Each element and each of its compounds has chemical reactions special to itself, by which it can be recognised and distinguished from all others. The number of these chemical reactions is therefore very great, but there are a few reactions which are common to a great number of the elements. We shall find that something similar occurs in their physiological reactions.

The number of possible actions which may be exerted on the body by the elements and their compounds is very great, yet we shall find that there are certain physiological reactions which are common to so many that their repetition under the head of each drug becomes monotonous.

Chemical Reactions

Although the chemical reactions of the metallic elements are numerous and varied, vet there are certain reactions which are common to a very large number, and by these the class of metallic elements may be subdivided into sub-classes. Other reactions again are common to a few elements only, and by these the sub-classes may be subdivided into groups. Other reactions again are peculiar to each individual element, and by them it may be distinguished from all others.

Thus, by the use of hydrogen sulphide, or ammonium sulphide, we at once divide the class of metallic elements into two sub-classes:

A. Metals which give a precipitate with one or other of these reagents.

B. Metals which give no precipitate with either.

Physiological Reactions

It is probable that, if our knowledge of physiological chemistry were sufficient, we might be able to classify physiological reactions according to the chemical relation between substances introduced into the organism and the various constituents of the organism itself. At present we are quite unable to do this; but, as albuminous substances form an essential part of all living organisms, we may roughly divide the elements physiologically, by their relation to albumen, just as we do it chemically, by their relation to sulphur, into two sub- classes:

A. Those which precipitate albumen.

B. Those which do not.

Just as in the case of sulphides, we might further sub-divide sub-class A into two sections :

(a) Those which precipitate albumen in acid solutions.

(b) ,, ,, ,,in neutral or alkaline solutions.

Section (b) may be further sub-divided into groups according to the kind of albuminous bodies which its members precipitated, e.g., myosin, globulin, serum-albumen, albumoses, peptones, etc.

We might also divide sub-class B in two sections:

(a) Substances which, though they do not precipitate albumen, have a marked affinity for fatty substances or other constituents of the organism, and especially of the nervous system (p. 144).

(b) Substances having no such action.

It is evident that such a classification as this, although it might form the groundwork of a system to be perfected at some future time, is at present so imperfect that it is generally more convenient to divide physiological reactions according to the organs affected: e.g., muscles, nerve-centres, respiration, circulation, secretion, etc.

A. This group contains substances which paralyse muscles and motor nerves. The number of these substances is very great (p. 126 et seq., p. 150).

This large group can again be subdivided into those which

(a) paralyse muscle, while affecting the nerves but slightly, or

(b) paralyse the nerves and leave the muscle uninjured.

B. Another large group is that which acts specially on nerve-centres, and has little effect either on muscles or motor nerves. This contains sub-groups of substances which affect the brain, medulla, or spinal cord by exciting, paralysing, or disturbing the functions of each.

C. Another group is that which affects the secretions, with sub-groups of substances affecting the secretions from the sweat and mammary glands, salivary, gastric, or intestinal glands, liver, or kidneys.

D. Another group still is that which acts chiefly upon the circulation.

These groups are all more or less distinct, although they, to a certain extent, may run into, or overlap, each other.

Individual members of the same group may differ very widely in their physiological action, even when they all finally paralyse muscle, nerves, and nerve-centres. For while they may produce the same final result, the course of their action will be different, and the symptoms they occasion will depend very greatly upon the part of the organism which they affect first. Thus atropine and curare both completely paralyse motor or efferent nerves, but, while a very large dose of curare is required to paralyse the cardiac and vascular nerves, a very small dose paralyses those going to the muscles, and produces increasing weakness, gradually passing into death. On the other hand, an enormous dose of atropine is required to paralyse the motor nerves of muscles, but very small doses are sufficient to affect the nerves of the heart and other involuntary muscles, and thus we get rapid circulation, dilated pupil, and restless delirium.

The physiological action of any drug depends to a great extent, not merely on its general affinities for classes of tissues, but upon its particular affinity, or power of acting on one tissue or organ first. The organ first affected may, through its functional activity, greatly alter the effects of the drug upon the others.

As an example of this we may take the effects produced by very large and by moderate doses of veratrine on the frog. A moderate dose will produce great stiffness of the muscles, while a very large dose may have comparatively little effect. Yet if the large dose were applied directly to the muscles it would act more powerfully than the moderate dose. The reason that it does not do so in the living body is that the large dose paralyses the heart so quickly that the circulation stops, and therefore the poison, not being conveyed to the muscles, has no action upon them.