Symptoms of sensory irritation which do not usually amount to actual pain, or are unlike in kind from the usual feeling experienced from similar stimuli, are grouped under the term paresthesia. They are usually due to abnormal conditions in the nerves themselves and may include one or all of the elementary forms of sensation, defined by such terms as formication, pricking, numbness, burning, etc. True paresthesia is indicative of grave central or peripheral nervous disorders. While the abnormalities of sensation may be said to include a multitude of sensory perversions, in many instances they merge into hyperaesthesia or anesthesia or both. The condition known as dermatalgia is, strictly speaking, a painful paresthesia while pruritus is a form of paresthesia in which itching is the sole symptom. These conditions are treated separately in this book.