-" 2 
590 THE NERVOUS SYSTEM. 
perforates the deep fascia on the inner side of the arm, and is distributed to the skin 
of the upper half or more of the upper arm in its inner side. 
The nerve varies considerably in size. It may be absent, its place being taken by branches 
of the intercosto-humeral or by branches from the internal cutaneous branch of the musculo- 
spiral. It generally bears a distinct relation in size to the intercosto- humeral, due to the fact 
that the size of the latter depends upon the size of the part of the second thoracie nerve con- 
nected with the first in the thorax. If an intra-thoracic connexion occurs between the first and 
second thoracic nerves, the intercosto-humeral may be deprived of a certain number of its 
fibres, which im that case reach the upper limb through the lesser internal cutaneous nerve 
When traced up to the plexus the lesser internal cutaneous is found to have an origin from the 
back of the cord formed by the eighth cervical and first thoracic nerves, and usually recelves 
fibres from the first thoracic nerve only. In cases where “axillary arches” ave present they may 
be supplied by this nerve. ‘ } 
CIRCUMFLEX NERVE. 
The circumflex nerve (n. axillaris), at its origin is just below the supra- 
scapular and comes from the same spinal nerves—the fifth and sixth cervical 
nerves (Fig. 429, p. 581). Extending downwards and outwards behind the axillary 
artery, it leaves the axilla by passing round the external border of the subscapularis 
muscle, in company with the posterior circumflex artery, in a quadrilateral space 
bounded by the humerus, subscapularis, triceps (long head), and teres major. 
Winding round the surgical neck of the humerus from within outwards, it 
terminates under the deltoid by supplying that muscle (Fig. 454, p. 589). 
Branches.—Muscular branches are supplied to the teres minor and deltoid 
muscles. The nerve to the teres minor enters the outer side of the muscle. It 
possesses a pseudo-ganglion, a thickening of fibrous tissue, on its trunk. 
Articular branches enter the back part of the capsule of the shoulder joint. 
A cutaneous branch (n. cutaneus brachii lateralis) of considerable size passes 
obliquely downwards and forwards from beneath the deltoid muscle, becoming 
superficial at its posterior border. Sometimes the branches pierce the muscle. It 
supplies the skin over the insertion of the deltoid and the upper half of the arm 
on the outer side (Fig. £53, p. 588). 
MUSCULO-SPIRAL NERVE. 
The musculo-spiral nerve (n. radialis) appears to be the continuation into the 
upper limb of the posterior cord of the brachial plexus. It usually takes origin 
from all the nerves which form the posterior cord—the fifth, sixth, seventh, and 
eighth cervical nerves (Fig. 429, p. 581). In a minority of cases the first thoracic 
contributes a few fibres, and more frequently the fifth cervical nerve is excluded 
from it. It extends from the axilla round the back of the humerus to the bend of 
the elbow, where it ends by dividing into its terminal branches. 
In the axilla it lies behind the axillary artery, in front of the subscapularis, teres 
major, and latissimus dorsi muscles. 
In the arm, in the upper third, it hes on the inner side of the humerus behind 
the brachial artery, and upon the long head of the triceps. In the middle third of 
the arm it courses obliquely outwards and downwards in the spiral groove of the 
humerus, along with the superior profunda artery, separating the long, external, 
and internal heads of the triceps muscle (Fig. 454, p. 589). In the lower third of 
the arm, piercing the upper part of the intermuscular septum at the outer border 
of the triceps muscle, it descends to the bend of the elbow in front of the external 
condyle of the humerus, in the interval between the brachio-radialis and brachialis 
anticus muscles. Under cover of the former muscle, in the hollow of the elbow, it 
divides into its two terminal branches, the radial and posterior interosseous nerves. 
The collateral branches are in three sets, arising (2) on the inner side, (0) on 
the back, and (¢) on the outer side of the humerus (Fig. 435). 
Branches arising internal to the humerus.—1. Internal cutaneous (n. cutaneus 
brachii posterior).—This branch, arising in common with one of the following, or 
independently, pierces the fascia on the inner side of the arm near the axilla. It 
supplies the skin of the inner side of the arm in the upper third, above and behind 
the area supplied by the lesser internal cutaneous nerve (Fig. 453, p. 588). This 
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