Extending up on either side of the upper trachea in the neck are the two lobes of the _thyroid gland_, the isthmus, which connects the lobes, covering the trachea below anteriorly. The function of the gland is obscure, but it has an internal secretion of great importance in the metabolic processes. Its removal or disease is followed by general disturbances of mind and body. The injection of thyroid extract has proved effective as treatment. In _goiter_ the gland becomes enlarged. The _thymus gland_ lies below the thyroid gland at birth, in front of and at the sides of the trachea, and runs down behind the sternum. It is largest at the end of the second year, after which it atrophies, being almost absent at puberty. It, too, is ductless and its function is not well understood. =The Bronchi.=--The two bronchi, of which the right is the larger and shorter, resemble the trachea in structure. As they enter the root of the lung they divide, the right into three and the left into two branches, one for each lobe, after which they divide and subdivide, the bronchioles becoming smaller and smaller and finally ending in the _infundibula_, pouch-like places lined with air cells, in which cilia keep the air in motion. As they grow smaller the bronchioles gradually become wholly membranous. [Illustration: FIG. 49.--Relation of lungs to other thoracic organs. (Ingals.)] =The Lungs.=--The lungs themselves, two in number, lie each in a serous sac or _pleura_, similar in structure to the pericardium and serving a like purpose. The outer layer of the pleura is reflected back over the thoracic wall and diaphragm. There is no pleural cavity in health between the two layers of the pleura, the two surfaces being in close contact, though moistened with lymph to prevent friction during respiration. In inflammation of the pleuræ or pleurisy, they become thickened and roughened and friction results, as is shown by the sounds heard through the stethoscope. Friction causes effusion and fluid collects. This generally absorbs again, but occasionally the serous fluid becomes pustular and _empyema_ results. In front, between the two pleuræ, which are wholly separate, is the _mediastinal space_ or _mediastinum_, which extends from the sternum to the spinal column and contains all the thoracic viscera except the lungs and heart, that is, the trachea, esophagus, thoracic duct, and many large vessels and nerves. [Illustration: FIG. 50.--Diagrammatic representation of the termination of a bronchial tube in a group of infundibula: _B_, Bronchial tube; _LB_, bronchiole; _A_, atrium; _I_, infundibulum; _C_, alveoli. (de Nancrede.)]