(_a_) Muscular atrophy due to lesions of some part of the nervous apparatus (thus excluding directly traumatic or inflammatory atrophy), as seen in the course of progressive muscular atrophy, of various forms of poliomyelitis, of lead paralysis, of neuritis, and injuries of nerves. These muscular atrophies are characterized by reduction in the size of the affected muscles, by altered electrical reactions, and by histological {55} changes. Reduction in size may affect one muscle or a muscular group shortly after paralysis (as in poliomyelitis, lead paralysis, affections of nerve-trunks), or without evident paralysis it may involve only small fasciculi of a muscle at a time (as in progressive muscular atrophy); so that we speak of diffused and fascicular atrophy, of secondary (post-paralytic) and primary atrophy. In some cases the wasting proceeds to such a degree that apparently no muscular tissue remains, and the skin seems to rest directly on the bones; in others the reduction in bulk is moderate, and is compensated by increase in the intra- and extra-muscular fat (thus in stout children the atrophy of severe poliomyelitis may be quite masked to the eye); in still another category the increase in connective tissue with fatty accumulation overbalances the muscular wasting, and we have the pseudo-hypertrophic condition. Alterations in electro-muscular contractility are most important signs of impaired muscular innervation and nutrition. They present numerous and somewhat complicated variations of value for diagnosis and prognosis, but which cannot here be noticed at length. The general principle underlying these variations is this: that a muscle deprived of its normal innervation (centrifugal or motor) soon loses its capacity to respond to the induced (faradic) current, and reacts in an abnormal way to the galvanic current. This constitutes the reaction of degeneration of Erb.[6] The chief abnormality in the galvanic reaction of a paralyzed and diffusely atrophied muscle is its slow or wave-like contraction, which on a graphic, or even to the observer's eye, presents a striking contrast to the sharp, jerking, complete contraction of a healthy muscle. Besides, the reactions to the two poles become equal, or even reversed, so that we may obtain in testing by the polar method such formulas as CaCC = AnCC, or CaCC < AnCC. With reference to the failure of atrophied muscles to respond to the induced or faradic current, this qualification must be added: If the poles are applied on the moistened skin, as usual in testing or treating cases, no reaction occurs even with most intense currents; but if a needle be made one pole of the battery and inserted into the affected muscle, slight and limited contractions may be obtained with a moderate current for many months. Of this we have repeatedly satisfied ourselves in cases of poliomyelitis, lead paralysis, and traumatic peripheral paralysis. In