_Inserted_ into the first costal cartilage near its middle, into the second costal cartilage near its sternal end, and into the sternum between the first and fourth cartilages. _Relations._--Outer surface of the cranial part of the muscle and its tendon with the pectoralis minor (Fig. 68, _o_) and the transversus costarum (Fig. 73, _j_); caudad the muscle is covered by the outer layer of the rectus sheath. Lateral edge with the obliquus externus (Fig. 68, _p_), obliquus internus (Fig. 73, _o_), and transversus abdominis (Fig. 69, _l_). Medial edge with the muscle of the opposite side. Inner surface with the internal intercostals (Fig. 69, _k_) and the rib cartilages; the inner layer of the rectus sheath and the peritoneum. _Action._--Retracts the ribs and sternum and compresses the abdomen. IV. MUSCLES OF THE THORACIC LIMBS. The muscles connecting the thoracic limbs with the rest of the body have been described. 1. Muscles of the Shoulder. A. MUSCLES ON THE LATERAL SURFACE OF THE SHOULDER. =M. deltoideus.=--The deltoid muscle of the cat is divided into two (or three) portions which are together equivalent to the human deltoid. These are the spinodeltoid, the acromiodeltoid, and possibly the clavobrachial, which is frequently called clavodeltoid. =M. spinodeltoideus= (Fig. 75, _e_; Fig. 68, _i_).--A rather thick, flat muscle between the scapular spine and the deltoid ridge of the humerus. It forms a chord of the angle between the glenoid border of the scapula and the humerus. _Origin_ by short tendon-fibres from the glenoid border of somewhat more than the middle third of the spine of the scapula (Fig. 76, _g_), and from a tendinous raphe between the spinotrapezius (Fig. 68, _j_), acromiotrapezius (Fig. 68, _h_), and infraspinatus (Fig. 75, _c_). The origin may pass toward the vertebral border of the scapula onto the infraspinatus muscle (Fig. 75, _c_). _Insertion_ (Fig. 81, _h_) by a flat tendon upon the deltoid ridge of the humerus, nearly parallel to that of the pectoralis major. _Relations._--Outer surface with the integument and at the insertion with the acromiodeltoideus (Fig. 75, _f_). Inner surface with the infraspinatus (_c_), teres minor (Fig. 80, _c_), caput laterale (Fig. 75, _h_), and caput longum (Fig. 75, _g_) of the triceps muscle. _Action._--Flexes the humerus and rotates it outward. =M. acromiodeltoideus= (Fig. 75, _f_; Fig. 68, _g_).--A flat muscle which overlies the distal end of the spinodeltoideus (Fig. 75, _c_). It connects the acromion with the humerus. _Origin_ (Fig. 76, _d_).--From the glenoid border of the acromion, and sometimes the adjacent metacromion as far as the tip.