UPPER EXTREMITY AN> THORAX, ANTERIOR. 269 



tal ridge, for a distance of two inches below the greater 

 tuberosity of the humerus. The tendon of insertion is in 

 two layers, the anterior layer belonging to the clavicular 

 portion of the muscle ; the posterior layer to the sternocos- 

 tal portion. These layers are open above but joined below 

 along the lower border of the tendon. The fibres of the 

 muscle (sternocostal) inserted into the posterior layer of the 

 tendon are twisted on themselves until the lower fibres of 

 origin take the highest point of insertion, and the highest 

 fibres of origin take the lowest point of insertion. 



Nerve Supply. From the anterior thoracics : The ex- 

 ternal thoracic nerve arises from the outer cord and enters 

 the muscle above the pectoralis minor ; the internal nerve 

 comes from the inner cord and pierces the minor to end in 

 the major muscle. 



Action. (i) On the humerus. The muscle adducts and 

 flexes the humerus if previously extended ; when the arm is 

 at the side it will rotate it internally and draw it across the 

 front of the chest (adduction and flexion). (2) On the 

 shoulder. When the arm is fixed the muscle will draw the 

 shoulder downward and forward. (3) On the chest. When 

 the body is supported on the arms the pectoral muscles sling 

 the chest between them. If the arms are fixed upward 

 and forward the muscles will raise the body (as in climbing). 



The sternocostal portion of the muscle may have some 

 action in enlarging the chest in difficult respiration, and thus 

 become an accessory muscle of inspiration. 



The Cephalic Vein. Figs. 60, 61. 



(Now dissected from the middle of the arm ; see below 

 for its formation at the elbow.) 



The cephalic vein starts just above the elbow-joint by 

 the confluence of the median cephalic and the radial veins ; 



