These are remedies which induce sleep. Although many of them are also narcotic, yet we may distinguish between hypnotics and narcotics. Pure hypnotics are substances which in the doses necessary to produce sleep do not disturb the normal relationship of the mental faculties to the external world.

In sleep the cerebro-spinal system, with the exception of the medulla oblongata, is to a great extent functionally inactive, and even the respiratory centre and the vaso-motor centre in the medulla, undergo a diminution in their functional activity, so that the respiration becomes slower, the vessels of the surface dilate, and the arterial tension falls.

Certain parts of the nervous system may still remain functionally active, so that, for example, when the nose is tickled with a hair, reflex movements of the face or hand may occur without awakening the sleeper; and certain parts of the brain may also be active so that dreams occur, which may be afterwards remembered as distinctly as real occurrences, or may produce at the time various movements of the body.

But while individual parts may be active, the whole cerebrospinal system is not active together, and thus any co-ordination which may occur between either sensations or motions is incomplete; the dreams are incoherent, and the motions do not affect the whole body, as is seen in sleeping dogs, where the legs make a movement of running, but the animal continues to lie on its side. The functional inactivity of the whole or of the greater part of the cerebrospinal system is associated with a condition of anaemia, and probably depends to a certain extent upon it. At the same time it is probable that sleep depends also on functional inactivity of the cerebral cells due to accumulation of the products of tissue-waste in or around them.

The arteries of the brain during sleep are contracted, the brain is anaemic, and its bulk is small. On awakening, the arteries become dilated, the circulation becomes rapid, and the brain increases in bulk. Where parts of the brain are active, as in dreaming, increased circulation occurs, but probably this is local and not general.

In considering the circulation of the brain, however, a marked distinction must be drawn between the condition of the arteries and veins. So long as the blood is in the arteries it is available for the nutrition of the nervous structures; but once it is in the veins it is no longer available, and its accumulation there will tend to impair nutrition, both by the pressure it exerts on the nervous structures, and by its interference with the supply of arterial blood.

In normal sleep the arteries and veins are both contracted, and the brain appears anaemic. In the very act of waking the brain may slightly contract, and this has been thought by Mosso, to whom we owe the observation, to show that sleep does not depend upon anaemia of the brain; but this contraction may be due to the removal of venous blood, preparatory to further arterial supply.

Observations on the brain by trephining appear to show that during ordinary sleep, whether it has come on naturally, or has been induced by narcotics, such as a small dose of opium, the brain is anaemic. During functional activity, either of the whole or of its parts, there is arterial dilatation, with a free supply of blood. During coma the veins become dilated and the brain congested.1 This congestion, however, is utterly different from the arterial congestion of functional activity, for in coma the blood, though abundant in quantity, is stagnating in the veins, and useless for the tissues.

In order to produce sleep, then, two things are necessary :1st. To lessen the circulation in the brain as much as possible by diverting blood from it or quieting cardiac action.

2nd. To lessen the functional activity of the organ.

Blood may be diverted from the brain by dilating the vessels elsewhere. In weak conditions of the body, with feeble vascular tone, this may occur simply from position, and such persons become drowsy when standing or walking about, or when sitting. As soon as they lie down, however, the cerebral vessels having little or no tone, the blood floods the brain, and they are unable to sleep. In such persons, sleep may be sometimes obtained by raising the head with high pillows. In such cases, also, vascular tonics, such as digitalis, by increasing the contractile power of the arteries leading to the brain, may enable them to resist the increased pressure in the recumbent position, and thus prevent the brain being flooded with blood and allow sleep to be obtained.

1 Hammond, On Wakefulness, 1866, p. 20.

Fig. 72. - Tracings from the brain of a dog after trephining, showing the influence of position on the cerebral circulation. In the upper tracing the vertical line shows when the head of the dog was lowered, and in the lower tracing when the head was raised. (Salathe.)1

The largest vascular area into which the blood may be drawn away from the brain is that of the intestinal canal. When the vessels in the intestine are contracted, it is almost impossible to obtain sleep. Consequently both man and animals, when exposed to cold, which acting through the thin abdominal walls would cause contraction of the intestinal vessels and drive the blood to the brain, instinctively keep the intestines warm by curling themselves up before going to sleep, and thus covering the abdomen with the thick muscles of the thighs.

Warmth to the abdomen by means of a large poultice outside will also tend to produce sleep; or, in place of a poultice, a wet compress, consisting of linen or flannel wrung out of cold water, and covered with oil-silk, and with two thicknesses of dry flannel placed above it, tends greatly to induce sleep and is most useful for this purpose, especially in children.

Warmth to the interior of the stomach has a somewhat similar action, but it differs from warmth to the exterior in this, that it may, to a certain extent, stimulate the heart as well as dilate the abdominal vessels. Stimulation of the heart is of course objectionable, as it tends to maintain the activity of the brain.

On this account the food or drink should be tolerably warm, but not very hot. Warm milk, either alone, or with bread soaked in it, warm gruel, thin corn-flour, or ground rice, sago, or tapioca, warm beef-tea or soup, or a glass of hot wine and water or spirits and water at bed-time, may all act as soporifics by withdrawing the blood from the brain to the stomach. In the sleeplessness of fever a wet pack, by restraining the movements and by diverting blood from the brain to the body generally, is often an efficient soporific.

1 Marey's Travaux, 1876, p. 397.

Cold feet also tend to keep up the tension in the vessels and prevent sleep, and therefore they ought to be warmed either by the use of an india-rubber bag filled with hot water, and covered with flannel, or by rubbing them briskly in cold water and drying them thoroughly before going to bed, or by both means combined.

Cardiac excitement may be lessened by sedatives, one of the most useful of which is cold. After hours of weary tossing sleep may sometimes be induced by walking about in a nightdress until cool, or by sponging the surface either with cold or hot water.

The chief hypnotics or soporifics are Opium. Morphine. Chloral-hydrate. Butyl-chloral-hydrate (crotonchloral). Hyoscyamus. Cannabis. Paraldehyde. Urethane.

Hypnone.

Bromide of potassium.

Bromide of sodium.

Bromide of calcium.

Bromide of zinc.

Monobromo-camphor.

Hop.

Lettuce.

Lactic acid.

The most powerful hypnotics that we possess are undoubtedly opium and morphine, and they seem to act by depressing the functional activity of the brain itself, although along with this depression an anaemic condition of the organ sets in. Besides their action in producing sleep, even in health opium and morphine have the power of lessening pain and thus removing the effect which painful stimuli have in maintaining a wakeful condition. Bromide of potassium and bromide of ammonium in large doses have also a hypnotic action, and even in smaller doses, when they would not of themselves produce sleep, they appear to lessen cerebral excitement, and allow sleep to come on when other conditions are favourable. Chloral probably causes sleep both by acting on the brain itself and by causing dilatation of the vessels generally. It is therefore a useful hypnotic in persons suffering from Bright's disease, in which there is high tension of the vessels and consequently a tendency to sleeplessness.

A combination of hypnotics sometimes answers much better than any one singly. Thus morphine or opium alone sometimes simply cause excitement; but when chloral is given, either along with, or after them, the excitement is quieted and sleep occurs.

A combination also of small quantities, such as five or ten minims, of solution of opium or morphine with five grains of chloral and ten to thirty of bromide of potassium, is sometimes more useful than any one of the three used alone.

Indian hemp also is sometimes used to procure sleep, and lettuce and lactucarium are also said to have a hypnotic action. Lettuce certainly does seem to have such an action, but how much of it depends upon the juice and how much upon the mechanical effect of the indigestible fibres of the lettuce upon the stomach in drawing blood to it, it would be hard to say. Hops are said to be hypnotic, and their combination with lettuce in the form of a supper consisting chiefly of beer and salad has sometimes a very marked soporific action.