230 DISSECTION OF THE UPPER LIMB. 



artery runs to the side of the chest ; and taking the same direction, though 

 nearer the middle of the hollow, are the small external mammary artery 

 and vein. 



Extending along the posterior fold, within its lower margin and in con- 

 tact with the edge of the subscapularis muscle, are the subscapular vessels 

 and nerves (d and 7 ) ; and near the humeral end of the subscapularis the 

 posterior circumflex vessels and nerve (e) bend backwards beneath the 

 large axillary trunks. 



On the inner boundary, at the upper part, are a few small branches of 

 the superior thoracic artery, which ramify on the serratus muscle ; but 

 these are commonly so unimportant, that this part of the axillary space 

 may be considered free from vessels with respect to any surgical operation. 

 Lying on the surface of the serratus magnus, is the nerve to that muscle ; 

 and perforating the inner boundary of the space, are the lateral cutaneous 

 nerves of the thorax two or more offsets of which are directed across the 

 axilla to the arm, and receive the name intercosto-humeral. 



The lymphatic glands of the axilla are arranged in two sets : one is 

 placed along the inner side of the bloodvessels ; and the other occupies the 

 lower and hinder parts of the space, lying near and along the posterior 

 boundary. Commonly they are ten or twelve in number; but in number 

 and size they vary much. Small vascular twigs from the branches of the 

 axillary vessels are furnished to them. 



The glands by the side of the bloodvessels receive the lymphatics of the 

 arm ; and those along the hinder boundary are joined by the lymphatics 

 of the fore part of the thorax and posterior surface of the Back, as well as 

 by some from the mamma. Most of the efferent ducts unite to form a 

 trunk, which opens into the lymphatic duct of the neck of the same side ; 

 some may enter separately the subclavian vein. 



The PECTORALIS MAJOR, A, is triangular in shape, with the base at the 

 thorax and the apex at the arm. It arises internally from the front of 

 the sternum, and the cartilages of the true ribs except the last ; superiorly 

 from the sternal half of the clavicle ; and inferiorly from the aponeurosis 

 of the external oblique muscle of the abdomen. From this wide origin 

 the fibres take different directions those from the clavicle being inclined 

 obliquely downwards, and those from the lower ribs upwards beneath the 

 former ; and all end in a tendon, which is inserted into the outer edge of 

 the bicipital groove of the humerus for about two inches. 



This muscle bounds the axilla anteriorly, and is connected sometimes 

 to its fellow by fibres in front of the sternum. Besides the superficial 

 structures and the mamma, the platysma covers the pectoralis major close 

 below the clavicle. A lengthened interval, which corresponds with a de- 

 pression on the surface, separates the clavicular from the sternal attach- 

 ment. One border (outer) is in contact with the deltoid muscle, and with 

 the cephalic vein and a small artery ; and the lower border forms the 

 margin of the anterior fold of the axilla. The parts covered by the muscle 

 will be seen subsequently. 



Action. If the humerus is hanging, the muscle will move forwards the 

 limb until the elbow reaches the front of the trunk, and will rotate it in. 



When the limb is raised, the pectoralis depresses and adducts it ; and 

 acting with other muscles inserted into the opposite side of the humerus, 

 it may dislocate the head of that bone when the lower end is fixed, as in 

 a fall on the elbow. 



