THE MUSCLES OF THE THORAX 283 
Nerve-supply.—Intercostal nerves. 
2. External intercostals (Mm. intercostales externi)—Each of these occupies 
an intercostal space, from the levatores to the sternal extremity of the rib. They 
do not occupy the intercartilaginous spaces. 
Origin.—The posterior borders of the ribs. 
Insertion —The anterior borders and lateral surfaces of the succeeding ribs. 
Action.—To draw the ribs forward in inspiration. 
Structure —The fibers are directed downward and backward. There is a 
considerable admixture of tendinous tissue. The thickness of the muscles grad- 
ually diminishes toward the lower ends of the spaces. 
Relations.—Superficially, the serratus ventralis, latissimus dorsi, serratus 
dorsalis, longissimus dorsi, longissimus costarum, rectus thoracis, deep pectoral, 
obliquus abdominis externus, and cutaneous muscles; deeply, the internal inter- 
costals and (in the upper part of the spaces) the intercostal vessels and nerves. 
Blood-supply.—Intercostal and internal thoracic arteries. 
Nerve-supply.—Intercostal nerves. 
3. Internal intercostals (Mm. intercostales interni)—These extend the en- 
tire length of the intercostal spaces, including their interchondral portion. 
Origin.—The anterior borders of the ribs and their cartilages. 
Insertion —The posterior borders of the preceding ribs and cartilages. 
Action—To draw the ribs backward in expiration. It seems probable, how- 
ever, that the intercartilaginous portion is inspiratory. 
Structure-—The direction of the fibers is oblique downward and forward. 
There is a smaller amount of tendinous tissue than in the external set, and the 
thickness diminishes dorsally. In the upper part of the spaces fibers sometimes 
cross a rib in a fashion similar to the subcostals of man. A thin fascia separates 
the internal from the external intercostal muscle in each space. 
Relations.—Superficially, the levatores costarum and the external intercostals; 
deeply, the endothoracic fascia and pleura, the transversus thoracis, diaphragm, 
transversus abdominis, and the internal thoracic and musculo-phrenic vessels. 
In the upper part of the intercostal spaces the intercostal vessels and nerves lie 
between the internal and external intercostal muscle, but below they lie chiefly on 
the deep face of the internal muscle. 
Blood-supply.—Intercostal and internal thoracic arteries. 
Nerve-supply.—Intercostal nerves. 
J The muscles in connection with the costal cartilages are sometimes distinguished as Mm. 
intereartilaginei; their direction is similar to that of the internal intercostal, and they cover the 
cartilages of the asternal ribs more or less. At the ventral ends of some of the intercostal spaces 
there is a layer of longitudinal muscle. 
4. Retractor coste.—This is a small triangular muscle which lies behind the 
last rib, chiefly under cover of the serratus dorsalis. 
Origin.—The transverse processes of the first three or four lumbar vertebrze 
by means of the lumbar fascia. 
Insertion.—The posterior border of the last rib. 
Action.—To retract the last rib. 
Structure-—The muscle arises by a thin aponeurosis. Its fibers are parallel 
to those of the adjacent internal oblique. 
Relations.—Superficially, the serratus dorsalis and external oblique; deeply, 
the transversus abdominis. 
Blood-supply.—Lumbar arteries. 
Nerve-supply.—Lumbar nerves. 
5. Rectus thoracis.°—This is a thin muscle which lies under cover of the deep 
The function of the intercostal muscles is still a subject of much discussion. The state- 
ments made above seem to represent the view most commonly held in regard to their action. 
= : : 
* Also known as the transversus costarum or lateralis sterni. 
