230 LATERAL THORACIC REGION. 



Lateral Thoracic Region. 

 Serratus magnus. 



The SERRATUS MAGNUS (serratus, indented like the edge of a saw,) 

 arises by fleshy serrations from the nine upper ribs excepting the first, 

 and extends backwards upon the side of the chest, to be inserted into 

 the whole length of the base of the scapula upon its anterior aspect. 

 In structure the muscle is composed of three portions, a superior por- 

 tion formed by two serrations attached to the second rib, and inserted 

 into the inner surface of the superior angle of the scapula, a middle 

 portion composed of the serrations connected with the third and fourth 

 ribs, and inserted into the greater part of the posterior border, and an 

 inferior portion consisting of the last five serrations which indigitate 

 with the obliquus externus and form a thick muscular fasciculus which 

 is inserted into the scapula near its inferior angle. 



Relations. By its superficial surface with the pectoralis major and 

 minor, the subscapularis, and the axillary vessels and nerves. By its 

 deep surface with the ribs and intercostal muscles, to which it is con- 

 nected by an extremely loose cellular tissue. 



Actions. The serratus magnus is the great external inspiratory 

 muscle, raising the ribs when the shoulders are fixed, and thereby in- 

 creasing the cavity of the chest. Acting upon the scapula, it draws 

 the shoulder forwards, as we see to be the case in diseased lungs, where 

 the chest has become almost fixed from apprehension of the expanding 

 action of the respiratory muscles. 



Anterior Scapular Region. 

 Subscapularis. 



The SUBSCAPULARIS muscle arises from the whole of the tinder sur- 

 face of the scapula excepting the superior and inferior angle, and ter- 

 minates by a broad and thick tendon, which is inserted into the lesser 

 tuberosity of the humerus. The substance of the muscle is traversed 

 by several intersecting membranous layers from which muscular fibres 

 arise, the intersections being attached to the ridges on the surface of 

 the scapula. Its tendon forms part of the capsule of the joint, glides 

 over a large bursa which separates it from the base of the coracoid 

 process, and is lined by a prolongation of the synovial membrane of 

 the articulation. 



Relations. By its anterior surface with the serratus magnus, coraco- 

 brachialis, and axillary vessels and nerves. By its posterior surface with 

 the scapula, the subscapular vessels and nerves, and the shoulder, 

 joint. 



Action. It rotates the head of the humerus inwards, and is a 

 powerful defence to the joint. When the arm is raised, it draws the 

 humerus downwards. 



