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is carried upwards to the lower border of the pons, but is often rendered very shallow by numerous external arcuate fibres which emerge upon the surface
between its lips and then curve laterally to reach the posterior part of the medulla oblongata. At the lower margin of the pons it expands slightly and ends in a blind pit, which receives the name of the foramen cæcum
Pontine part of floor of Vicq d'Azyr.
of 4th ventricle
The fissura mediana posterior is present only on the lower half of the medulla oblongata. As it ascends it rapidly becomes shallower. Half-way up, where the central canal opens into the fourth ventricle, the lips of the posterior median fissure are thrust apart from each other and constitute the boun
FIG. 479.—POSTERIOR VIEW OF THE MEDULLA, PONS, AND MESENCEPHALON daries of a triangular
OF A FULL-TIME HUMAN FETUS.
field, which is seen when the epithelial roof of the lower part of the fourth ventricle is removed. This triangular field is the lower part of the fossa rhomboidea, or the floor of the fourth ventricle of the brain. The lower half of the medulla oblongata, containing as it does the continuation of the central canal of the spinal medulla, is frequently termed the closed part of the medulla oblongata; the upper half, above the opening of the canal, which contains the lower part of the fourth ventricle, is called the open part of the medulla oblongata.
The examination of the floor of the fourth ventricle will be deferred for the present, and the appearance presented by the surface of the medulla oblongata may now engage our attention. In the spinal medulla the corresponding surface area is divided into three districts or funiculi by the emerging motor roots and the entering sensory roots of the spinal nerves. Of these the sensory enter along the bottom of the sulcus lateralis posterior, whilst the motor fila are spread over a relatively broad surface area and have no groove in connexion with their emergence from the spinal medulla. In the case of the medulla oblongata corresponding rows of fila enter and emerge from the surface of each side. The fila of the hypoglossal nerve carry up the line of the anterior nerve-roots of the spinal medulla. In one respect, however, they differ: they emerge in linear order and along the bottom of a distinct furrow, termed the sulcus lateralis anterior, which proceeds upwards on the surface of the medulla oblongata. The fila which carry up the line of the posterior nerve-roots on the surface of the medulla oblongata are the root-bundles of the accessory, the glosso-pharyngeal, and the vagus nerves. These are attached along the bottom of a furrow which is the direct continuation upwards of the sulcus lateralis posterior of the spinal medulla, and therefore receives the name of the sulcus lateralis posterior of the medulla oblongata. The root-bundles of these nerves differ, however, in so far that they are not all composed of afferent fibres which spring from ganglionic cells placed without and enter the medulla. Certain of them are purely efferent (roots of accessory), whilst others contain a considerable number of efferent as well as afferent fibres, and are therefore to be regarded as mixed roots.
By the sulci laterales, and also by the two rows of fila attached along the
bottom of these furrows, the surface of the medulla oblongata on each side is divided into three districts, viz., an anterior, a lateral, and a posterior, similar to the surface areas of the three funiculi on the side of the spinal medulla. Indeed, at first sight, they appear to be direct continuations upwards of these three portions of the spinal medulla; this, however, is not the case, because the fibres of the three funiculi of the spinal medulla undergo a rearrangement as they proceed upwards into the medulla oblongata.
Anterior Area of the Medulla Oblongata-Pyramis.-The district between the anterior median fissure, and the sulcus lateralis anterior, along the bottom of which the root-fila of the hypoglossal nerve issue from the medulla oblongata, receives the name of the pyramid. An inspection of the surface is sufficient to show that the pyramid is composed of a compact strand of longitudinally directed nerve-fibres. It represents, in fact, the portion of the great cerebro-spinal fasciculus which is destined to carry fibres from the cerebral hemisphere to all the motor nuclei on the other side of the medulla oblongata and medulla spinalis. Somewhat constricted at the place where it emerges from the pons (Fig. 478) it swells immediately to form a prominent rounded column, which passes vertically downward, separated from the pyramid of the other side by the fissura mediana anterior. Towards the lower part of the medulla oblongata it gradually tapers.
Although the pyramid at first sight appears to be continuous with the anterior funiculus of the medulla spinalis, only a very small proportion of the fibres contained in the latter are derived from the pyramid. This at once becomes manifest when the lips of the anterior median fissure are thrust apart at the place of junction between the medulla oblongata and spinal medulla. The pyramid is then seen to divide at this level into two parts, viz., a small portion composed of a variable number of the most lateral fibres of the pyramid, termed the fasciculus cerebrospinalis anterior (O.T. direct pyramidal tract), and a much larger portion, situated next the median fissure, called the fasciculus cerebrospinalis lateralis (O.T. crossed pyramidal tract). The anterior cerebro-spinal fasciculus is continued down into the anterior funiculus of the medulla spinalis, and in this it takes up a medial position next the median fissure. The lateral cerebro-spinal fasciculus is broken up into three or more coarse bundles, which sink backwards and at the same time cross the median plane, to take up a position in the posterior part of the opposite lateral funiculus of the spinal medulla. The term decussatio pyramidum (decussation of the pyramids) is applied to the intercrossing of the corresponding bundles of the lateral cerebrospinal fasciculi of opposite sides.
The anterior cerebro-spinal fasciculus is, therefore, the only part of the pyramid which has a place in the anterior funiculus of the spinal medulla. The much larger part of this funiculus, termed the fasciculus anterior proprius, as it is traced up into the medulla oblongata, is seen to be thrust aside by the decussating bundles of the lateral cerebrospinal fasciculus. It thus comes to occupy
a deep position in the substance of the medulla FIG. 480.-DIAGRAM OF THE DECUSSATION OF oblongata, behind and to the lateral side of the pyramid.
THE PYRAMIDS (modified from van Gehuchten).
pharyngeal nucleus; FS, Tractus solitarius; NA,
Lateral Area of the Medulla Oblongata. -This is the district on the surface of the medulla oblongata which is included between the two rows of nerve-roots, viz., the hypoglossal roots in front, and the root-bundles of the accessory, the vagus, and th
transmission to the cerebral cortex, which is concerned with the conscious appreciation of sensations) all impulses of pain, heat, and cold coming from the skin upon the opposite side of the body. The fasciculus spinothalamicus anterior conveys impulses of touch and pressure from the opposite side.
The spino-cerebellar fasciculi [anterior and posterior] convey to the cerebellum respectively homolateral and bilateral unconscious afferent impulses underlying muscular co-ordination and reflex tone.
Among the descending tracts that establish connexions between various parts of the brain (see Fig. 475) and the motor nerve-cells in the anterior column may be mentioned the cerebrospinal, the rubro-spinal (from the red nucleus), the tectospinal (from the corpora quadrigemina), the vestibulo-spinal (from the terminal nucleus of the vestibular nerve), and the bulbo-spinal tracts. The last-mentioned forms a peculiar triangular area upon the surface immediately to the lateral side of the anterior nerve-roots (Fig. 473), but there is great uncertainty as to its mode of origin: it is often called the fasciculus olivospinalis, from the fact that its discoverer, Helweg, believed it to originate from the olivary nucleus in the bulb or medulla oblongata. It may be regarded as an outlying part of the vestibular (or cerebellar) tract to the motor nuclei of the spinal medulla.
The fasciculus cerebrospinalis lateralis (O.T. crossed pyramidal tract) is a large well-defined descending tract which lies immediately in front of the posterior column of gray matter, and subjacent to the posterior spino-cerebellar fasciculus, which shuts it out from the surface. Below the point where the posterior spino-cerebellar fasciculus begins the cerebrospinal fasciculus becomes superficial, and in this position it can be traced as low as the fourth sacral nerve, at which level it ceases to exist as a distinct strand. The cerebro-spinal fasciculus is composed of an admixture of both large and small fibres. These arise in the brain from the large pyramidal cells of the motor or precentral area of the cerebral cortex, and pass downwards through various subdivisions of the brain to gain the spinal medulla. As they enter the spinal medulla they cross the median plane from one side to the other, and it thus happens that the cerebro-spinal tract in the right lateral funiculus of the spinal medulla has its origin in the cortex of the left cerebral hemisphere, and vice versa. As the tract descends in the spinal medulla it gradually diminishes in size; and this is due to the fact that, as it traverses each spinal segment, numerous fibres leave it to enter the anterior column of gray matter, and end in connexion with the anterior motor cells from which the fibres of the anterior nerve-roots arise. The entire strand is ultimately exhausted in this way. Numerous collateral fibrils spring from the cerebro-spinal fibres, and, entering the gray matter, end in a similar manner. In this way a single cerebro-spinal fibre may be connected with several spinal segments before it finally ends. The lateral cerebro-spinal fasciculus must be regarded as a great motor strand which brings the spinal motor apparatus under the control of the will.
Schäfer believes that many of the fibres of the cerebro-spinal fasciculus end in connexion with the cells of the nucleus dorsalis.
In many marsupials, rodents, and ungulates the lateral cerebro-spinal fasciculus lies in the posterior funiculus of the spinal medulla.
The fasciculus lateralis proprius represents the remainder of the lateral funiculus. Its fibres are largely derived from the cells situated in all parts of the gray matter, and also from the nerve-cells of the opposite side of the spinal medulla. After a course of very varying length in the fasciculus lateralis, these fibres turn medially and re-enter the gray matter. Such fibres may thus be regarded as inter-segmental association fibres binding two or more segments of the spinal medulla together. It may be mentioned that the association fibres which link together segments of the spinal medulla which are near to each other lie close to the gray matter, whilst those which connect the more distant segments are situated further out in the lateral funiculus.
Funiculus Anterior.-One well-defined tract is situated in the funiculus anterior. This is termed the fasciculus cerebrospinalis anterior. The remainder of the funiculus receives the name of the fasciculus anterior proprius.
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 there 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