There are certain abnormal mental states that deserve a passing notice. The chief physical change in sleep is a large reduction of blood in the brain. Its external features are the suppression of voluntary action and of the action of the senses. There may always remain, and there certainly often remains, the play of the imagination known as dreaming. The mental action seems to be sympathetic with the bodily state, and to be attended with very little control. While complete sleep involves the large arrest of voluntary life incident to muscular repose, there are many partial forms of it. The senses may remain cognizant of very many events; a slight uneasiness or a gentle push may call forth a change of position. "Words may be spoken; or, more rarely, words may be listened to and answered, if introduced in the line of existing impressions.