2. _Affections of the nerves of common sensation._--Chief among the causes of sleeplessness thus produced is pain. This is a modification of feeling, caused by excessive or extraordinary excitement of the peripheral nerves of sensation. The seat of the excitement may be in the skin or in the deeper tissues of the body. Cutaneous pain may be caused by the activity of various insects, like flies, mosquitos, fleas and bedbugs, or by the presence of certain parasites, such as the itch-mite, or by ordinary diseases of the skin, of which notable examples are found in erysipelas, erythema, urticaria, lichen, prurigo, certain varieties of eczema and psoriasis. The troublesome forms of pruritus which accompany icterus, or which may occur without any clearly defined cause, are frequent causes of wakefulness. The last mentioned disorder must, however, be sometimes recognized as a consequence of central nervous disorder, rather than a result of peripheral disease. Witness the frightful itching sometimes experienced during the progress of chronic myelitis. All kinds of injuries, wounds, ulcers, and other local inflammations are common causes of insomnia by reason of the painful impressions transmitted from them to the brain. Hence the great importance of anodynes and hypnotic remedies in the course of surgical practice. Diseases or injuries of the various peripheral nerves are notable causes of sleeplessness. Witness the horrible wakefulness caused by neuritis and by neuralgia. The development of neuromata in the stumps of amputated limbs may thus become a most painful cause of insomnia. Inflammations which encroach upon sensitive nerves produce intense pain with consequent loss of sleep. Of this very conspicuous examples are furnished by spinal meningitis, and by the effects of local periostitis causing compression of the branches of the fifth pair of nerves.