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branches of the windpipe in higher vertebrates (Amniota) and man. In the majority of animals the windpipe, or trachea, is divided into two main bronchi. Only in the Hatteria does a longitudinal fissure in the posterior section of the trachea mark the paired bronchi, which do not have separate cavities. In all other reptiles, and also in birds and mammals, the bronchi are well developed and extend inside the lungs. In reptiles, bronchi of the second order emerge from the main bronchi; these in turn divide into bronchi of the third and fourth orders, and so on; division of bronchi is especially complex in turtles and crocodiles. In birds, second-order bronchi are connected by parabronchi—canals along whose radii the so-called bronchioles branch off, branching and becoming a network of pneumatic capillaries. The bronchioles and the pneumatic capillaries of each parabronchus unite with the corresponding formations of the other parabronchi, thus forming a system of continuous air paths. The main bronchi, as well as certain side bronchi, widen at their ends into what are called air sacs. In mammals, from each main bronchus emerge secondary bronchi, which divide into ever smaller branches, forming the so-called bronchial tree. The very smallest of these branches become alveolar ducts, which end in alveoli. In mammals, besides the usual secondary bronchi, prearterial secondary bronchi are distinguished, which emerge from the main bronchi before the place where the pulmonary arteries cross them. Most often there is just one right prearterial bronchus, which in most Artiodactyla emerges directly from the trachea. The fibrous walls of the large bronchi contain cartilaginous half-rings joined in the rear by transverse bundles of smooth muscle. The mucous membrane of the bronchi is covered with ciliated epithelium. In the small bronchi the cartilaginous half-rings are replaced by individual nodules of cartilage. In the bronchioles there is no cartilage, and ring-like bundles of smooth muscle lie in a dense layer. In the majority of birds the first rings of the bronchi participate in the formation of the lower larynx.
In man the division of the trachea into the two main bronchi occurs at the level of the fourth and fifth thoracic vertebrae. Each bronchus then divides into ever smaller ones, ending in microscopically small bronchioles, which become the alveoli of the lungs. The walls of the bronchi are composed of smooth muscles and of hyaline cartilaginous rings that prevent the collapse of the bronchi; the bronchi are lined inside with mucous membrane. Along the branching paths of the bronchi are located numerous lymph nodes, which collect lymph from the lung tissues. The bronchi are supplied with blood by the bronchial arteries that emerge from the thoracic aorta; innervation is effected by branches of the vagus, sympathetic, and spinal nerves.