Materia Medica is that branch of medical science which refers to and describes the methods and substances known as "medicinal agents," which are employed in the prevention and treatment of disease; also their source or derivation, preparation, composition, and properties.

Therapeutics is that branch of medical science which comprises the doctrine of the management of disease. Generally, however, the term is restricted to a description of the modus operandi of medicines, or, in other words, their use, application, effects, and doses, when applied in the treatment of various morbid conditions.

Pharmacology is the science of the action of medicines, and is expressed by what is termed their "physiological action."

Pharmacy is the art of preparing medicines, and dispensing them by direction of the therapeutist.

An accurate knowledge of the principles and rules which govern the administration and action of medicinal substances enables the practitioner to restore disordered functions, and to so impress the organism as to maintain harmonious conditions, by means of which the various functions, in a state of health, are intimately connected by relation and sympathy.

a destructive influence upon the tissues. Some medicinal agents affect the nervous system, and others are so irritant in their effects, as to cause their speedy expulsion; while others, again, have a particular affinity for certain organs, and are eliminated by them, the effect ceasing as soon as the evacuation is completed. Other medicinal agents prevent septic decomposition, and the growth of micro-organisms.

What are known as topical or external remedies act directly upon the parts to which they are applied, and their general effects are produced through the nervous system.

The methods of treatment which have for their objects the prevention, and relief of pain, and the cure of disease, include preventive or prophylactic treatment, which embraces all hygienic conditions which will obviate any tendency to disease, prevent its extending to others, and the employment of antiseptic agents; for example, in dental practice attention to the hygiene of the oral cavity, which consists in keeping the teeth and associate parts clean, and preventing dental caries, and affections of the oral mucous membrane: also palliative treatment, which affords relief from pain, such as that of odontalgia, neuralgia of dental origin, etc.; also curative treatment, which eradicates the disease completely; for example, in dental practice the proper preparation and filling of carious cavities, the cure of alveolar abscesses, alveolar pyorrhoea, etc.

The indications for treatment will depend upon the nature and location of the affection, and the symptoms present. The condition of vital organs, such as the heart, lungs, and kidneys, also influences the treatment; for example, in the administration of anaesthetics.

Medicines enter the circulation by either external application or internal administration. Externally by such methods as the epidermic, or by inunction - the application of medicines to the skin by rubbing or friction; the en-epidermic, - the application of plasters, poultices, lotions, etc., to the skin; the endermic, - the application of a blister, which is followed by that of a medicine to the raw surface thus produced; the hypodermic, - the injection of medicines into the skin and mucous membrane to obtain a more rapid effect than when administered by the mouth internally. Internally, by the moutb; by inhalation; by the rectum, when it is inadvisible to administer them by the mouth.

The changes which medicines undergo when taken into the system depend upon the condition of the system, the temperature of the body, the food and drink, and the tendency of medicines to combine with other substances and form different compounds. The rapidity with which medicines enter the circulating fluid is governed by their composition. The crystalloid substances pass into the blood very readily, while colloid substances enter slowly, or not at all. Arsenious acid is an example of the crystalloid. Corrosive poisons not only destroy life, but also the parts with which they come in contact, while other poisons destroy life alone. The blood conveys medicines to different parts of the body, and during such a passage they change the character and composition of the blood; and are eliminated from the system by the excretory organs, such as the lungs, kidneys, skin, urinary and salivary organs. The form in which medicines are administered and applied, also modify their effects.

The action of drugs is also modified by pharmaceutical combination, as the joint effect of two medicines is different from that one of them may cause, for opium given with mercury will prevent the purgative action of the mercury. The action of medicines may be direct so as to produce local effects; or it may be general or indirect when the entire body, or remote organs are affected. A large number of medicines operate physically on the body and affect remote parts through the agency of the nervous system; among such are electricity, cold, heat, mechanical irritants, etc., which affect remote parts by the influence of the cerebro-spinal and ganglionic systems. The afferent or sensory nerves convey impressions to the nerve-centre, and the efferent or motor nerves transmit impressions from the centre to muscles, vessels, glands, etc., and the effects of medicines are transmitted to the brain, producing an excitement of a nerve-centre and a reflex action is carried along the efferent nerves, producing certain symptoms. The effects of medicines are modified by the age of the patient, as young children require small doses as a general rule; by the sex, as females possess greater susceptibility of the nervous system, and more excitability of the vascular system than males; they have also less energy, and medicines act on them more powerfully and rapidly and for a less period than on males; also climate influences the effects of medicines, as some, such as narcotics for example, act more energeticallv in hot climates, and others less energetically, calomel for example; habit also influences the action of medicines, as some become inert after long continued use; also diseased conditions modify the effects of medicines, such as mercury in fevers ; the idiosyncrasy also, as is shown by the effects manifested in different individuals; mercury for example will cause profuse salivation in susceptible patients when a very small quantity is administered; also the mind, as the cheerful convalesce sooner than the despondent.

Some medicines have a specific action on certain tissues and organs of the body, - as alcohol on the brain, strychnine on the spinal cord.

Temperaments are peculiarities of organization characterizing the different classes of individuals, the nervous patient being more easily affected by medicinal, than he is by other agencies, while the phlegmatic patient is not.

Idiosyncrasies are peculiarities belonging to single individuals, and they are so numerous that a knowledge of them is important for the practitioner.