As to the portions of the cord in which impulses are conducted downwards front the brain, and upwards to the brain, to put it broadly, motor impulses descend from the brain in the anterior and lateral columns of white matter, the white matter consisting of nerve-fibres running longitudinally in the cord, while sensory impulses pass upwards in the posterior columns. In the figure the shaded parts (A) bounding the anterior median fissure and the shaded part (n) in the lateral column are the tracts down which motor impulses descend from the brain, while the shaded parts (x) bounding the posterior median fissure, and the unshaded portions (n) on both sides be tween the fissure and the posterior roots, are the tracts up which sensory impulses pass to the brain, leading to sensation. If the anterior portion of the cord be cut through on one side, the parts below the section on the same side are paralysed as regards motion, though sensation is not disturbed ; while if the cut involved the whole anterior portion of the cord the body would be paralysed on both sides below the level of the section. To put it in another way, motor impulses from the brain to the one side of the body, with certain ex ceptions that need not be further noticed, pass clown the antero-lateral columns of the cord on the side to which they are distributed. As to the tracts of sensory conduction the facts are not so clear. If a posterior root (that is a tract by which sensory impulses enter to pass up the cord) be divided inside the ganglion of the posterior root, the part of the root connected with the cord degenerates, and the degenera tion can be traced in the posterior columns right up the cord to the medulla, where it ceases, and always on the same side of the cord, never crossing to the opposite side. This would lead to the opinion that sensory impres sions coming from one side of the body passed up the posterior column of the same side of the cord. On the other hand, cases of disease involving the posterior portion of half of the cord have been numerous in which sensation was diminished on the opposite side of the body. Thus injury or disease limited to one half of the cord, but involving both anterior and posterior portions, produces loss of motion on the same side of the body, and loss or at least diminution of sensation on the opposite side. This fact suggests that sensory impulses, entering the cord from one side of the body, soon after entrance cross and ascend to the brain on the opposite side. It may be noted that there are different sensations coming from the skin, sensations of touch merely, or of pressure, or of heat and cold, or of pain, and that from muscles there is a sensation of re sistance, which is called the muscle sense.
Certain facts seem to indicate that these various sensations. pass upwards in the cord to the brain along different nerve-fibres run fling in the posterior columns, so that disease affecting limited areas of the spinal cord may destroy the conducting paths for one sensation, leaving the other tracts intact. For example, limited disease might destroy the conducting paths for sensations of pain, leaving intact the tracts along which sensations of touch, pres sure, temperature, and muscular resistance pass. In such a case a pin thrust deeply into the skin would not give rise to a sensation of pain, but merely to one of contact.
a whole, it has been explained that Lefore one can become conscious of an impression reach ing the spinal cord, it must pass up the cord to a centre for consciousness in the brain, and that the impulse to voluntary movements begins in centres in the brain, and travels downwards to lower centres in the cord, through which the movement is ultimately effected. What is known about the pathways of sensory impulses up the cord to the brain, and of motor impulses down the cord from the brain/ The spinal portion of the pathway has been noted above, we must now note the brain portion, and first the pathway of motor im pulses, for these have been most clearly deter mined. In 1870 two German physiologists, Fritsch and Hitzig, discovered that the stimu lation, by a galvanic current, of certain areas on the surface of the cerebral hemispheres gave rise to certain definite muscular move ments. Since then a great many other ob servers, abroad and in this country; notably Dr. Ferrier of London, have repeated and extended these experiments. The results have been corroborated by the effects of disease, and a summary of the accepted facts will now be given.
Fig. 97 is a diagrammatic view of the left side of the brain of the monkey. A marked fissure, the fissure of Rolando, dips deeply into the brain substance, running downwards from the middle line above. Stim ulation of small areas in the convolution in front of this fissure, and in the one behind, produce certain definite movements, and the diagram itself indicates the parts that are moved. Destruction of these areas leads to loss of power of voluntary movement of the specified parts, and from the destroyed area degeneration can be traced downwards through the brain and into the cord. That is to say, it is in the cells of these areas that impulses arise, when the person wills to perform the particular movement, and the impulse thus