570 THE BLOOD-VASCULAR SYSTEM 



Behind, it rests upon the first intercostal space and first intercostal muscle, the first digita- 

 tion and sometimes a portion of the second digitation of the serratus anterior (magnus) muscle, 

 and a part of the second rib. The long thoracic nerve, on its way to the serratus anterior 

 muscle, passes behind it. 



To its lateral side, and somewhat on a higher plane, are the cords of the brachial plexus. 



To its medial side, and on a slightly anterior plane, is the axillary vein. The internal 

 anterior thoracic nerve courses between the vein and the artery. 



The Second Part of the Axillary Artery 



The second part of the axillary artery (fig. 464) lies beneath the pectoralis 

 minor deep in the axilla. It measures 3 cm. (a little more than 1 in.) in length. 



Relations. — In front, in addition to the pectoralis minor, it is covered by the pectoralis 

 major and the integuments. 



Behind, it is separated by a considerable interval, containing loose connective tissue and 

 fat, from the subscapularis muscle; whilst behind, and in contact with it, is the posterior cord 

 of the brachial plexus. 



To the medial side, but separated from the artery by the medial cord of the brachial 

 plexus, is the axiUary vein. 



To the lateral side is the lateral cord of the brachial plexus, and at some little distance 

 the coracoid process. 



It is thus seen that the second portion of the auxiliary artery is surrounded on three sides 

 by the cords of the brachial plexus — one behind, one medial, and one lateral. 



The Third Part of the Axillary Artery 



The third part of the axillary artery (fig. 464) extends from the lower border 

 of the pectoralis minor to the lower border of the teres major. Its upper half lies 

 deeply placed within the axilla, beneath the lower edge of the pectoralis major 

 muscle, but its lower half is in the arm external to the axilla, and is uncovered by 

 muscle. It measures about 7.5 cm. (3 in.) in length. 



Relations. — In front it has, in addition to the skin and superficial fascia, the pectoralis 

 major above, and lower down the deep fascia of the arm. It is crossed obliquely by the medial 

 root of the median nerve and by the lateral brachial vena comitans. 



Behind, it lies successively upon the subscapularis, the latissimus dorsi, and teres major 

 muscles. From the first-named muscle it is separated at first by a considerable mass of fat 

 and cellular tissue. The radial (musculo-spiral) and axillary (circumflex) nerves intervene 

 between the artery and the muscles. 



On its lateral side it is separated from the bone by the coraco-brachialis, by which it is 

 partly overlapped, this muscle and the short head of the biceps serving as a guide to the artery 

 in ligature. For a part of its course it has also the musculo-cutaneous nerve and the lateral 

 root of the median nerve to its lateral side. 



To its medial side it has the axillary vein, the ulnar nerve, the medial antibrachial (internal) 

 and brachial (lesser internal) cutaneous nerves, and the medial root of the median nerve. 

 The ulnar nerve is between the artery and the vein. The medial antibrachial (internal) 

 cutaneous nerve is a little in front of the artery as well as medial to it. 



Branches of the Axillary Artery 



The branches of the axillary artery are exceedingly variable. In fig. 465 is 

 shown what Hitzrot has found the usual type, in which the second portion of 

 the artery has no named branches. The figure brings out the segmental relation 

 of the branches of the axillary artery to tlie chest wall and suggests how one of the 

 branches may supply the place of another. If the lateral thoracic arises directly 

 from the axillary, it is generally from the second part as described below. In 

 addition to the larger branches of the artery small twigs are supplied to the ser- 

 ratus anterior, coraco-brachialis, and subscapularis; also to the axillary lymph- 

 nodes. 



The first part gives off. — (1) The superior thoracic; and (2) the thoraco- 

 acromial. 



The second part gives off: — (3) The lateral thoracic. 



Th(; third part gives off: — (4) The subscapular; (5) the anterior humeral 

 circumflex; and (6) the posterior humeral circumflex. 



Branches of the Axillary Artery 



1. The superior thoracic [a. thoracalis suprcma] is variously given off from the 

 axillary artery, usually either as a common trunk with the next branch, the 



