(_c_) Under the head of degenerative atrophies should be included the secondary changes which affect certain nerve-tracts within the nervous system itself. It is impossible in this introduction to treat fully of this interesting category of trophic changes; the following summary must suffice: (1) When the cerebral motor cortex or any part of the associated pyramidal (or motor) tract is destructively injured, there occurs, in from three to six weeks, a degeneration of the whole tract caudad of (below) the brain. The myeline becomes granular and disappears, the cylinder-axes are broken up and vanish, the connective tissue (neuroglia) increases in amount; the atrophied and degenerated area appears in a transection rather translucent in contrast to the pearly white of the normal medullary tissue, and when the preparation is stained with carmine the patch takes up an abnormal amount of pigment. This is the so-called descending degeneration, or centrifugal atrophy of the central nervous system. (2) After total transverse lesions of the spinal cord, besides the above-described centrifugal degeneration caudad of the lesion, we observe frontad of (above) it similar changes in the posterior median columns, and in the direct cerebellar tracts—centripetal degeneration. At present we have no knowledge of centripetal (ascending) degenerations in the cerebrum and in nerve-trunks; and if the results of von Gudden's experimental method be cited against this statement, it must be replied that its effects are best seen in newly-born animals, and that its pathology is yet unknown. (3) Lesions of the anterior gray matter (ventral cornua) of the spinal cord, and of mixed nerve-trunks produce only centrifugal or descending degeneration. All the nerve-fibres deriving their innervation from the injured area in the cord, or in case of nerves all fibres below the injury, perish—_i.e._ their myeline breaks up and undergoes granular and fatty degeneration, their cylinder-axes are segmented and disappear, while at the same time the connective tissue of the nerve becomes abnormally active and increases. Furthermore, in cases of this category there are, inevitably, degenerative and atrophic changes in the attached muscles, and peculiar electro-muscular reactions (vide supra). All these central and peripheral nervous degenerations, due to a local lesion, are conveniently grouped under the name of Wallerian degeneration.