_The Anterior Thoracic Air-sac._—The anterior thoracic air-sac is located above the clavicles and the interclavicular space, in the cavity of the thorax. It is related superiorly with the trachea and the esophagus; laterally with the lungs and the cervical air-sacs; inferiorly with the sternum, the clavicle, and the interclavicular aponeurosis; posteriorly with the heart and the anterior diaphragmatic reservoir; and anteriorly with the integuments of the neck. It contains the inferior larynx and the two primary bronchi, and large vascular trunks from which are given off vessels supplying the neck and the wings. Three prolongations, called subpectoral, subscapular, and middle, or humeral, arise from the lateral walls of this air-sac. These prolongations cross the walls of the thorax and pass around the articulation of the shoulder. The _subpectoral prolongation_ extends from the thoracic reservoir by an orifice situated behind the coracoid, and passes beneath the tendon of the great pectoral muscle. When the pectoralis major contracts, this contraction dilates the subjacent cell and draws into it a greater quantity of air. The _subscapular_ and the _humeral prolongations_ communicate with the thoracic air cell by a common opening situated behind the small adductor muscle of the humerus. The subscapular prolongation, after leaving this point, spreads under the scapular and the subscapular muscle, which it separates from the ribs and corresponding intercostal muscles, and extends in a longitudinal direction. The humeral prolongation, smaller than the subscapular, occupies the axilla, and is in shape triangular. It has from its summit into an infundibular fossa, an extension which enters the canal of the humerus. The walls of this cell form the lining of the air space in the humerus. The thoracic air-sac thus possesses numerous membranous folds which divide its cavity. The contiguous structures which it overlies, as the trachea, the esophagus, the muscles of the inferior larynx, as well as the arteries and veins, make its outer walls irregular. This orifice is dilated during inspiration, by the contraction of the two first fasciculi of the diaphragm.