The direct continuation of the spinal cord is called the medulla oblongata. It consists of representatives of the various parts of the cord, with some additional gray matter. The relationship of the different parts of the medulla to those of the spinal cord may be best understood by supposing the posterior median fissure and underlying nerve substance at its upper limit to be split vertically down to the central canal, and the lateral masses separated, so that the gray part becomes spread out on the posterior surface, and there forms the floor of the fourth ventricle. The gray matter of the medulla oblongata consists of two sets of nuclei; one being the continuation of the gray columns of the spinal marrow, and the other made up of certain additional gray nodules embedded here and there among the white strands.
The anterior motor gray columns, which are cut off from the central gray substance by the passage of the pyramidal tract to the opposite side, are continued along the floor of the fourth ventricle near the median line. The posterior gray columns are continued upward to form the nucleus of Rolando, and are spread out on the lateral part of the floor of the ventricle. Important nuclei of gray matter lie in the olivary bodies, and numerous collections of cells forming the nuclei from which arise the chief cranial nerves. For an adequate description of these groups of nerve cells and their connections, works on anatomy must be consulted.
The various white columns of the spinal cord are so distributed in the medulla that their course gives some indication of the channels by which impulses are carried through it.
In ascending to the medulla the posterior white columns become differentiated into three, (1) Goll's column is more distinctively marked off, and enlarges to form the funiculus gracilis, containing the clavate nucleus; the funiculus gracillis tapers away to nothing above. (2) Burdach's column widens in a wedge-like fashion, and is catted funiculus cuneatus, which contains the cuneate nucleus. It passes on to help to form the inferior peduncle of the cerebellum. (3) By the projection of the enlarged posterior gray column, Tubercle of Rolando, a prominence is produced called the funiculus of Rolando. This also helps to form the inferior peduncle of the cerebellum.
The greater part of the lateral while columns of the spinal cord pass, at the decussation of the pyramids, to the opposite side to form the pyramidal prominence on the front of the medulla, and are thence continued upward directly to the motor areas of the cortex. The direct cerebellar tract which forms the superficial part of the lateral column joins the cuneate and Rolando's bands to form the inferior cerebellar peduncle. The majority of the fibres of the anterior gray columns pass into the medulla beneath the pyramids by which they are quite concealed. They can be traced some distance through the pons Varolii. The fibres of the direct pyramidal tracts join the pyramid of their own side.
It must be remembered that the medulla is the only route between the spinal cord and the upper nerve centres.
Fig. 25. Diagram of Brain and Medulla Oblongata. (Cleland).
a, Spinal cord; b,b, Cerebellum divided, and above it the valve of Vieussens partially divided; c, Corpora quadrigemina; d, d, Optic thalami; e, pineal body; f,f, Corpora striata; g, g, Cerebral hemispheres in section; h, Corpus callosum; i, Fornix; l, l, Lateral ventricles; 3, Third ventricle; 4, Fourth ventricle; 5, Fifth ventricle, bounded on each side by septum lucidum.
Through it all the afferent and efferent channels must pass, as they do through the spinal cord. From it, and the prolongation of its gray nuclei in the pons Varolii, several cranial nerves take origin. Thus, the medulla is to the cranial nerves (from the fifth to the ninth) as the spinal cord is to the spinal nerves, but their mode of distribution is different.