242 FASCIA AND MUSCLES OF THE HORSE 



3. Internal intercostals (Mm. intercostales interni). — These occupy the 

 entire 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 ol^lique downward and forward. 

 There is a smaller amount of tendinous tissue than in the external set, and the 

 thickness diminishes from below upward. In the upper part of the spaces fibers 

 sometimes cross a rib in a fashion similar to the subcostals of man. A thin 

 aponeurosis 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 asternal vessels. In the 

 upper part of the intercostal spaces the intercostal vessels and nerves lie between 

 the internal and external intercostal muscle, l)ut below the}' lie chiefly on the deep 

 face of the inner muscle. 



Blood-supply. — Intercostal and internal thoracic arteries. 



Nerve-supply. — Intercostal nerves. 



4. Retractor costae. — This is a small triangular muscle which lies behind the 

 last rib, chiefly under cover of the serratus posticus. 



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 ril). 



Structure. — The muscle arises by a thin aponeurosis. Its fibers are parallel 

 to those of the adjacent internal oblique. 



Relations. — Superficially, the serratus posticus and external oblique; deeply, 

 the transversus al)dominis. 



Blood-supply. — Lumbar arteries. 



Nerve-supply. — Lumbar nerves. 



5. Rectus thoracis (M. transversus costarum; lateralis sterni). — This is a 

 thin muscle which lies under cover of the deep pectoral muscles. It is directed 

 obli(iuely backward and downward, and crosses the lower part of the first 

 three intercostal spaces. 



Origin. — The outer surface of the first rib, below the scalenus. 



Insertion. — The cartilage of the third or fourth rib. The aponeurosis usually 

 joins the rectus abdominis. It may reach the sternum. 



Action. — It may assist in inspiration or concur with the rectus abdominis. 



Relations. — Superficially, the deep pectoral muscles; deeply, the intercostal 

 muscles and the ribs. 



6. Transversus thoracis (Triangularis sterni). — This is a flat muscle situated 

 on the thoracic surface of the st(>rnum and the cartilages of the sternal ribs. 



Origin. — The sternal ligament. 



Insertion. — The cartilages of the ribs, from the second to the eighth inclusive. 



Action. — It draws the costal cartilages inward and backward, thus assisting 

 in expiration. 



Structure. — Each muscle has the form of a scalene triangle, of which the base 

 is the strongly serrated external border. The muscle contains a good deal of tend- 

 inous tissue. The anterior bundles are directed forward and outward; the poste- 

 rior backward and outward. 



^ The function of the intercostal muscles is still a subject of much discussion. The statements 

 made above seem to represent the view most commonly held in regard to their action. 



