1 88 PHYLUM CHORD ATA 



tached to it in the medial area. The lateral ends of the subhyoid 

 iind their origin in the hyoid cartilage. 



At the forward end of the trunk is a group of four muscles 

 which radiate from the base of the foreleg on each side toward 

 the medial area of the body and may be called the pectoral group. 

 Beneath the medial ends of these muscles between the forelegs 

 will be seen the delicate sternum, or breastbone. The foremost 

 of these muscles is the coracoradial. It is a wide muscle, the 

 hinder half of which is concealed beneath the muscle next be- 

 hind it. From its distal end a long tendon passes along the 

 humerus to the forearm. The function of this muscle is to bend 

 the forearm. 



The other three muscles of this group are divisions of the pec- 

 toral muscle. The anterior two divisions, like the coracoradial, 

 have their origin in the breastbone near the median line. The 

 hindermost division, which is the largest, has its origin in the 

 outer edge of the broad aponeurosis, which occupies the median 

 area of the abdomen. All the divisions of the pectoral muscle 

 have their insertion near the proximal end of the humerus in the 

 upper arm and serve to bend the arm back. 



Lying in front of the pectorals is the deltoid muscle. It con- 

 sists of two principal portions, an anterior and a posterior 

 portion, both of which arise near the median line along the 

 forward border of the pectorals. They are inserted in the hu- 

 merus and cover the insertions of the pectoral muscles. Deter- 

 mine the action of the deltoid muscle. 



Back of the breastbone are the abdominal muscles, which form 

 the ventral and lateral walls of the abdomen. Three pairs are 

 present, the rectus abdominis, the external oblique, and the 

 transversus. 



Extending from the hinder end of the breastbone to the hinder 

 end of the trunk is the aponeurosis just mentioned — a broad 

 median band of connective tissue covering the midventral area 

 of the abdomen. A similar aponeurosis is also present in the mid- 

 dorsal area. 



The rectus abdominis muscles are a pair of longitudinal muscles 

 which lie in the midventral area beneath the aponeurosis. A 



