Consciousness.-This in a purely mental state, partly consisting of perceptions, and partly inseparably associated with the emotions, the intellect, and the will. Consciousness depends on the perfectness of the whole sensory apparatus, but from a practical point of view it may be considered to reside in the cerebral part of the same, i.e. in the convolutions, where it is readily reached by the therapeutist.

Sleep. - We cannot account perfectly for natural sleep, but we are probably right in associating it with diminished metabolism of grey matter, whether due to deficient blood supply, to impaired quality of blood, or to the molecular inactivity of the tissues following exhaustion. Sleep bears a definite relation to work, food, and the time of the day, and brings rest and refreshment to the exhausted system.

II. Pharmacodynamics

When we come to consider how far the nervous system is under our influence, we enter upon a field of enormous proportions, of which we can take but a few examples.

1. Sensation

Sensation. We have a remarkable power over both common sensibility and the special senses, increasing or diminishing their activity at our pleasure, by means respectively of local stimulants and local anaesthetics.

a. Local stimulants.-This name is given to a great and mixed group of agents, which increase common sensibility or common sensation so much as to cause pain. The majority of them act directly upon the nerve fibrils in the tissues, such as extreme heat, extreme cold (for a time), faradic electricity, and many drugs, including: Iodine and Bromine; Alcohol, Ether, and Chloroform, when the vapour is confined; Carbolic Acid and Creasote; volatile oils, e.g. Turpentine, Cajuput, Menthol, Thymol; acrid essential oils, e.g. Mustard and Mezereon, and Cantharides in the first stage. Mineral Acids and Ammonia; Metallic salts, such as those of Silver, Lead, Zinc, Antimony, Mercury, Arsenic, and Copper, also stimulate the nerves and cause severe pain, but not when supplied in sufficient strength to interfere markedly with the vessels and protoplasm of the part as caustics or astringents. Possibly some local stimulants act primarily upon the vessels, and many of them no doubt excite the circulation as well as the nerves. It must be carefully noted that the effect of local irritation on the sensory apparatus is really a central one. The sensation of pain, although it may be referred to the periphery, is a cerebral state. It therefore affords us a means of rousing the highest centres. What is even more important therapeutically, the whole of the impression conveyed from the irritated spot does not become converted into a painful sensation or act of consciousness. A portion of it, whilst traversing the grey matter of the spinal and medullary centres en route, disturbs these and causes reflex impulses, which rouse the muscles and viscera. In this way sensory, and especially painful impressions are powerful and readily available means of stimulating not only consciousness but the cardiac, vaso-motor, and respiratory centres, and through them the great viscera themselves. Thus the cold douche produces a sensation of cold referred to the part, rouses consciousness and so excites the respiratory centre as to cause the gasping movements of breathing familiar under the circumstances. In other words, local stimulants may become powerful general stimulants.

b. Local Anaesthetics.-Pursuing an exactly opposite line of action, we can readily diminish the sensibility of the origins of nerves until their power of receiving impressions is lost, and thus remove sensations by preventing the very contact of the influence with the nervous system. The measures which have this effect are called local anaesthetics (II Pharmacodynamics 10 without, and II Pharmacodynamics 11 sensibility), or if pain be relieved, local anodynes (II Pharmacodynamics 12 without, and II Pharmacodynamics 13 pain). Some of these agents directly depress the nerve fibrils, such as Belladonna, Aconite, Veratria, and Opium; and Ether, Alcohol, Chloroform, Carbolic Acid, Volatile Oils, and Cantharides, when their application is prolonged. Moderate cold, especially such as is induced by evaporation, is decidedly anaesthetic; and Ether, Spirits, Acetic Acid, Water, and various Saline solutions, e.g. of Chloride of Ammonium, possess this property. Prolonged or extreme cold directly reduces the functions of the nerves, causing first numbness, and then absolute anaesthesia. Warmth reduces, and extreme heat destroys, the irritability of the nerves. Other anodynes act partly or wholly through the vessels. Thus moderate heat relieves pain partly by dilating and relieving the blood vessels, and by increasing the blood-supply, the osmosis, and the migration of corpuscles in the tissues-an effect which is assisted by moisture, as familiarly seen in poultices. Cold partly acts by reducing excessive blood supply. The galvanic form of electricity often removes pain very quickly, probably by acting on the nerves, muscles, vessels, and even the metabolism of the part.

The influence of local anaesthetics and anodynes is not confined to the sensorium. With the arrest of sensation, the whole brain passes into a state of rest, and sleep readily occurs. The in-travelling impressions being reduced in strength, the spinal and medullary centres through which they pass, or into which they previously radiated, are no longer excited, and the action of the organs, such as the lungs and heart, becomes more automatic, and, as a rule, but not invariably, more quiet. Thus, as with local irritants, we possess in local anaesthetics and anodynes, a powerful means of influencing the functions of the highest centres, the visceral centres, and the viscera themselves. In other words, local sedatives may become powerful general sedatives.