Wherever a surface occurs in the connective tissues it is generally covered by a single layer of thin cells with 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.

Section through ossifying cartilage and young bone.

Fig. 34. Section through ossifying cartilage and young bone. (Cadiat).

a. Cartilage cells.

b. Degenerating cartilage cells.

c. Cell space, empty.

d. Spiculae of calcareous deposit.

e. Blood corpuscles.

f. Osteoblasts.

g. Ditto, of periosteum. h. Bone cells.

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.