Morbid Anatomy.—The lesion is limited to the spinal cord, the brain being unaffected. An inflammatory process attacks the anterior cornua and produces certain changes in the gray matter itself, in the roots of the nerves which take their origin in this situation, and in the muscles, tendons, bones, and joints to which they are distributed.
In the gray matter the changes are not appreciable by the naked eye, except that in old standing cases a certain diminution in bulk, with increased consistence of the affected parts, can be sometimes detected. By careful microscopic examination, however, the changes can be distinctly recognised.
The inflammatory process is diffused through the gray matter forming the anterior horns ; but is more intense at certain points, notably the cervical and lumbar enlargements. As a consequence, areas of softening can be seen, more or less sharply defined, seated towards the front of one or both cornua. In these areas the tissue is soft and friable, the blood vessels are fuller than natural, and numerous granulation cells are seen with an. increase in the amount of connective tissue. The most striking change consists, however, in the fact that the large ganglion cells have almost completely disappeared, and the few which are left are greatly atrophied and degenerated. The nerve fibres and axis cylinders are also destroyed, and the anterior roots are degenerated and wasted. As a consequence of these changes the anterior horns look small and shrunken at the spots where these diseased foci are situated. Although the diseased process is thus concentrated in certain patches, the gray substance generally is not com pletely healthy. Throughout the whole dorsal portion of the cord the gray matter is often more or less affected. Granulation cells may be seen to be scattered through the tissue ; the nuclei are multiplied ; the blood vessels are dilated and ganglion cells here and there have disappeared.
The above changes constitute the first stage—that of active inflamma tion. As the acute process subsides improvement takes place in parts where the gray matter has not undergone entire destruction. But in other regions, where the disintegrating process has been complete, further changes ensue. These consist in a more extreme wasting and shrinking of the anterior horns, so that the diminution in bulk becomes visible to the naked eye. The disease is most marked in the cervical and lumbar
enlargements. In the affected areas there is complete destruction of all nerve fibres and ganglion cells. Even if a few are left, they are degener ated and shrivelled. The area becomes filled with a fine fibroid connec tive tissue, rich in nuclei, and the blood-vessels are hypertrophied. Even the anterior white columns become more or less degenerated. Their neuroglia is thickened, their nerve fibres are atrophied, and the develop ment of the columns is retarded, so that they look small and narrow. This is, however, probably a secondary affection, and is not necessary for the complete development of the symptoms. Stated briefly, the lesion which constitutes infantile paralysis may be said to be an acute myelitis of the anterior gray cornua, leading to circumscribed patches of sclerosis with complete destruction of the large ganglion cells and other nerve elements.
The changes which have been described supply an explanation of the peculiar phenomena observed in the disease. The striking limitation of the paralysis to certain muscles, or groups of muscles, and the complete immunity of others, is due to the concentration of the lesion into certain circumscribed areas ; while the early resolution of the inflammation in the larger portion of the tissue attacked accounts for the disappearance of the first severe symptoins, and the restitution of power in many of the muscles primarily affected.
The paralysed muscles also undergo atrophy and degeneration. They become at first paler and softer, then grayish or reddish yellow, with bands of connective tissue, and yellow lines or streaks of fatty tissue. The micro scope shows at different stages the fibres wasted, and their striation indis tinct, with hyperplasia of the cells of the sarcolemma ; then the fibres cloudy with numerous fat molecules ; finally, almost complete absence of muscular fibre. The normal structure is often replaced by an increased formation of connective tissue, so that what was once a muscle becomes a mere fibrous bundle ; in other cases we find substitution of the normal muscular substance by adipose tissue, and by this means the original volume of the muscle may be actually increased.