(_a_) Pain, the most distinct and frequent of paræsthesiæ, is by most authors classed as a hyperæsthesia, yet a careful analysis will show the difference. Pain and hyperæsthesia often coexist and are inseparable, yet in a large proportion of cases of nervous diseases the former sensation occurs independently, sometimes in regions where absolute anæsthesia exists (anæsthesia dolorosa), and even apparently in lost parts (neuralgia after amputations). We are consequently justified in considering most pains as paræsthesiæ. Pain assumes many forms, some real and typical, others as various as the lively imagination of nervous patients can make them. Thus we have sharp, cutting, darting pains in neuralgia, posterior spinal sclerosis, etc.; aching, throbbing, pounding pains in cephalalgia, inflammatory and traumatic conditions; boring, crushing, distending, constricting, burning pains, etc. etc. In some cases the sensation is only semi-painful, and more akin to paræsthesia (neuritis, parenchymatous lesions). (_b_) Numbness, prickling, and formication usually coincide. They may easily be produced experimentally by pressure upon a nerve-trunk or by the exhibition of aconitia, so that any one may study these sensations for himself. By taking one-fiftieth of a grain of Duquesnel's crystallized aconitia the experimenter will soon find himself the possessor of intense subjective sensations of prickling, numbness, vibrations, and cold, lasting several hours. He will be able to satisfy himself that though the finger-tips feel numb, as if there was a coating or layer of something interposed between the skin and objects, he can distinguish tactile perceptions very well. In the more serious experiment of compression of a nerve-trunk a most interesting succession of phenomena will be observed: the first effects of pressure are various paræsthesiæ in the parts supplied by the nerve; then these sensations (prickling, numbness, swelling, vibration, heat, and cold) cease; paralysis and anæsthesia occur. If the compression be now interrupted, after a few moments the paræsthesiæ reappear, more intensely, as a rule, and as they gradually fade a normal state of sensibility is re-established. By making such experiments it is easy to convince one's self that anæsthesia and numbness are different conditions: indeed, during the stage of recovery from nerve-pressure distinct hyperæsthesia may be demonstrated. These results throw much light on the origin and diagnostic value of paræsthesiæ as expressions of irritation, central or neural, of nervous elements. At the same time, in practice, we occasionally meet with slight dulness of tactile sensibility in numb {34} parts. Another point to be remembered is that while patients usually complain loudly of paræsthesiæ, they are sometimes wholly unaware of anæsthesia (hysterical analgesia, for example); therefore sensibility should be tested even if the patient does not mention sensory disturbances.