CONNECTIONS BETWEEN THE BRAIN AND SPINAL CORD.
It will be useful here to note how the brain and spinal cord are connected. In the general view of the functions of the nervous system as originated passes by efferent, i.e. motor fibres, through the brain and medulla oblongata to the spinal cord, down which it passes to reach the cells of the anterior cornu, from which efferent fibres proceed to the particular muscles to be moved. By means of the degeneration, which follows when any area has been de stroyed, the pathway of the motor fibres pro ceeding from the area can be traced. Fig. 98 shows the median aspect of the left hemisphere when it has been separated from the right by a cut in the middle line from front to back, and it illustrates the fact that the motor areas, as they are called, are not limited to the outer side of the hemisphere, but are found also in the middle surface, the part next to the great longitudinal fissure (see p. 134). Now when the experiments are conducted on the left side of the brain, the movements occur on the right side of the body ; when the right side of the brain is stimulated the movements occur on the left side of the body. When the explanation of this is sought, it is found that the motor fibres from the left side of the cere bral surface, after passing through the brain, reach the upper part of the spinal cord—the medulla oblongata— and there cross to the opposite side of the cord. The left side of the brain thus sends motor impulses to the right side of the body, and the right side of the brain to the left side of the body. Destruction of these areas, as has already been stated, causes loss of power of voluntary movement in the particular parts of the body they con trol, but, because of this crossing of fibres, destruction of the surface of the left aide of the brain will cause paralysis on the right side of the body. These discoveries have been of infinite value in the treatment of disease. For, owing to them, in certain cases of paralysis of particular parts of the body or of convulsive movements of particular muscles, it is now frequently possible to determine the exact site in the brain of the disturbing cause, and surgeons have thus been enabled to open into the part of the brain indicated and successfully remove the growth, splinter of bone, or collec tion of matter causing the disturbance.
From these motor areas the efferent fibres pass towards the base of the brain through the corpus striatum, thence through the crura cerebri and pons Varolii to the medulla oblongata. (These parts are described on p. 134.) When these fibres reach the medulla they form a well-marked strand, called the pyramidal tract. From each side of the brain
there is a pyramidal tract. In the lower part of the medulla the tracts from each side cross one another, interlacing as they do so, so that the pyramidal tract from the right side of the brain crosses to the left side of the cord, and vice versa. This crossing is called the decussation of the pyramids. After crossing, the fibres pass down in the lateral column of the cord, forming the column of the crossed pyramidal tract (n of Fig. 96). Subsequently these fibres become connected with the cells of the anterior horn of the gray matter of the cord, and then from these cells other fibres pass off to the muscles. Thus when one wills to move the right leg, an im pulse starts in an area on the surface of the left side of the brain, passes through the brain to the base, reaches the medulla, crosses there to the right side of the cord, down which it passes along fibres of the lateral column, till it reaches the lumbar enlargement of the cord. 'There it enters nerve-cells of the right anterior cornu of the gray matter, which it excites. In conse quence, the cells discharge energy along fibres of the anterior root, down the nerves passing to the muscles of the right leg, which there upon contract, and the leg is moved. All the fibres of the pyramidal tract do not pass from one side to the other in the medulla; a few fibres pass down the cord on the same side, but ultimately cross in the cord. These fibres form a strand in the cord called the uncrossed or direct pyramidal tract, A in Fig. 96. It is not so easy to trace the course of sensory impulses up the cord to the brain. But it is known that such impulses pass up to the medulla, where they may reach nerve-cells, from which by other afferent fibres they may pass to the cere bellum or through the pons and crura to the cerebral hemispheres. In some part of their course sensory impressions cross like motor impulses from one side to another. Experi ments have been directed to discover where the centres for the perception of sensations are situated in the brain, and the results are indi cated in the diagrams (Figs. 97, 98). It is sufficient to say that though the areas in the brain for the conscious perception of sensory impulses have not been so accurately defined as the motor areas, the evidence is conclusive that definite groups of nerve-cells in the cere brum are set apart for receiving all the varie ties of sensations, and that if any of these areas be destroyed the power of perceiving the par ticular sensation is lost.