THE AXILLARY ARTERY. 783 
shoulder the axillary artery is curved over the head of the humerus, and the vein 
lies still more in front of it. 
For descriptive purposes the artery is divided into three parts; the first part 
lies above, the second behind, and the third part below the pectoralis minor 
muscle. 
Though we have followed the usual custom in describing three parts of the 
axillary artery, a division which is perhaps of practical interest in so far as it 
emphasises the fact that the axillary artery is surgically accessible above the 
pectoralis minor, it is to be noted that the upper border of the pectoralis minor is 
usually exactly opposite the outer border of the first rib, at the point where the 
axillary artery begins. In the strict sense, therefore, no part of the artery is 
above the pectoralis minor. 
Relations of the first part.—Posterior.—The first part of the artery is enclosed, 
together with the vein and the cords of the brachial plexus, in a prolongation of the 
cervical fascia known as the axillary sheath, behind which is the upper serration of the 
serratus magnus muscle, the contents of the first intercostal space, the internal anterior 
thoracic and the posterior thoracic nerves, the latter descending vertically between the 
artery and the serratus magnus.  Anterior.—It is covered in front by the costo-coracoid 
membrane, the upper part of which splits to enclose the subclavius muscle. The mem- 
brane intervenes between the artery and the cephalic vein, the branches of the external 
anterior thoracic nerve, the branches of the acromio-thoracic artery with their accom- 
panying veins, and the clavicular part of the pectoralis major muscle, superficial to which 
are the deep fascia, the platysma, the descending clavicular branches of the cervical 
plexus, and the superficial fascia and the skin. Behind the costo-coracoid membrane the 
artery is crossed by a loop of communication between the external and internal anterior 
thoracic nerves. Lateral.Above and to the outer side are the cords of the brachial 
plexus and the external anterior thoracic nerve. Below and to the inner side is the 
axillary vein, the internal anterior thoracic nerve intervening. 
Relations of the second part.—/osterior.— Behind this portion of the artery is 
the posterior cord of the brachial plexus and a layer of fascia which separates it from ‘the 
subscapularis muscle. Anterior.—In front is the pectoralis minor, and more superficially 
the pectoralis major, the fascize and skin. Lateral.—To the outer side lies the outer cord 
of the brachial plexus. On the inner side the inner cord of the plexus lies in close relation 
to the artery, and intervenes between it and the axillary vein. 
Relations of the third part.— Posterior.— The third part of the artery rests 
posteriorly upon the lower border of the subscapularis, the latissimus dorsi, and the 
teres major. It is separated from the fibres of the subscapularis by the circumflex and 
musculo-spiral nerves, and from the latissimus dorsi and teres major by the musculo- 
spiral nerve alone. Anterior.—It is crossed in front by the inner head of the median 
nerve. In its upper half it lies under cover of the lower part of the pectoralis major, 
the fascia and skin, whilst its lower part, which is superficial, is covered by skin and 
fascia only. Lateral.—-To the outer side lie the median and musculo-cutaneous nerves 
and the coraco-brachialis muscle. To the inner side is the axillary vein. The two vessels 
are, however, separated by two of the chief branches of the inner cord of the brachial plexus. 
In the angle between the vein and the artery, and somewhat in front of the latter, lies 
the internal cutaneous nerve; in the angle behind is the ulnar nerve. The feceer 
internal cutaneous nerve lies internal to the vein, and the venz comites of the brachial 
artery ascend along the inner side, to terminate in the axillary vein at the lower border 
of the subscapularis muscle. 
BRANCHES OF THE AXILLARY ARTERY. 
(1) The superior thoracic (a. thoracalis suprema, Fig. 561), a small branch 
which arises from the first part of the axillary at the lower border of the sub- 
clavius. It runs downwards and inwards across the first intercostal space, pierces 
the inner part of the costo-coracoid membrane, and supplies branches to the sub- 
clavius, the peetoralis major and minor, and to the serratus magnus and the inter- 
costal muscles ; it anastomoses with branches of the suprascapular, the internal 
mammary, and the acromio-thoracic arteries. 
(2) The acromio-thoracic (a. thoraco-acromialis, Fig. 561) arises near the upper 
border of the pectoralis minor, from the second part of the axillary artery. It isa 
