respiratory tract, which causes bronchodilation. The action of breathing takes place because of nerve signals sent by the respiratory center in the brainstem, along the phrenic nerve from the cervical plexus to the diaphragm. Variation The lobes of the lung are subject to anatomical variations. A horizontal interlobar fissure was found to be incomplete in 25% of right lungs, or even absent in 11% of all cases. An accessory fissure was also found in 14% and 22% of left and right lungs, respectively. An oblique fissure was found to be incomplete in 21% to 47% of left lungs. In some cases a fissure is absent, or extra, resulting in a right lung with only two lobes, or a left lung with three lobes. A variation in the airway branching structure has been found specifically in the central airway branching. This variation is associated with the development of COPD in adulthood. Development The development of the human lungs arise from the laryngotracheal groove and develop to maturity over several weeks in the foetus and for several years following birth. The larynx, trachea, bronchi and lungs that make up the respiratory tract, begin to form during the fourth week of embryogenesis from the lung bud which appears ventrally to the caudal portion of the foregut. The respiratory tract has a branching structure, and is also known as the respiratory tree. In the embryo this structure is developed in the process of branching morphogenesis, and is generated by the repeated splitting of the tip of the branch. In the development of the lungs (as in some other organs) the epithelium forms branching tubes. The lung has a left-right symmetry and each bud known as a bronchial bud grows out as a tubular epithelium that becomes a bronchus. Each bronchus branches into bronchioles. The branching is a result of the tip of each tube bifurcating. The branching process forms the bronchi, bronchioles, and ultimately the alveoli. The four genes mostly associated with branching morphogenesis in the lung are the intercellular signalling protein – sonic hedgehog (SHH), fibroblast growth factors FGF10 and FGFR2b, and bone morphogenetic protein BMP4. FGF10 is seen to have the most prominent role. FGF10 is a paracrine signalling molecule needed for epithelial branching, and SHH inhibits FGF10. The development of the alveoli is influenced by a different mechanism whereby continued bifurcation is stopped and the distal tips become dilated to form