4 ey}? Pe ut ¥ , 
Composition of the Brachial Plexus.—In an analysis of the brachial plexus 
four stages may be always seen :— 
(1) The undivided nerves. 
(2) The separation of the nerves into ventral (anterior) and dorsal (posterior) 
trunks; and the formation of three primary cords. 
(3) The formation of three secondary cords—outer, inner, and posterior. 
(4) The origin of the nerves of distribution. 
THE BRACHIAL PLEXUS. 581 
LAT 
CrTe. 
vA OUTER CORD ; i 
AN ) 
———— 
\Subev. — 
PY: ae 
eS ~_SNIVER CORD \ 
= rf SS \ 
/. 
va 
> / 
: a 
> ‘ 
WA, 
Fic. 429.—THk NERVES OF THE BRACHIAL PLEXUS. 
Sy, Sympathetic gangliated cord ; Phr, Phrenic nerve ; C.4, 5, 6, 7, 8, T.1, 2, 3, Anterior primary divisions of the 
lower cervical and upper thoracic nerves; M!, M?, Muscular branches to axial muscles ; P.T, Posterior 
thoracic nerve; Rh, Nerve to rhomboids (posterior scapular); Subel, Nerve to subclavius muscle ; 
Int, Intercostal nerves ; 8.Sc, Suprascapular nerve, ‘The intercostal part of the first thoracic nerve is 
omitted. 
Outer Cord.—E.A.T, External anterior thoracic nerve ; M.C, Musculo-cutaneous nerve ; Cb, Nerve to coraco- 
brachialis ; M, Median nerve. 
Inner Cord.—1.A.T, Internal anterior thoracic nerve ; U, Ulnar nerve ; I.C, Internal cutaneous nerve ; L.I.C, 
Lesser internal cutaneous nerve. 
Posterior Cord.—Cire, Circumflex nerve ; M.S, Musculo-spiral nerve ; 8.Sub, Short subscapular nerve ; M.Sub, 
Lower subscapular nerve ; L.Sub, Long subscapular nerve ; I.H, Intercosto-humeral nerve ; Lat, Lateral 
branch of third intercostal nerve. 
(1) The undivided nerves have only a very short independent course at the side 
of the neck, after passing between the scalene muscles. 
(2) Almost immediately after entering the posterior triangle there are formed 
three primary cords: the first cord is formed by the union of the fifth and sixth 
nerves together; the second, by the seventh nerve alone; and the third, by the 
union of the eighth cervical and first thoracic nerves together. While these cords 
are being formed, a division occurs in each of the last four cervical nerves, into 
ventral (anterior) and dorsal (posterior) trunks; the first thoracic nerve usually 
gives rise to no dorsal trunk. The ventral and dorsal trunks of the fifth, sixth, 
and seventh nerves are nearly equal in size. The dorsal trunk of the eighth 
cervical nerve is much smaller. 
(3) The secondary cords of the plexus are formed by combinations of these 
ventral and dorsal trunks in relation to the axillary artery. They are three in 
number. The outer cord (fasciculus lateralis) is formed by a combination of the 
ventral trunks of the fifth, sixth, and seventh nerves, and lies on the outer side of 
the axillary artery. The inner cord (fasciculus medialis) is formed by a combina- 
tion of the ventral trunk of the eighth cervical with the whole of the first thoracic 
nerve engaged in the formation of the plexus; it lies on the inner side of the 
axillary artery. The posterior cord (fasciculus posterior) is made up of all the 
