The Rhombencephalon

The Medulla Oblongata

The medulla oblongata lies on the basilar part of the occipital bone. It is quadrilateral in outline, but much wider in front than behind, and compressed dorso-ventrally. Its length, measured from the root of the first cervical nerve to the pons, is about two inches (ca. 5 cm.).

Its ventral surface is convex in the transverse direction. It is marked by a median fissure (Fissura mediana ventralis) which is continuous behind with the similar fissure of the spinal cord. The posterior part of the fissure is faintly marked, but in front it becomes deeper and ends in a small depression (Foramen cæcum) behind the central part of a transverse band, the corpus trapezoideum. On either side of the fissure is a rounded tract, the pyramid (Pyramis), which is bounded externally by a faint lateral groove (Sulcus intermedius ventralis). The pyramids join the pons in front; behind they become narrower and disappear into the substance of the medulla, in which their fibers intercross, forming the decussation of the pyramids (Decussatio pyramidum).

The superficial origin of the sixth cranial nerve (N. oculomotorius) is just lateral to the anterior end of the pyramid. The corpus trapezoideum is a transverse band which extends across the surface immediately behind the pons. It is crossed by the pyramids, which cut off a small central part. The lateral part extends out to the roots of the seventh and eighth nerves on either side. Behind the outer part of the corpus trapezoideum there is a variably developed rounded eminence, the tuberculum faciale. The root-fibers of the twelfth or hypoglossal nerve form an oblique linear series lateral to the posterior part of the pyramid.

The dorsal surface is largely concealed by the cerebellum and forms the greater part of the floor of the fourth ventricle. The dorsal median fissure (Fissura mediana dorsalis), the direct continuation of the corresponding groove of the spinal cord, extends forward to about the middle of the surface. Here the restiform bodies, which constitute the lips of the fissure, diverge to form the lateral boundaries of a triangular depression; this is the posterior part of the rhomboid fossa or floor of the fourth ventricle of the brain. The dorso-lateral fissure winds outward and forward to the lateral aspect of the medulla, where it presents the roots of the ninth, tenth, and eleventh cranial nerves. External to it is a distinct oval eminence on the anterior part of the lateral column, termed the tuberculum cinereum. The central canal of the cord is continued in the posterior part of the medulla, inclines dorsally, and opens in the posterior angle of the fourth ventricle. Hence it is customary to distinguish a closed and an open part of the medulla. The dorsal aspect of the latter, which is concealed at present, will be considered later in the description of the fourth ventricle.

The lateral surface is narrow behind, wider and rounded in front. From it the root-fibers of the ninth, tenth, and eleventh cranial nerves arise in a linear series, and alongside of it the spinal part of the eleventh nerve passes forward to join the medullary root. Close inspection reveals the presence of striæ which curve obliquely downward and backward from the surface of the restiform body toward the hypoglossal root-fibers; these are the external arcuate fibers (Fibræ arcuatæ externæ). The recess between the lateral aspect of the medulla and the cerebellum is occupied by an irregular mass of villous projections of the pia mater, containing tufts of vessels; this is the chorioid plexus of the fourth ventricle, and is the lateral edge of the tela chorioidea of the ventricle. On raising the chorioid plexus it is seen that the tela chorioidea is attached to the dorsal aspect of the medulla, and reinforces here the wall of the fourth ventricle; also that the restiform body terminates in front by entering the base of the cerebellum, forming its posterior peduncle.

In the medulla the fiber tracts of the spinal cord either terminate in the nuclei of the gray matter or undergo changes in their relative position, and new tracts appear. The gray matter is highly modified and forms masses which have no homologues in the cord. The central canal of the closed part of the medulla is surrounded by a thicker layer of gray matter than is the case in the spinal cord. As the medulla opens out this gray matter is naturally spread in the floor of the fourth ventricle. The dorsal horns of gray matter become wide, spread apart, and are broken up to a great extent in the formatio reticularis. Two elongated masses of cells appear above the central gray matter; these are the nucleus gracilis and nucleus cuneatus, and in them the fasciculi of like name gradually end. In front of these are the terminal nuclei of the afferent or sensory cranial nerves and of the sensory portions of the mixed nerves. The ventral horns are succeeded by the nuclei of origin of the efferent or motor cranial nerves and the motor root-fibers of the mixed nerves. Of the twelve pairs of cranial nerves, the last eight are connected with nuclei in the medulla and pons. The posterior olivary nucleus (Nucleus olivaris caudalis) is a conspicuous gray mass which lies above the pyramid on each side. On cross-sections it appears as a wavy layer of gray matter which is folded on itself so as to inclose a mass of white matter. It is about 2 cm. long in the horse, but is smaller in circumference than in man and hence does not cause any very distinct external enlargement (olivary eminence), as in the latter. It is connected with the opposite side of the cerebellum by the cerebello-olivary fibers. At its inner side are two small accessory olivary nuclei. The pyramidal tracts, which are small in ungulates, send most of their fibers across to the opposite side in the posterior part of the medulla, forming the pyramidal decussation. Some fibers, however, continue in the ventral column of the same side of the cord, and others are connected with the nuclei of the motor nerve-roots. From the nucleus gracilis and nucleus cuneatus fibers arise which are traceable forward to the thalamus. These are the internal arcuate fibers (Fibræ arcuatæ internæ), which curve across the median plane below the central gray matter and form with those of the opposite side the decussation of the fillet (Decussatio lemniscorum). Beyond the decussation the fibers form an important longitudinal tract which extends forward in the mid-brain. This is the fillet or lemniscus, the chief continuation of the sensory conducting path from the dorsal roots of the spinal nerves. The external arcuate fibers, some of which were seen on the lateral aspect of the medulla, have a similar origin. Part of them (Fibræ dorsales) pass directly to the dorsal aspect of the restiform body of the same side; others (Fibræ ventrales) cross to the opposite side, descend close to the ventral fissure, and then curve upward and forward to the restiform body. The decussation of the arcuate fibers forms the distinct median raphé seen on cross-sections of the medulla anterior to the pyramidal decussation. The restiform body, situated dorso-laterally, contains, in addition to the arcuate fibers, the cerebello-olivary fasciculus before mentioned, and the cerebellospinal fasciculus or direct cerebellar tract. The dorsal longitudinal fasciculus corresponds to the ventral ground-bundle of the spinal cord, displaced dorsally by the decussation of the pyramids and fillet. In the posterior part of the medulla it is not marked off from the fillet, along the dorsal edge of which it lies. From the level of the hypoglossal nucleus forward it is distinct and can be traced as a conspicuous tract in the ventral margin of the gray matter of the floor of the fourth ventricle and of the central gray matter of the mid-brain. Below the restiform body and related externally to the external arcuate fibers, there is a considerable bundle of longitudinal fibers, the spinal root (Tractus spinalis) of the fifth nerve; internal to it is the terminal nucleus of the sensory root of the nerve.