The =brachial plexus= (Plexus brachialis) (Fig. 433) results from
anastomoses established between the ventral branches of the last three
cervical and first two thoracic nerves. It appears as a thick, wide band
between the two parts of the scalenus muscle, and is covered by the
anterior deep pectoral and subscapularis muscles. The largest root of
the plexus is derived from the first thoracic nerve, while the root
supplied by the sixth cervical nerve is quite small. Each of the three
chief roots, _i. e._, those from the last two cervical and the first
thoracic nerve, is connected with the sympathetic by a ramus
communicans.
The =branches= emanating from the plexus go for the most part to the
thoracic limb, but some are distributed on the chest-wall. The names of
the branches, and their arrangement so far as they can be conveniently
examined before removal of the fore limb, are as follows:[207]
1. The large =suprascapular nerve= (N. suprascapularis) turns outward
and forward and disappears between the supraspinatus and subscapularis.
2. The much smaller =subscapular nerves= (Nn. subscapulares), usually
two primary trunks, arise close behind the suprascapular, run backward a
short distance, and divide into several branches which enter the lower
third of the subscapularis.
3. The =anterior thoracic= or =pectoral nerves= (N. pectorales
craniales), three or four in number, arise from the anterior part of the
plexus and from the loop formed by the musculo-cutaneous and median
nerves. One enters the anterior deep pectoral muscle. Another passes out
between the divisions of the deep pectoral to supply the superficial
pectoral, giving a twig usually to the posterior deep muscle. The latter
receives one or two other nerves.
4. The =musculo-cutaneous nerve= (N. musculocutaneus) arises from the
anterior part of the plexus and passes over the outer face of the
brachial artery, below which it is connected by a large but short branch
with the median nerve, thus forming a loop in which the artery lies. One
or two branches to the pectoral muscles are given off from the nerve or
the loop.
5. The =median nerve= (N. medianus) is usually the largest branch of the
brachial plexus. It lies at first above the brachial artery, then passes
between the artery and vein to reach the anterior border of the former.
It is easily recognized by its large size and the loop which it forms
with the musculo-cutaneous nerve.
6. The =ulnar nerve= (N. ulnaris) arises with the median by a short
common trunk. It descends behind the brachial artery and is accompanied
a short distance by the radial nerve.
7. The =radial nerve= (N. radialis) arises from the posterior part of
the plexus and is sometimes the largest branch. It descends behind the
ulnar nerve over the origin of the subscapular artery and the lower part
of the teres major, and dips into the interstice between that muscle and
the long and internal heads of the triceps.
8. The =axillary= or =circumflex nerve= (N. axillaris) arises behind the
musculo-cutaneous. It passes downward and backward on the inner face of
the subscapularis and disappears between that muscle and the subscapular
artery.
9. The =long thoracic nerve= (N. thoracalis longus) is wide and thin. It
passes backward across the surface of the serratus magnus, to which it
is distributed. The branches which enter the muscle are given off both
upward and downward in fairly regular fashion.
10. The =thoraco-dorsal nerve= (N. thoracodorsalis) passes upward and
backward across the subscapularis muscle to ramify in the teres major
and the latissimus dorsi.
11. The =external= (or subcutaneous) =thoracic nerve= arises by a common
trunk with the ulnar. It runs backward and downward across the inner
face of the tensor fasciæ antibrachii, communicates with the anterior
pectoral branches, and gives twigs to the deep pectoral muscle. It then
runs backward in company with the external thoracic or “spur” vein,
gives branches to the latissimus dorsi and the deep pectoral, and
ramifies in the panniculus and the skin of the abdominal wall. It
communicates with perforating branches of the intercostal nerves. A
branch from it, accompanied by a large perforating intercostal branch,
winds around the lower border of the latissimus dorsi and ramifies in
the panniculus on the outer surface of the arm.
The term posterior thoracic or pectoral nerves (Nn. pectorales
caudales) may be used to include 8, 9, and 10.