_Insertion._—All along the inner margin of the arytenoid bone, and to the apex of the mid-cricoidal segment. _Shape._—Thin, flat sheet extending outward and upward, and then inward. _Relation._—Interiorly with the larynx. _Action._—The two muscles of this kind acting together, close the glottis by drawing the apices of the arytenoids to the median line. =Thyreo-arytenoideus.= _Location._—Supero-lateral portion of the superior larynx. _Origin._—From the entire outer margin of the thyroid plate and the outer margin of the cricoid bone. _Insertion._—All along the outer margin of the arytenoid bone and the outer border of the central cricoid piece. _Shape._—A thin, flat sheet. _Relations._—Infero-laterally with the superior larynx. _Action._—The two muscles of this kind acting together, open the glottis by pulling the arytenoid bone outward. =Tracheo-lateralis= (Fig. 20, No. 2). _Location._—Along the lateral side of the trachea. _Origin._—By the union of the bronchio-trachealis posticus and bronchio-trachealis anticus, on the lateral side of the trachea, about 1½ centimeters from the bifurcation of the trachea. _Insertion._—In delicate fan-like structure to the side of the trachea, near the superior larynx. _Shape._—Thin, ribbon-shaped, closely attached to the trachea. _Relations._—Internally with the trachea, and externally with the skin and other integument. _Action._—Acts as a brace to the sides of the trachea, and contracting, approximates the tracheal rings and thus shortens the trachea. =Broncho-trachealis Posticus= (Fig. 21, No. 3). _Location._—Postero-superior portion of the trachea. The fibers pass downward and backward. _Origin._—From the inferior end of the tracheo-lateralis. _Insertion._—To the end of the third half of the bronchial ring of the same side. _Shape._—A small fasciculus. _Relations._—Internally with the trachea, and externally with the skin and other integument. [Illustration: FIG. 20.—The anterior pectoral region. 1, Sterno-trachealis. 2, Tracheo-lateralis. 3, Broncho-trachealis brevis. 4, Costal process of sternum. 5, Right and left carotid converging and occupying same sheath at 6. 7, The heart showing branches of coronary artery. 8, Cervical air-sac. 9, Left brachio-cephalic artery. 10, Right brachio-cephalic artery. 11, Trachea. 12, Inferior larynx. 13. Subclavian artery. 14, Carotid trunk. 15, Internal thoracic artery. 16 Thyroid gland. 17, Anterior vena cava. 18, Auricular appendage. ] [Illustration: FIG. 21.—The inferior tracheal region. 1, Bronchialis anticus. 2, Bronchialis posticus. 3, Broncho-trachealis posticus. 4, Broncho-trachealis anticus. ] _Action._—A tensor of the true or inferior larynx. =Broncho-trachealis Anticus= (Fig. 21, No. 4). _Location._—Anterior portion of the inferior extremity of the trachea. _Origin._—The anterior branch of the bifurcated tracheo-lateralis.