As a general rule, unless a current that causes energetic and painful action in the muscles of the ball of the thumb produces some contraction, irritability is impaired. If, in our examination of the muscle, we find irritability normal to both currents, we have proof of the integrity of the muscular tissue. If the muscle also responds by contraction to electrization of its nerve, we have further proof that the nerve is healthy, and also the spinal cord at the point of origin of the nerve. [SN: Electro-Irritability diminished.] If we find the irritability lessened, there is disease of either muscular tissue, nerve or cord; and, as a rule, this will be in direct proportion to the amount of diminished irritability. [SN: Electro-Irritability increased.] Increased irritability points to increased vascularity or irritative lesion; but in such cases we very seldom require the aid of electricity to complete our diagnosis. [SN: Voltao-Irritability increased.] The reactions of Faradism and Voltaism are usually equal, but in some cases in which muscular response to Faradism is diminished or abolished the muscular reaction to an interrupted Voltaic current is not only preserved but increased. The diseased muscle will respond much more readily to the Voltaic current than the corresponding muscle of the healthy side; while at the same time it will be found on examining the nerve that there is equal diminution to both currents. [SN: Diagnosis of Peripheral from Central Disease.] This increased muscle reaction is due to the special irritability of muscular tissue, and is quite independent of any nerve influence. It exists only in peripheral and never in central lesion, and by it we obtain an absolute diagnosis of such cases as local paralysis of the facial nerve from facial hemiplegia, paralysis of the extensors of the wrist and fingers, due to lead poisoning, from commencing muscular atrophy, or progressive muscular atrophy from paralysis from section of a nerve--all important questions as affecting treatment. [SN: “The Degenerative Nerve Reaction.”] It is known as the “degenerative nerve reaction,” as it always follows when a nerve trunk is involved in the lesion. There is then diminution of Faradaic irritability through both nerve and muscle, diminution of reaction to the Voltaic current when the rheophores are applied over the nerve, and increase of reaction when they are applied over the muscle; but there is no relation between these interesting variations in the electrical condition of nerve and muscle, and the degree of paralysis to the will, for there may be perfect muscular paralysis, as in hemiplegia, when the electrical condition of both nerve and muscle is unchanged; but, on the other hand, in muscles equally paralyzed we may find absolute electrical changes in some, and none in others. We may often positively predict that these latter will rapidly recover--one instance of the use of electricity in prognosis.