The simple elements from which the various parts and organs are built. All the body originates from the union of a pair of CELLS, but as growth proceeds the new cells produced from these form tissues of varying character and complexity. It is customary to divide the tissues into five groups:

• Epithelial tissues, including the cells covering the skin, those lining the alimentary canal, those forming the secretions of internal organs. (See EPITHELIUM.)

• Connective tissues, including fibrous tissue, fat, bone, cartilage. (See under these headings.)

• Muscular tissues (see MUSCLE).

• Nervous tissues (see NERVE).

• Wandering corpuscles of the BLOOD and LYMPH.

Many of the organs are formed of a single one of these tissues, or of one with a very slight admixture of another, such as cartilage, or white fibrous tissue. Other parts of the body that are widely distributed are very simple in structure and consist of two or more simple tissues in varying proportion. Such are blood vessels (see ARTERIES; VEINS), lymphatic vessels (see LYMPHATICS), lymphatic glands (see GLANDS), SEROUS MEMBRANES, synovial membranes (see JOINTS), mucous membranes (see MUCOUS MEMBRANE), secreting glands (see GLANDS; SALIVARY GLANDS; THYROID GLAND) and SKIN.

The structure of the more complex organs of the body is dealt with under the heading of each organ.