It seems natural to commence the description of the molecular changes that take place in the various tissues and organs of the body with a brief account of the chemical composition of the most characteristic substances found in animal textures, because none of the processes of cell life, or tissue activity, can be satisfactorily studied without familiarity with the more common terms occurring in physiological chemistry.

The chapter on this subject here introduced, is intended rather to give the medical student a general view of the chemical composition and characters of the substances most frequently met with in the chemical changes specially connected with animal life, than to supply a complete or systematic account of the relationships of the chemical bases of the body, for which reference must be made to more advanced text-books, or treatises on the special subject of physiological chemistry. This review must, for the sake of brevity, be inadequate in the case of many substances, but these will be again referred to when speaking of the function with which they are associated.

It has already been stated that of the seventy elements known to chemists, a comparatively small number form the great bulk of the animal body, although traces of many are constantly present. Thus, we shall see that four elements, namely, (i) oxygen, (2) carbon, (3) hydrogen, (4) nitrogen, are present in large proportions in every tissue, and together make up about 97 per cent, of the body; and sulphur, phosphorus, chlorine, fluorine, silicon, potassium, sodium, magnesium, calcium, iron, and in certain animals copper, are indispensable to the economy, and are widely distributed, but are found in comparatively minute quantities. Occasionally traces of zinc, lead, lithium, and other minerals may be detected, but these must be regarded rather as accidental than indispensable ingredients.

The attempt to investigate the composition of a living tissue by chemical analysis, must cause its death, and thus alter the arrangements of its constituents, so that its true molecular constitution during life cannot be determined.

We know that the composition of all living textures is extremely complicated, having a great number of components, most of which contain many chemical elements associated together in very complex proportions.

But as has already been pointed out, the complexity of their chemical constitution is not so wonderful as the fact, which indeed sounds paradoxical, that in order to preserve their elaborate composition, they must constantly undergo a change or renewal, which is necessary for, and forms the one essential characteristic of, their life. In fact, their complexity and instability is such, that they require constant reconstruction to make up for the changes inseparable from their functional activity.

Their chemical constituents are easily permanently dissociated, and the various components are themselves readily decomposed, generally uniting with oxygen to form more stable compounds.

The investigation of the chemical changes known as assimilation forms a great part of physiological study, and therefore will occupy many chapters of this book. Here we can only call attention to the chief characteristic substances to be found in the animal body, as the result of the primary dissociation or death of the textures, and briefly enumerate the products of their further decomposition as obtained by the analysis of the different substances.

The tissues of the higher animals present a great variety of substances, materially differing in chemical composition; they have all been made from protoplasm, and contain a proportion of some substance forming a leading chemical constituent of protoplasm. Every living tissue contains either protoplasm or a derivative of it, and the special characters of each tissue depend upon the greater development of some one of these substances.

It is of little use'to classify the numerous chemical constituents found in the animal body in such a systematic manner as to satisfy the rules of modern chemistry, because their classification, from a strictly chemical point of view, does not set forth their physiological importance or express adequately the relation they bear to the vital phenomena of organisms.

The following enumeration of the chief chemical ingredients found in the tissues has regard to their physiological dignity as well as to their chemical construction, and will thus, it is hoped, assist the student to distinguish the different groups, and give him a better idea of their vital relationships, than a more strictly systematic classification.

(A) Nitrogenous

I. Complex bodies forming the active portion of many tissues - Plasmata, e. g., protoplasm, blood plasma. II. Bodies entering into the formation of and easily obtained by analysis from Group I, Albumins, e. g., serum albumin.

III. Bodies the outcome of differentiation, manufactured in the tissues by Group I, Albuminoids, e. g., gelatin, etc.

IV. Bodies containing nitrogen, being intermediate, bye, or effete products of tissue manufacture, e. g., lecithin, urea, etc.

(B) Non-Nitrogenous

V. Carbohydrates in which the hydrogen and oxygen exist in the proportion found in water, e. g., starch and sugar. VI. Substances containing oxygen in less proportions than the above, e. g., fats.. VII. Salts. VIII. Water.