310 THE MUSCULAR SYSTEM. 
passing outwards along the upper border of the pectoralis minor, it reaches the 
coracoid process. The part of the membrane extending directly between the first 
costal cartilage and the coracoid process is thickened and forms the costo-coracoid 
ligament. The costo-coracoid membrane is otherwise thin and of comparatively 
small importance. It is pierced by the cephalic vein, thoracic axis artery, and 
branches of the external anterior thoracic nerve. By its deep surface it is connected 
to the sheath of the axillary vessels. 
At the lower border of the pectoralis minor there is a further extension of the 
deep fascia beneath the pectoralis major. It passes downwards to join the fascia 
forming the floor of the axilla, and is continued externally into the fascia covering 
the biceps and coraco-brachialis muscles. 
MUSCLES OF THE PECTORAL REGION. 
The muscles connecting the upper limb to the axial skeleton anteriorly comprise 
the pectoralis major, pectoralis minor, subclavius, serratus magnus, and sterno- 
cleido-mastoid (the last described in a later section). 
The pectoralis major is a large fan-shaped muscle arising in three parts :—(1).- 
a clavicular portion arising from the front of the clavicle in its inner half or two- 
thirds ; (2) @ costo-sternal portion, the largest part of the muscle, arising from the 
anterior surface of the pre-sternum and meso-sternum by tendinous fibres decus- 
sating with those of the opposite muscle: and more deeply from the cartilages of the 
first six ribs; (5) an abdominal portion, a small and separate slip, arising from the 
aponeurosis of the obliquus externus muscle. The abdominal slip, at first separate, 
soon merges with the costo-sternal portion, but a distinct interval® usually remains 
between the two first-named parts of the muscle. The fibres converge towards the 
upper part of the arm, and are inserted into (1) the outer border of the bicipital groove 
of the humerus, extending upwards to the great tuberosity, and blending externally 
with the insertion of the deltoid, internally with the insertion of the latissimus 
dorsi; (2) from the upper border of the insertion a membranous band extends 
upwards to the capsule of the shoulder joint, enveloping at the same time the 
tendon of the biceps; and (3) from the lower border a band of fascia passes down- 
wards to join the fascia of the arm. 
The arrangement of the fibres of the muscle at its insertion is peculiar. The — 
muscle is twisted on itself, so that the lower (costo-sternal) fibres are directed _ 
upwards and outwards behind the upper (clavicular) part of the muscle; in conse- 
quence the clavicular part is attached to the humerus lower down than the costo- 
sternal portion, and is inserted also into the fascia of the arm. 
Placed superficially in the chest, the pectoralis major, by its lower or outer — 
border, forms the anterior fold of the axilla. Its upper border is separated from the 
edge of the deltoid muscle by an interval in which lie the cephalic vein and 
humeral artery. Its deep surface is in relation with the ribs and intercostal 
muscles, the costo-coracoid membrane and the structures piercing it, the pectoralis 
minor, the axillary vessels, and the nerves of the brachial plexus. 
Sternalis muscle.—The sternalis muscle is present in 4:4 cases out of 100. It is slightly 
more frequent in the male than in the female. It has been regarded by different observers as (1) 
a vestige of the panniculus carnosus, (2) an homologue of the sterno-mastoid, or (8) a displaced slip of 
the pectoralis major. It is placed, when present, parallel to the sternum upon the sterno-costal origin 
of the pectoralis major, and has attachments which are very variable, above and below, to the : 
costal cartilages, sternum, rectus sheath, sterno-mastoid, and pectoralis major. Its nerve supply 
is from one or both of the anterior thoracie nerves. In certain rare eases it has been said to be : 
innervated by intercostal nerves. : 
Chondro-epitrochlearis, dorso-epitrochlearis, axillary arches, costo-coracoideus.—One 
or other of the above-named slips is occasionally present, crossing the floor of the axilla in the 
interval between the latissimus dorsi and the pectoralis major. They take origin from the costal ; 
cartilages, ribs, or borders of the pectoralis major (chondro-epitrochlearis, axillary arches, costo- 
coracoideus), or from the border of the latissimus dorsi (dorso-epitrochlearis, axillary arches, costo- 
coracovdeus). Their insertion is variable. The chondro-epitrochlearis and dorso-epitrochlearis are 
inserted into the fascia of the arm on the inner side, the internal intermuscular septum, or the 
internal condyle of the humerus. The azillary arches are inserted into the border of the pectoralis 
major, the fascia of the arm, or the coraco-brachialis or biceps muscle. The costo-coracoideus, 
arising from the ribs or the aponeurosis of the obliquus externus, or detaching itself from the 
