The lung is supplied with air through a bronchus. In mammals and reptiles the membrane of the lung takes the form of numerous sacs (see alveolus) that are connected to the bronchus via bronchioles. The lungs themselves contain no muscular tissue and are ventilates by respiratory movements, the mechanisms of which vary with the species.

Luxuriance. The somatic vigour of hybrids that does not add to their capability to compete.

Lymphatic system. The network of vessels that conveys lymph from the tissue fluids to the blood stream. Tiny lymph capillaries drain into larger tubular vessels that converge to form the right lymphatic duct and the thoracic duct, which connect with the venous blood supply to the heart. Associated with the lymphatic vessels at intervals along the system are the lymph nodes.
The lymph capillary walls are very permeable, so lymph bathing the body’s tissues can drain away molecules, so lymph is pumped by cycles of contraction and relaxation of the lymphatic vessels and also by the action of adjoining muscles.

Lymphokine. A soluble mediator released by lymphocytes on contact with specific.

Lymphoma. A tumor of lymphoid tissue.

Lyrate. Shaped like a lyre.

Lysin. A type of Antibody.

Lysis. Destruction of cells through damage to or rupture of plasma membrane, allowing escape of cell contents.

Lysogenic bacterium. A bacterium that carries a prophase of a temperate virus.