MUSCLES 31 



It is inserted into the head of the humerus. (Plate 

 IV.) 



d. The posterior division takes its origin from 

 the sternum, between the cartilages of the fifth and 

 eighth ribs, and is inserted principally on the inner 

 side of the deltoid ridge of the humerus. (Plate 

 IV.) 



e. The smallest and most anterior part arises 

 from the side of the manubrium and is inserted on 

 the humerus just above the insertion of b, from 

 which it can be separated with difficulty. This 

 division also sends some fibres to the clavicle. 



To expose the remaining muscles of the thorax the 

 pectoralis group must be reflected. 



2. The Sternalis Muscle is a small triangular mus- 

 cle arising from the fascia of the rectus abdominis, 

 at the point where the cartilages of the third and 

 fourth ribs unite with the sternum. Its fibres 

 pass forward and outward and are inserted on the 

 first rib. A second, smaller, sternalis muscle lies 

 just outside the rectus abdominis, at the level of 

 the sixth rib. (Plates III. and IV.) 



3. The Scalenus Primus Muscle is long and narrow. 

 It takes its origin from the fourth and fifth cervi- 

 cal vertebrae, and .is inserted on the cartilages of 

 the fourth, fifth, and sixth ribs. (Plates III. and 

 IV.) 



4. The Scalenus Secundus Muscle lies deeper than 

 the last. It takes its origin from the transverse 



