FIG. 34.

Section through ossifying cartilage and young bone. (Cadiat.)

a. Cartilage cells.

b. Degenerating cartilage cells.

c. Cell space, empty.

d. Spicule of calcareous deposit.

e.

Blood corpuscles. f. Osteoblasts.

g. Ditto of periosteum.

h. Bone cells.

a characteristic outline, which can only be made visible by staining the intervening cement substance with silver nitrate. This tissue, which forms the immediate lining of all vessels and spaces developed in the tissues arising from the mesoblast, is called endothelium, in contradistinction to the epithelium developed from the epi- and hypo-blast.

The Vascular System is developed in the mesoblast with the earliest stages of the connective tissue. The blood vessels, which are chiefly made up of connective tissues, soon traverse all parts of the body, and distribute the nutrient fluid or blood. The blood may be considered as an outcome of the connective tissues, since the corpuscles of the blood are at first formed from the cells of the mesoblast, and later from the connective tissue corpuscles.

An arrangement of special cells, such as epithelial or muscle cells, with a special function, constitutes an organ. However, in the higher animals and man an organ is almost invariably a complex structure, having various tissues entering into its construction. Thus a skeletal muscle is made up of a quantity of muscle fibres held together by sheets of connective tissue, and attached to bones by connecting bands. It is further traversed by many blood vessels, and the fibres are in immediate relation to certain nerves which terminate in them. The various secreting organs are made up of epithelial cells, held together by connective tissue in close relation to blood vessels and nerves, and are so arranged that they pour their secretion into a duct. The bones, which are the organs which give the body support, contain, in addition to the bone tissue of which they are composed, a great quantity of indifferent cells, fat cells, nerves and blood vessels. They are covered on the outside with a tough vascular coat, which gives them strength, assists their nutritive repair and reproduction, and acts as a point of attachment for the muscles and ligaments. Where the bones are in contact at the joints, they are tipped with hyaline cartilage.

If, then, we analyze anatomically the architecture of the

human body, we find that it is made up of a number of complex parts, each adapted to some special function, and composed of an association of simple tissues such as the requirements of the special part demand.

The general arrangements of these organs and their modes of action will be discussed in future chapters.

CHAPTER III.

CHEMICAL BASIS OF THE BODY.

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, (1) 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 sub

stances.

It is of little use to classify the numerous chemical constituents found in the animal body in such a systematic manner as to