294 THE SPINAL NERVES. 



B. BRANCHES BELOW THE CLAVICLE. 



Anterior thoracic nerves. The anterior thoracic nerves, two in number, 

 supply the pectoral muscles. 



The external, or more superficial branch, arising from the outer cord under cover 

 of the clavicle, is derived from the sixth and seventh nerves, very frequently also the 

 fifth. It crosses inwards over the axillary artery, and, after giving off a branch to 

 join the inner nerve, terminates in the great pectoral muscle. 



The internal, or deeper branch, springing from the inner cord, mostly receives 

 fibres from both the last cervical and first dorsal nerves. It comes forwards between 

 the axillary artery and vein, and is joined by the communicating branch from the 

 external nerve, with which it forms a plexiform loop embracing the axillary artery. 

 From this loop offsets proceed to the small and to the lower part of the large 

 pectoral muscles. 



Varieties. The internal branch occasionally has no fibres from the first dorsal nerve. 

 Cutaneous filaments from the anterior thoracic nerves to the mammary region and to the arm 

 have been described by various observers. The external branch is said to supply twigs 

 sometimes to the clavicular part of the deltoid ; and, according to Bock and Valentin, a 

 filament is given to the acromio-clavicular articulation. 



Subscapular nerves. The subscapular nerves are usually three in number, 

 and supply the muscles forming the posterior wall of the axilla. 



The upper nerve, the smallest of the three, is derived from the fifth and sixth 

 cervical nerves, and penetrates the upper part of the subscapularis muscle. This 

 branch is often double. 



The middle or long subscapular nerve (n. thoraco dor sails) is the largest of the 

 three, and obtains its fibres mainly from the seventh, to a less extent in many cases 

 also from the sixth or eighth cervical nerves. It descends in company with the 

 subscapular artery to enter the deep surface of the latissimus dorsi muscle. 



The lower subscapular nerve is derived from the fifth and sixth, or less frequently 

 the sixth and seventh cervical nerves. It gives one or two branches to the lower 

 part of the subscapularis, and ends in the teres major muscle. 



Varieties. The upper nerve occasionally proceeds from the fifth or sixth cervical nerve 

 alone. The most frequent origin of the long subscapular ncrrc is from the seventh alo.ie, or 

 from the seventh and eighth nerves ; it rarely receives fibres from the fifth. The lower nrrcc 

 seldom proceeds from the fifth or sixth nerve alone, or from the fifth, sixth, and seventh 

 nerves. The branches to the lower part of the subscapularis and the teres major muscles are 

 sometimes independent offsets of the brachial plexus. The nerve to the teres major is 

 occasionally given off by the commencement of the circumflex nerve. 



Circumflex nerve. The circumflex nerve (n. axillaris), one of the terminal 

 branches of the posterior cord of the plexus, obtains its fibres from the fifth and 

 sixth cervical nerves. It is placed at first behind the axillary artery, resting on the 

 subscapularis muscle, at the lower border of which it turns backwards with the 

 posterior circumflex vessels. It then appears at the back of the shoulder in the 

 quadrilateral space between the two teres muscles (fig. 194), external to the long 

 head of the triceps, and divides into branches, which are distributed to the deltoid 

 and teres minor muscles, the integument of the shoulder, and the shoulder-joint. 



(a) The upper branch winds round the upper part of the shaft of the humerus, 

 extending nearly as far as the anterior border of the deltoid muscle, to which it is 

 distributed. One or two cutaneous filaments penetrate between the muscular fibres, 

 and are bent downwards to supply the integument over the lower part of the muscle. 



(b) The lower branch supplies offsets to the back part of the deltoid, and 



