External

It is a powerful antiseptic. Locally applied it is irritant, causing vesication. Chloral rubbed up with equal parts of menthol and camphor is a local anodyne.

Internal

Alimentary canal. - Unless diluted, chloral is a gastric irritant; large doses, therefore, may give rise to vomiting and purging.

Blood

It is readily absorbed, and circulates in the blood unchanged. It was formerly thought that as alkalies convert it into chloroform and formic acid, this change would take place in the blood, and consequently Liebreich suggested its use as an hypnotic. It is now known that this view is wrong, for no chloroform can be found in the blood of chloralized animals, nor in the urine unless that fluid is alkaline, in which case chloral is decomposed by the alkali in the urine.

Circulation

Chloral depresses the heart, large doses having this action to a considerable degree. This is due to a local effect on the organ itself; probably both the muscular substance and the nerves contained in it are affected. The pulse, which may at first be slightly quickened, soon becomes slow, feeble, and irregular, and the heart finally stops in diastole. The vaso-motor centre is depressed, and consequently the vessels dilate. As a result of these actions on the heart and the vessels the blood-pressure falls.

Respiration

After large doses the respirations become slow and full, and after toxic doses they become irregular and shallow before finally ceasing. This is due to the action of chloral on the respiratory centre.

Temperature

Large doses cause this to fall, probably by diminishing the production of heat.

Brain

Chloral is a powerful hypnotic, acting directly on the brain. The stage of excitation, if it exists, is very short. Soon after taking a moderate dose the patient is overcome by sleep, which lasts several hours, and is indistinguishable from natural sleep. On waking there is neither confusion nor headache, and he feels refreshed. Large doses produce coma. The pupil is always contracted.

Spinal Cord

At first the anterior cornua may be slightly stimulated, but soon they are depressed, and there is consequently paralysis and loss of reflex excitability. The motor nerves and the muscles are not affected, nor are the sensory nerves unless the dose is very large, when there may be anaesthesia.

It will be observed that chloral is a powerful general depressant, chiefly of the cerebrum, but also of the respiratory centre, the vaso-motor centre, the anterior cornua, of the production of heat, and the heart. It is only because it depresses the cerebrum much earlier than any other part of the body that we can use it as an hypnotic.