INTERNAL OBLIQUE GROUP 



433 



column; and (4) to rotate the column toward the opposite side. With the thorax fixed it 

 serves to flex and rotate the pelvis. 



Relations. — It lies superficial to the lower ventro-lateral margin of the thorax and the 

 internal oblique muscle. It is partly covered by the latissimus dorsi muscle behind. Other- 

 wise it is subcutaneous. 



Variations. — It may have a more or less extensive origin from the ribs. Broad fasciculi 

 not infrequently are separated by loose tissue from the main belly of the muscle either on its 

 deep or superficial surface. Occasionally tendinous inscriptions are found. These transverse 

 inscriptions are constant in many of the smaller mammals. The supracostalis anterior is a 

 rare fasciculus sometimes found on the upper portion of the thoracic wall. It is usually supplied 

 by branches of the upper thoracic nerves and seems to be a continuation upward of the external 

 oblique muscle. In some prosimians the external oblique extends normally to the first or sec- 

 ond rib. 



3. Internal Oblique Group 



The intercostales interni (figs. 385, 386, 388). — These extend in the intercostal spaces from 

 the angles of the ribs to the sternal ends of the spaces. The upper and lower muscles are usually 

 continued dorsally slightly beyond the angles of the ribs, while the intermediate muscles fre- 

 quently do not quite reach them. Dorso-medially the internal intercostals are continued in 



Fig. 389. — Structures op Inguinal Region. 



Spermatic cord "ft 



Aponeurosis of 

 '^ \\\ obliquus externus 



Inguinal ligament 



Subcutaneous 

 inguinal ring 



Fossa ovalis 



the form of thin fascial sheets across the inner surface of the external intercostals and become 

 fused with the subcostals. 



Origin. — Near the angles of the ribs they arise from the internal lip of the costal sulcus. 

 More ventrally they arise mainly from the external lip of the sulcus, but also in part from the 

 internal lip. 



Structure and insertion. — The fibre-bundles take a parallel course downward and dorsal- 

 ward to the upper margin of the rib below. They are less obHquely placed than those of the 

 external intercostals. The muscles are thicker in front and grow thinner dorsaUy. They con- 

 tain less fibrous tissue than the external intercostals. 



Nerve-supply. — From numerous branches of the corresponding intercostal nerves. 



Action. — Investigators disagree as to the functions. It is probable that the portions of 

 the muscles between the ribs serve to contract, those between the costal cartilages to expand, 

 the thorax. 



Relations. — Between the ribs they are covered by the external intercostal muscles and be- 

 tween the costal cartilages by the external intercostal ligaments. Between the internal and 

 external muscles there is some loose areolar tissue. Proximally, for a short distance, the inter- 

 costal nerve in each interspace runs between the external and internal intercostal muscles, but 

 more distally it runs first in the substance of and then on the internal surface of the internal 

 intercostal. Eisler distinguishes that portion of the internal intercostal muscle which lies 

 external to the nerve as the intercostalis intermedius, that which lies internal as the true in- 

 ternal intercostal. The terminal branches of the first six nerves, however, pass through the 

 muscle on their way to the skin, while the last six pass beneath the inferior margin of the thorax. 

 Internal to the internal intercostal muscles lie the transversus (triangularis sterni) and sub- 



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