The _canaliculi_ or _tertiary tubules_ given off by these secondary bronchial tubes do not differ greatly in caliber in the various parts. They are given off at right angles from the pulmonary wall of each bronchus, and extend perpendicularly into the lung substance. Thus we find three kinds of conduits, the primary, the secondary, and the peripheral, or tertiary. The first are like the barbs of a feather on its shaft; and the second and parenchymatous are implanted on the pulmonary walls of the first, like the hairs of a brush on their common base. Thus instead of the branching of the bronchi being dichrotomous, as in mammals, it is piniform. The canaliculi, or finer tubules, communicate with one another. The inner microscopic appearance of the canaliculi indicate that they are divided into areola, which gives them a cellular aspect. These tertiary bronchi open on a dense labyrinth of blood capillaries (Fig. 61, _C_). At this point the ciliated epithelial cells give way to simple squamous epithelium. Thus we find three kinds of bronchi, or their ramifications, as follows: the primary, the secondary, and the tertiary. =The Air-sacs= (Fig. 61, _A_).—The air-sacs are bladder-like structures consisting of a delicate cellulo-serous membrane, an extension from the bronchial tubes, in some places strengthened by an external envelope of elastic fibrous tissue. Long thin blood-vessels are distributed in the substance of these walls. They are branches from vessels of the general circulation and not extensions from those of the lungs. No lymphatics have been found in the air-sacs. [Illustration: FIG. 61, _A_.—Diagram of air-sacs and their location. 1, The proximal end of the humerus. 2, The proximal end of the right clavicle. 3, The cervical air cell. 4, The right coracoid bone. 5, The anterior thoracic air cell. 6, The right side of the sternum. 7, The right side of the liver. 8, The peritoneum. 9, The right abdominal air cell. 10, The coccyx. 11, The proximal end of the right femur. 12, The right supero-posterior air-sac. 13, The right infero-posterior air-sac. 14, The right lung. 15, The axillary extension of the air-sac. 16, The obturator foramen. 17. The pelvis. ] These sacs do not communicate with each other and normally they are not fully inflated. In some locations they extend into the bones and are in communication with the extensions of the bronchial tubes. In fact, by some anatomists they have been called “bladder-like, extra-pulmonary expansions of the bronchial tubes, free from cartilage.” The air-sacs make the bird’s body lighter, thus making long-continued flight possible. They are best developed in those birds which fly most. There are four pairs of cells and one single cell from which all other expansions and extensions are made. These sacs are as follows: a single anterior thoracic, and, in pairs, cervical, anterior diaphragmatic, posterior diaphragmatic, and abdominal.