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594 THE NERVOUS SYSTEM. 
lateral border of the sternum, it passes forwards in front of the internal mammary 
artery and through the internal intercostal muscle, and the aponeurosis of the 
external intercostal muscle, and ends by supplying the skin of the front of the 
chest over the second intercostal space. 
The nerve supplies the following branches :— 
1. Muscular branches to the muscles of the second intercostal space. 
2. Cutaneous branches. (a) Anterior terminal branches (rr. cutaneus anterior) to 
the skin over the second intercostal space (Fig. 438). (4) A large lateral cutaneous braneh, 
the intercosto-humeral nerve (n. intercosto-brachialis) (Fig. 429, p. 581). This nerve 
pierces the intercostal muscles and the serratus magnus, and, crossing the axilla, extends 
to the arm. It pierces the deep fascia just beyond the posterior fold of the axilla, and 
can be traced down the arm as far as the interval between the internal condyle of the 
humerus and the olecranon process. It supplies an area of skin stretching across the 
armpit and along the posterior surface of the arm on the inner side as far as the elbow 
(Fig. 432, p. 586). 
The intercosto-humeral nerve varies in size. It may pierce the first intercostal space, and it 
is often divisible into anterior and posterior branches, like the lateral branch of an ordinary 
intercostal nerve. 
Communications. — (1) The intercosto-humeral nerve communicates with two 
adjacent nerves. Either before or after piercing the fascia of the axilla it is joined by the 
lesser internal cutaneous nerve of the brachial plexus. It also communicates with the 
posterior part of the lateral branch of the third intercostal nerve by means of the branches 
distributed to the floor and boundaries of the axilla. It may supply the axillary arches, 
when present. (2) Besides the branches referred to, the second thoracic nerve in many 
cases transmits a nerve to the brachial plexus, which becomes incorporated with the first 
thoracic nerve after passing over the neck of the second rib. This branch is inconstant. 
As already mentioned, it may join only the intercostal part of the first thoracic nerve, it 
may join the brachial plexus only, or it may send branches to both parts of the first thoracic 
nerve. (3) Besides the communications effected by branches of the second thoracic nerve 
ee Poa in its course, it also receives a gray ramus 
BRANCH BRANCH communicans from the second thoracic gan- 
glion of the sympathetic cord in the thorax. 
It probably also sends to the sympathetic 
GST Rich the first white ramus communicans, though 
PRIMARY bal, 7 : 
DIVISION this is not known with certainty. 
ROOT 
MIXED 
SPUIMAL 
NERVE, 
The third thoracic nerve only differs 
from a typical thoracic nerve in one 
ANTERIOR 
(VENTRAL) ANTERIOR respect. Its lateral branch divides in 
LOOT COM. PRIMARY ‘ a 4 
sy COMMUNICA. \NOIVISION the usual way into anterior and posterior 
parts, of which the latter is carried to the 
arm and supplies an area of skin on the 
posterior half of the inner side near the 
root of the imb. It effects a junction with 
the intercosto-humeral (Fig. 429, p. 581). 
The fourth, fifth, and sixth thoracic 
Cae nerves have a course and distribution 
which is simple and typical. Except 
1 net for the peculiarities above mentioned, 
fp sRiMcH the second and third thoracic nerves have 
a similar distribution. 
The nerves appear on the posterior 
wall of the thorax, in the subcostal groove 
of the corresponding rib. They extend 
forwards between the intercostal muscles 
as far as the middle of the chest wall, 
lying at a lower level than the intercostal vessels. At the side of the chest 
each nerve passes obliquely through the internal intercostal muscle, and comes to 
lie upon the pleura, triangularis sterni muscle, and internal mammary artery. 
Thereafter, piercing the fibres of the internal intercostal muscle and the 
aponeurosis of the external intercostal muscle, each nerve ends by supplying the 
POSTERIOR 
BRANC/T 
ANTERIOR 
TRON 
ANTIERIOR 
BRANCH : 
Fic. 437.—ScHEME OF THE DISTRIBUTION OF A 
TypicaL SPINAL NERVE. 
ilge 
