The symptoms produced by irritation of nervous organs are usually hyperkineses or paræsthesiæ in their various forms. The symptoms, spasm, pain, or numbness, are usually, though by no means always, intermittent. That paralysis and anæsthesia may occur from irritation, by an inhibitory process, is not to be denied, yet we must maintain the extreme rarity of such a mechanism. The phenomena of inhibition are so prominent in the normal nervous system that a physician who looks at nervous diseases from the standpoint of the physiologist may well be excused for seeing morbid inhibitory processes where others do not. The symptoms due to destructive lesions—_i.e._ those indicating destruction of parts of the nervous system—are paralysis, hyperkinesis, and anæsthesia, of absolutely or relatively constant presence. Thus, for example, the paralysis of common hemiplegia due to destruction of the motor part of the internal capsule is constant and permanent, while the accompanying tonic spasm (late contracture), which is considered an even more positive sign of serious destructive injury to the cerebral motor tract, is permanent, but intermittent—_i.e._ it is absent in profound sleep, and reappears as soon as the patient awakes. The association of symptoms of irritation and those of destruction is frequent but variable. In many cases, as in cerebral tremor and posterior spinal sclerosis, spasm and paræsthesiæ precede paralysis, anæsthesia, and ataxia. In other cases (in many at a certain period) they coexist. In a small group of cases the irritative symptoms follow those indicating destruction, sometimes occurring years afterward, as in post-hemiplegic epilepsy and neuralgia due to cicatrices. We would repeat that very often, more especially in organic nervous affections, much light is thrown on the diagnosis by careful noting of the topographic distribution and chronological order of appearance of the symptoms. In the second place, it is necessary to group the symptoms of disease of the nervous system in two great classes—viz. those representing demonstrable lesions, macro- or microscopic, and those dependent upon perverted functions or molecular malnutrition of the nervous organs or elements. The first group is designated as organic diseases; the second as functional affections. Mental diseases, so called, can also be classified, according to their symptoms, in either of these groups. The symptoms of the first group, that of organic nervous diseases, are characterized by definiteness of distribution, by permanency, by relative invariability, and by the predominance of objective signs. Another important characteristic of organic diseases is their progressive or fatal {61} tendency, either with reference to life in general or to that of parts or organs. A third peculiarity of these diseases is that they do not occupy the patient's attention as strongly or as constantly as neuroses: in other words, the Ego is less involved.