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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. The 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 communicates.
with the lateral ventricles by two small apertures, called the foramina interventricularia.
The cerebrum is connected with the parts in the posterior cranial fossa (pons, cerebellum, and medulla oblongata) by a narrow stalk called the mesencephalon or mid-brain. The mid-brain is built up of (1) the pedunculi cerebri, passing from the pons to the cerebrum; (2) the corpora quadrigemina, forming its dorsal part; and (3) the brachia conjunctiva (O.T. superior cerebellar peduncles), proceeding from the cerebellum to the cerebrum. It is tunnelled by a narrow passage, the aquæductus cerebri, which extends between the fourth and third ventricles.
In a view of the intact brain the greater part of the mesencephalon and diencephalon is hidden by the cerebral hemispheres; but a precise idea will be obtained of the inter-relationships of the various parts of the brain, if we study
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
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 fissure
glossopharyngeal nerves behind. It presents a very different appearance in its upper and lower parts. In its lower portion it simply appears to be a continuation upwards of the lateral area of the spinal medulla; in its upper part a striking oval prominence bulges out on the surface of the medulla, and receives the name of oliva (O.T. olivary eminence).
The lower part of this district, however, is very far from being an exact counterpart of the lateral funiculus of the spinal medulla. The large lateral cerebrospinal tract is no longer present, seeing that it forms, in the medulla oblongata,
FIG. 481.-LEFT LATERAL ASPECT OF A BRAIN FROM WHICH THE CEREBRAL HEMISPHERE (WITH THE EXCEPTION OF THE CORPUS STRIATUM) AND THE CEREBELLUM (EXCEPTING ITS NUCLEUS DENTATUS) HAVE
the greater part of the pyramid of the opposite side. Another strand of fibres, viz., the fasciculus spinocerebellaris (posterior), prolonged upwards in the lateral funiculus of the medulla spinalis, gradually leaves this portion of the medulla oblongata. This tract lies on the surface, and is frequently visible to the naked eye as a white band (Fig. 481), which inclines obliquely backwards into the posterior district of the medulla oblongata to join its upper part, or, in other words, to join the restiform body. The remainder of the fibres of the lateral funiculus, comprising the fasciculus lateralis proprius and the fasciculus anterolateralis superficialis, is continued upwards in the lateral area of the medulla oblongata, and at the inferior border of the olive the majority of these fibres disappear from the surface by dipping into
the substance of the medulla oblongata under cover of that projection. A small proportion of the fibres, however, are retained on the surface and travel upwards towards the pons in the interval, which exists between the posterior border of the olive and the roots of the vagus and glossopharyngeal nerves.
The olive is a smooth oval projection which bulges out from the upper part of the lateral area of the medulla oblongata. Its long axis is vertical and is about half an inch long. It marks the position of the subjacent nucleus olivaris inferior, a crumpled thin-walled sac of gray matter, which is separated from the surface only by a very thin layer of superficial white matter.
Posterior Area of the Medulla Oblongata.—In its inferior half, this district is bounded behind by the posterior median fissure, and in its superior half by the lateral margin of the medullary part of the floor of the fourth ventricle of the brain. In front it is separated from the lateral area by the row of root-fila belonging to the accessory, glossopharyngeal and vagus nerves. As in the lateral area, we recognise an inferior portion and a superior portion, which appear continuous but in reality are almost quite distinct the one from the other.
The inferior part of the posterior area corresponds more or less closely with the posterior funiculus of the spinal medulla. In the cervical region the posterior funiculus is divided by a septum of pia mater into a medial fasciculus gracilis and a lateral fasciculus cuneatus. These are prolonged upwards into the medulla oblongata, and in the lower part of the posterior area they stand out distinctly, and are separated one from the other by a continuation upwards from the spinal medulla of the sulcus intermedius posterior. In the medulla oblongata the medial of these strands is called the funiculus gracilis, whilst the lateral one is designated the funiculus cuneatus. When they reach the level of the inferior part of the floor of the fourth ventricle, each ends in a slightly expanded bulbous prominence. The swollen extremity of the funiculus gracilis is called the clava. This is thrust aside from its fellow of the opposite side by the opening up of the medulla oblongata to form the floor of the fourth ventricle, and the central canal opens on the surface in the angle between the two clavæ.
The elongated prominences formed on the surface of the medulla oblongata by these two strands and their enlarged extremities are due to the presence of two elongated nuclei or collections of gray matter which make their appearance subjacent to the strands, and represent the termini of these uppermost extensions of the spinal posterior root-fibres. These are termed respectively the nucleus gracilis and nucleus cuneatus. [As it is the slenderness of the one nucleus and the wedge-shape of the other in transverse section which gave rise to the terms gracilis and cuneatus respectively, it is clearly wrong to introduce the word funiculi into the B.N.A. terminology. The funiculi were named from the nuclei and not the nuclei from the funiculi.]
But a third longitudinal elevation is also apparent on the surface of the inferior part of the posterior area of the medulla oblongata. This is placed on the lateral side of the funiculus cuneatus-between it and the posterior row of nerve-rootsand it has no counterpart in the posterior funiculus of the medulla spinalis. It is called the tuberculum cinereum. It is produced by a mass of substantia gelatinosa coming close to the surface and forming a bulging in this situation. Extremely narrow below, it widens as it is traced upwards, and finally ends in an expanded extremity. A thin layer of white matter, composed of longitudinally arranged fibres, is spread over this district, and separates the substantia gelatinosa from the surface. These fibres constitute the tractus spinalis of the trigeminal nerve, which here assumes a superficial position as it descends in the medulla oblongata.
Corpus Restiforme.-The restiform body forms the upper part of the posterior area of the medulla oblongata. It lies between the floor of the fourth ventricle and the roots of the vagus and glossopharyngeal nerves. It is a large and prominent rope-like strand, which inclines upwards and laterally, and then finally takes a turn backwards and enters the cerebellum. It forms the great link of connexion between the cerebellum on the one hand and the medulla oblongata and spinal medulla on the other, and consequently it is also called the inferior cerebellar peduncle. A study of the surface of the medulla oblongata yields some important