior.
The tuber cinereum is a slightly-raised field of gray matter, which occupies the interval between the anterior portions of the optic tracts in front of the corpora mamillaria. Springing from the anterior part of the tuber cinereum, immediately behind the optic chiasma, is the infundibulum, or the stalk which connects the hypophysis cerebri with the base of the brain (Fig. 478).
Lateral to the limits of the anterior part of the interpeduncular space there is, on each side, a small depressed triangular field of gray matter, which leads laterally into the lateral cerebral fissure. It is perforated by the antero-medial and the antero-lateral groups of basal arteries, and receives the name of the substantia perforata anterior.
General Connexions of the Several Parts of the Brain. - The medulla oblongata, the pons, and the cerebellum occupy the posterior cranial fossa, and they are separated from the cerebral hemispheres, which lie above them, by a partition of dura mater, termed the tentorium cerebelli. Further, they surround a cavity, a portion of the primitive cavity of the early neural tube, which is termed the fourth ventricle of the brain, and they all stand in intimate connexion, one with the other. The medulla oblongata is for the most part carried upwards into the pons; but at the same time two large strands from its dorsal aspect, termed the restiform bodies, are prolonged into the cerebellum, and constitute its inferior peduncles, or the chief bonds of union between the medulla (oblongata and spinalis) and the cerebellum. The pons has large numbers of transverse fibres entering into its composition, and the great majority of these are gathered together on each side in the form of a large rope-like strand. This plunges into the corresponding hemisphere of the cerebellum, and constitutes its middle peduncle, which is known as the brachium pontis.
The cerebrum, which forms the great mass of the brain, occupies the anterior and middle cranial fossæ, and extends backwards into the occipital region above the tentorium and the cerebellum. The greater part of the cerebrum is formed by the cerebral hemispheres, which are separated from each other in the median plane by the longitudinal fissure. At the bottom of this fissure is the corpus callosum, a broad commissural band which connects the two hemispheres with each other. Each hemisphere is hollow, the cavity in its interior being termed the lateral ventricle of the brain. Between and below the cerebral hemispheres, and almost completely concealed by them, is the inter-brain or diencephalon. principal parts forming this portion of the brain are two large masses of gray matter, termed the thalami. Between these is the third ventricle of the brain-a deep narrow cavity occupying the median plane. The third ventricle communicate
FIG. 477.-THE PARTS OF THE BRAIN CUT THROUGH IN A MEDIAN SAGITTAL SECTION.
the relationship of these structures to the series of cavities in the interior of the brain as they are displayed in a median sagittal section (Fig. 477).
The central canal which tunnels the spinal medulla is seen to extend into the medulla oblongata for a short distance; then it expands into the irregular cavity of the fourth ventricle, the floor (anterior wall) of which is formed partly by the medulla oblongata and partly by its continuation upwards, the pars dorsalis pontis. Behind the fourth ventricle lies the cerebellum, but it forms only a small part of the roof (tegmen). The roof consists mainly of the velum medullare anterius above and the thin epithelial lamina (lamina chorioidea epithelialis) below.
The fourth ventricle is continued upwards into the aquæductus cerebri, which tunnels the mesencephalon, of which the thick mass of the tegmentum is placed in front of it and the lamina quadrigemina behind.
The aqueduct opens in front into the third ventricle, the major portion of each side wall of which is formed by the thalamus. Near the antero-superior
a corner of each side wall of the third ventricle the small foramen interventriculare (O.T. foramen of Monro) leads into the cavity of the corresponding cerebral hemisphere, which is known as the lateral ventricle.
The medulla oblongata is the continuation upwards of the medulla spinalis. It is a little more than 25 mm. (one inch) in length, and it may be regarded as beginning immediately above the uppermost root of the first cervical nerve, or,
FIG. 478.-FRONT VIEW OF THE MEDULLA OBLONGATA, PONS, AND MESENCEPHALON OF A
roughly, about the level of the foramen magnum. From this it proceeds upwards in a very nearly vertical direction, and ends at the lower border of the pons. At first its girth is similar to that of the spinal medulla, but it rapidly expands as it approaches the pons, and consequently it presents a more or less conical form. Its anterior surface lies behind the grooved surface of the basilar portion of the occipital bone, whilst its posterior surface is sunk into the vallecula of the cerebellum. The medulla oblongata is a bilateral structure, and this is indicated. on the surface by the presence of anterior and posterior median fissures, on the ventral and dorsal surfaces respectively.
The fissura mediana anterior, as it passes from the spinal medulla on to the medulla oblongata, is interrupted at the level of the foramen magnum by several strands of fibres which cross the median plane from one side to the other. This intercrossing is termed the decussation of the pyramids. Above this level the fissur