Obliquus Internus (Internal Abdominal Oblique)
A fibrous band or inscription frequently interrupts the internal oblique and the intercostal muscles. In rare instances a similar inscription has been seen opposite the twelfth or even the eight rib. They apparently represent tendinous intersections between the internal oblique and the internal intercostal muscles. An additional slip occasionally arises from the eight costal cartilage.
It has been reported that the internal oblique inserts onto the eleventh rib in 1% of cases, the tenth in 66.5%, the ninth in 31%, and the eighth rib in 1.5% of cases. Mori reported that in 200 Japanese males, the highest rib of insertion was the eleventh , on the right side, in 34% and on the left in 36% and the tenth, on the right side in 56% and on the left in 64%. In a comparison of Polish subjects by Loth compared with Japanese by Mori follows:
Absence of the inguinal portion or of the anterior superior part of the muscle has been recorded. Fusion of the lower part of the muscle with the transversus abdominis may occur. A deep slip was found inserted on the transverse process of the second lumbar vertebra in one case.
The part of the internal oblique that arises from the iliac crest is often thick and may be divided into an anterior and posterior part (accessory internal oblique). Macalister found a highly developed internal oblique that was traversed by the spermatic cord.
Syn.: m. obliquus abdominis internus, s. ascendens, s. oblique ascendens, Innerer schiefer Bauchmuskel, Petit oblique.
References
Anson, B.J., Ed. (1966) Morris' Human Anatomy, 12th ed., The Blakiston Division, McGraw-Hill Book Company, New York.
Gruber, W. (1873) Un cas de muscle oblique interne de l'abdomen, prive completement de sa portion inguinal. Bull. l'Acad. Imp. Sci. St. Petersbourg. 18:157-158.
Henle, J. (1871) Handbuch der Muskellehre des Menschen, in Handbuch der systematischen Anatomy des Menschen. Verlag von Friedrich Vieweg und Sohn, Braunschweig.
Latarjet, A. (1948) Testut's Traité D'Anatomie Humaine, 9th ed., G. Doin & Cie., Paris.