MUSCLES 31 



tilage of the fourth rib. It is inserted principally 

 on the anterior three-fourths of the humerus, on 

 the outside of the deltoid ridge. Anteriorly it is 

 covered by a and b, posteriorly it is superficial. 

 (Plates III. and IV.) 



d. The largest division takes its origin from the 

 sternum, between the cartilages of the second and 

 ninth ribs, and is inserted in two parts, one on the 

 head of the humerus, the other on the proximal 

 half of the ventral side of the humerus. (Plate 

 IV.) 



e. The posterior division arises from the median 

 raphe just posterior to the sternum and is inserted 

 principally on the tuberosities of the humerus.. 

 (Plate IV.) 



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 some- 

 times is found just outside the rectus abdominis, 

 at the level of the sixth rib. (Plates III. and IV.) 



3. The Scalenus Muscle is long and narrow. It 

 takes its origin from the transverse processes of the 

 cervical vertebrae, and is inserted on the cartilages 

 of the first nine ribs. (Plates III. and IV.) 



