THE MUSCLES OF THE THORAX 283 



Nerve-supply. — Intercostal nerves. 



2. External intercostals (Mm. intercostales extern!). — 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 ex^ternus, antl 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. 



The muscles in connection with the costal cartilages are sometimes distinguished as Mm. 

 intercartilaginei; 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 costae. — 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 vertebrae 

 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. 

 2 Also known as the transversus costarum or lateralis sterni. 



