29S THE MUSCLES 



the posterior division of the same fascia, the rhomboid hgament, the su])clavian 

 vessels, and the brachial plexus. 



Variations. — The subchivius may be absent, or its insertion may extend to the coracoid 

 process, transverse ligament, and upjjer border of the scapula. Occasionally the costo-clavicular 

 portion is sei^arate from the costo-scapular. Again, its origin may extend inwards to the sternum, 

 and the portion which arises from the sternum may be separate from the rest of the muscle. 



2. PECTORALIS MINOR 



The pectoralis minor — named from its being the smaller of the two muscles 

 which arise from the front of the chest — is a fan-shaped or triangular sheet, with its 

 inner edge divided into three teeth. 



Origin. — (1) The upper borders and outer surfaces of the third, fourth, and 

 fifth ribs near their anterior extremities; (2) the fascia covering the intercostal 

 muscles in the spaces between these ribs. 



Insertion. — (1) The upper surface of the coracoid process of the scapula; (2) 

 the upper ])art of the tendon of the coraco-brachialis along its inner border. 



Structure. — Its origin is by aponeurotic slips, which, after becoming fleshy, 

 converge ui)wards, outwards, and somewhat backwards, upon the flattened tendon, 

 which is attached chiefly to the coracoid process, but blends for an inch or more 

 VteloAv that jn'oeess with the origin of the coraco-brachialis. 



Nerve-supply. — From the inner cord of the brachial plexus (through the 

 eighth cervical nerve ) by the internal anterior thoracic, which enters its deep sur- 

 face near the upper liorder, the nerve subsequently piercing the muscle to send 

 filaments to the pectoralis major. 



Its action is to draw downwards and forwards the scapula, depressing at the 

 same time the point of the shoulder. Taking its fixed point from the coracoid 

 process, it draAvs upwards and outwards the ribs to which it is attached, and 

 so helps in forced inspiration. Its connection with the tendon of the coraco- 

 brachialis will enable it to act slightly as a flexor and adductor of the humerus. 



Relations, — Superficially, the pectoralis major; deeply, tbe external intercostal 

 muscles; and, near its insertion, the axillary vessels and brachial nerves. 



Variations. — The origin may extend upwards as far as the second, or downwards to the sixth 

 rib, and it may receive additions from the pectoralis major. It is occasionally altogether absent. 

 Its insertion may be continued over the coracoid process to the capsule of the shoulder-joint, the 

 greater tuberosity of the humerus, or the clavicle. 



Third Layer 



SERRATUS MAGNUS 



The serratus magnus — named from its serrated or saw-like anterior border 

 and large size — is an irregular quadrilateral sheet curved to the shape of the side of 

 the thorax. Its anterior attached l)order has a somewhat sinuous curve, and arises 

 from the side of the thorax by nine or ten digitations or teeth, which, by their saw- 

 like appearance, give the muscle its name. The muscle may be divided into an 

 upper, middle, and lower part. 



Origin. — First part, by two teeth from the middle of the outer surface of the 

 first and second ribs, and from the fascia covering the first intercostal space. 

 Second part, by two or three heads from the second, third, and sometimes the 

 fourth ribs u])on their outer surface. Third part, by far the largest and strongest 

 portion of the muscle, arises from tbe fom-tb or fifth to the eighth or ninth ril)s l\v 

 a series of teeth, which are attached in front near the upper border of each rib, and 

 behind to a line running backwards across tlie outer surface of the ril) from its 

 u])per to its lower l)()rder. These attachments form a curved line with the con- 

 vexity forwards, the attachment to the sixth rib l)eing the most anterior and 

 prominent. 



