THE MEDULLA OBLONGATA.
The medulla oblongata forms the transition from the spinal cord to the brain. The intimate make-up, relatively simple within the spinal cord, at the same time undergoes manifold modifications. Not only does the gray substance change its form, but new masses, large and small nuclei, appear. Coincidentally, a rearrangement of certain systems of the white substance occurs, certain tracts of fibres are suppressed and new ones make their appearance. Almost each cross-section presents a different picture. It would carry us too far to follow accurately at this place the structure of the medulla oblongata in its topographical relations, as shown in transverse sections. The study of the fibre tracts in the brain and spinal cord, without the use of serial sections, is impossible ; and especially the study of the fibre-tracts in the medulla oblongata offers difficulties as nowhere else.
The reader is particularly referred, therefore, to Part III of this book, in which, by means of the drawings of serial sections, the most important tracts may be followed through the entire brain-stem. By means of these microscopical pictures and the assist ance of the following diagrammatic figures, the reader will be able to find his way.
In the consideration of the morphology, the most important gray masses of the medulla oblongata have been mentioned ; we may limit ourselves, therefore, to the pres entation of the connection of these gray masses with other parts of the central nervous system. Then, as supplementary, the origin of the cerebral nerves will be more closely considered, since, as pointed out in the morphological section, the nuclei of most of the cerebral nerves are situated along the floor of the fourth ventricle.
We proceed most advantageously, if we trace upward the fibre-systems of the white substance of the cord, which were described in the preceding section. At the same time, we will ignore the fibre-systems, which come from the brain and descend in the cord and have been mentioned repeatedly in previous sections.
Let us first follow the tract of the posterior column. The fibres of Burdach's and of Goll's column end in the nuclei of the posterior column, that is, within the nucleus fasciculi cunati and the nucleus fasciculi gracilis. From these nuclei further tracts are
developed, of which one in particular, the tract establishing relations between the pos terior column nuclei and the thalamus, now concerns us. The fibres pass, as the axones of the cells within the cuneate and gracile nuclei, in ventrally directed courses, the fibrae arcuatae internae, toward the mid-line, where by decussation they form the raphe. After crossing, the fibres assemble close to the mid-line and, turning upward, pass longitudinally to higher levels. The field so formed is known as the interolivary stratum, on account of its position between the two inferior olivary nuclei. The fibre-bundles can be traced through the pons and the mid-brain as far as the thalamus, where they end in the nucleus lateralis and in the centrum medianum. This is the path usually called the medial fillet or lemniscus medialis ; it is also known as the tractus bulbo thalamicus.
The medial fillet is not composed exclusively of fibres which come from the nuclei of the posterior columns. During its course through the medulla oblongata, the fillet-tract is augmented by the fibres from the spinal cord, which we have studied as the tractus spino-thalamicus. In addition, as will be pointed out later, fibres come from the terminal nuclei of the cranial nerves. All these contributions collectively constitute the medial fillet, which ends within the thalamus.
Still other tracts arise from the posterior column nuclei and unite the latter with the cerebellum. Some of these fibres pass, as do the above-mentioned fibrae arcuatae interne, first towards the mid-line and there cross. They course, however, not longitu dinally within the interolivary stratum, but pass ventrally along the raphe as far as the anterior medial fissure, then around the pyramids and the olives as the fibrae arcuatae externae ventrales, and continue as constituents of the restiform bodies to the cerebellum. Other fibres issue dorsally from the posterior column nuclei and pass directly to the corpus restiforme as the fibrae arcuatae externae dorsales (Fig. 156). Perhaps these are joined by direct fibres from the posterior column tracts.