This is usually the result of arrest of function. It may be due to transverse section of the nerve, as in surgical neurectomy when the separated peripheral end of the nerve gradually wastes. It may come from contused wounds implicating the nerve and causing destruction of its substance. It may be from tumors or other neoplasms pressing on the trunk of the nerve and preventing the passage of nerve currents. Or, inflammatory effusion may press on the nerve, as happens often to the crural in hæmoglobinuria. Or the pressure may come from enlarged mediastinal glands, or even from the distended posterior aorta under habitual violent exertion so as to permanently incapacitate and atrophy the left recurrent laryngeal nerve as in chronic laryngeal paralysis (roaring). Similar wasting occurs in other nerves under corresponding conditions. Atrophy may, however, extend centrally from the peripheral end of a nerve when it can no longer remain functionally active. We find an example of this in the atrophy of the optic nerve up to the commissure when the eyeball has been excised. A similar condition is often seen in horses in which the integrity of the eye has been completely destroyed in connection with recurring ophthalmia. The _symptoms_ attendant on atrophy of a nerve are those of impaired function gradually advancing to complete paralysis of motion or sensation. In cases of a complete breach of continuity as in section or severe traumatism the entire loss of function necessarily precedes the atrophy. Again, when it comes from destructive changes in the coats and media of the eye, and of the ganglionic cells of the retina, the atrophy of the nerve trunk proceeds simultaneously with the lesions of the organ of vision. The _diagnosis_ will in many cases be easy as deduced from the traumatic or surgical lesion. In other cases it may be made with certainty from the complete muscular paralysis, wasting and degeneration of the muscles supplied by the nerve, and by the history of the case (hæmoglobinuria in atrophy of the triceps extensor cruris, roaring in atrophy of the laryngeal muscles and recurrent nerve). In other cases, as in the eye, we have the atrophy of the eyeball, the distortion or complete paralysis of the iris, the opacity of the lens, or the exudation into the vitreous, choroid and retina when these can still be observed. _Prognosis_ will depend on the cause. With a nerve severed with a knife or crushed in a part of its course and atrophied, without destructive changes in the organs in which it is distributed, repair is possible and to be expected in time. _Treatment_ is expectant, yet inflammation must be subdued, tumors removed, divided ends ligatured, etc. DISEASES OF THE URINARY ORGANS.