_Insertion._—To the anterior extremity of the third half ring of the bronchus of the same side. _Shape._—A small fasciculus. _Relations._—Internally with the trachea, and externally with the skin and other integument. _Action._—A tensor of the inferior larynx. =Broncho-trachealis Brevis= (Fig. 20, No. 3). _Location._—Posterior part of the inferior extremity of the trachea. _Origin._—From just beneath the broncho-trachealis anticus muscle. _Insertion._—Extends obliquely across the inferior larynx and is inserted to the posterior end of the second bronchial ring. _Shape._—Rather short, strong, straight, subcylindrical. _Relations._—Internally with the trachea, and laterally with the broncho-trachealis posticus. _Action._—A tensor of the inferior larynx. =Bronchialis Posticus= (Fig. 21, No. 2). _Location._—Supero-lateral side of the anterior end of the bronchi. _Origin._—From the lateral inferior margin of the last tracheal ring. _Insertion._—Passing obliquely across the larynx it inserts to the posterior extremity of the second half ring of the bronchus. _Shape._—Small, thick, spindle-shaped. _Relations._—Internally with the trachea, inferiorly with the bronchialis anticus, and superiorly with the broncho-trachealis brevis. _Action._—A tensor of the larynx. =Bronchialis Anticus= (Fig. 21, No. 1). _Location._—Inferior to the former muscle. _Origin._—From the last ring of the trachea. _Insertion._—The fibers pass obliquely forward and become inserted to the rim of the arytenoid cartilage of the inferior larynx, and the anterior extremities of the first and second half rings of the bronchus. _Shape._—Thick, spindle-shaped; about twice the size of the bronchialis posticus. _Relations._—It is crossed at its origin by the broncho-trachealis anticus and is related internally with the bronchus and end of the trachea. _Action._—A tensor of the larynx. =Sterno-trachealis= (Fig. 20, No. 1). _Location._—The inferior region of the trachea above the above-named muscles. _Origin._—From the side of the trachea beneath the posterior border of the broncho-trachealis anticus. _Insertion._—Passes downward and backward across the cavity of the chest and is inserted to the inner part of the costal process of the sternum. _Shape._—A delicate cord of fibers. _Relations._—Internally with the broncho-trachealis muscles. _Action._—A relaxor of the larynx, and hence, of the tympanic membrane. THE STERNAL GROUP =Triangularis Sterni.= _Location._—In the floor of the thoracic cavity. _Origin._—From the entire superior margin of the summits of the costal processes. _Insertion._—The fibers extend upward and backward, dividing into four digitations which cover the inner surface of the three principal sternal ribs, and are inserted to the first four as high as their articulations with the vertebral ribs. _Shape._—Flat and somewhat triangular. _Relations._—Interiorly with the floor of the thoracic cavity.