Delirium is a condition in which there is perversion of the mental processes. In its essential nature this symptom is analogous to insanity; insanity is, in short, a similar state, unaccompanied by the obvious conditions which are the causes of delirium.
In health the mental processes correspond to present sensory impressions or to the memory of those which are past, but in delirium this correspondence ceases, and the results of cerebral activity bear no true relation to reality. Delirium has three well-known mental phenomena, any or all of which may be present in any indi- vidual instance. The mind may be possessed by false ideas or delusions; sensory impressions may produce false perceptions or illusions; or there may be fictitious perceptions or hallucinations, without the presence of any sensory impressions.
The more common causes of delirium are four. (1) Local diseases of the brain or its envelopes, as in the case of inflammation of the lining membranes. (2) Toxic substances circulating in the blood, which may have their origin within the system, as in the retention of waste products during the final stages of kidney disease, or may be introduced from without, such as the specific poisons of the acute infectious diseases, or active substances like alcohol. (3) High body temperatures, which may occur apart from any blood-poison in a local inflammation of some distant organ. (4) Inanition, which may often be seen in the concluding periods of wasting diseases.