PECTORA L IS MA J OR 



295 



the sternum) lieing attaelied to the lower i)art of the horseshoe, where they become 

 continuous with those forming the anterior segment of the tendon. 



In consequence of this arrangement the lower til)res of the muscle disappear 

 from view soon after tlieir origin, and are concealed by the clavicular and upper 

 sterno-costal iibres. This arrangement gives to the anterior border of the axilla 

 a concave outline, ^^"ere it not for this decussation, the muscle would be l)roader 

 and thinner, and would cause an inconvenient and unsightly projection in front of 

 the axilla. 



The sterno-costal portion may usually ])e divided into a superficial and deep 

 plane, the superficial being composed of the sternal and external oljlique origin; 

 the deep, of the fibres which s]»ring from the cartilages and the sixth ril). 



Fig. 264. — The Pectoralis Major and Deltoid. 



Biceps 



Teres major 



Aponeurosis of external oblique 



External intercostal 



Nerve-supply. — From all the nerves of the brachial plexus: — (1) through the 

 external anterior thoracic nerve, which enters the deep surface of the muscle just 

 below the middle of its upper border; and (2) through the internal anterior 

 thoracic, which, after piercing and supplying the pectoralis minor, enters the deep 

 surface of the muscle a little lowcu- down, half-Avay between its origin and insertion. 



Action. — To adduct. Hex, and rotate inwards the upper arm. By its clavicular 

 ]»ortion it will draw the arm more directly forwards, as in round-hand bowling, 

 Avhile the sterno-costal portion will draw the arm more downwards than forwards. 

 The whole muscle will be used with great force in striking a blow downwards and 

 forwards. When the arm hangs close to the side, the pectoralis major will draw 

 the scapula forwards and downwards, so as to advance and depress the point of the 

 shoulder, as may be seen in a person shivering with cold, ^\'hen the arm is 



