The brachial plexus divides into two distinct parts. The first (Fig. 73, No. 20) the dorsal called the superior thoracic is distributed to the serratus and the rhomboideus muscle. The second is the inferior thoracic (Fig. 73, No. 21), which consists of the main plexus, and which gives off muscular branches especially to the sterno-coracoideus and to the main portion passing on as the nerve trunk to the structures of the wing. From the superior thoracic plexus there are several secondary plexuses formed. The _dorsalis_ or _serratus plexus_. This plexus is located adjacent to the anterior part of the main plexus and is formed of from two to four spinal nerve branches. The rhomboideus is supplied by branches which may be traced to the first root of this plexus. This nerve is called the rhomboideus superficialis. Another nerve is given off from the middle part of the dorsal side of the plexus and is called the rhomboideus profundus. A third, the superficialis serratus, is the largest of the branches. It breaks up into terminal branches, one going to each serration, or digitation, of the serratus muscle. Branches from the anterior portion of this plexus are distributed to the patagii muscles. The superior brachial plexus gives off the following nerves: The _subcoraco-scapularis_, which is purely a motor nerve, springs from the anterior roots of the plexus. It gives off a branch to the following three muscles: subcoracoideus, subcoracoido-scapularis, and scapulo-humeralis. The _scapulo-humeralis_ is distributed to the scapular humeral region. The _latissimus dorsi_ is located on the peripheral border of the scapulo-humeralis or supraspinatus. It originates from the second, and the fourth nerve roots of the plexus. It is located on the dorsal side of the plexus. It divides into two main branches, one going to the latissimus dorsi and the other to the scapulo-humeralis muscle. From the side are given off branches which supply the patagii muscle and enter into the formation of the dorso-cutaneous plexus. The _axillaris_ springs from the second and the third root of the brachial plexus, extends in a lateral direction, passing the ventral and distal rim of the insertional part of the posterior scapulo-humeralis or teres et infraspinatus and enters near the capsular ligament of the shoulder-joint. It gives off to this joint a small branch, called the recurrent articularis, which passes outward between the triceps brachialis and the humerus. It lies on the inner side of the major deltoid, and patagii muscles, and also the skin of the lateral shoulder and the upper arm region. The recurrent axillaris communicates with the main branch of the radial nerve.