Instead of following the same direction, then, let us turn a sharp switchback from the PSIS on the most posterior fibers of the internal oblique, which head upward and forward to the lower ribs. Laid over this is the more superficial track from the ASAS, consisting of the more posterior fibers of the external oblique, which go upward and backward. The fibers of both these muscles are nearly vertical along the lateral aspect of the trunk, but still take an oblique direction so that they form an “X” (Fig 5.9). If you pinch your waist at the side, the fibers of the external oblique, running up and back from the ASIS, will be more superficial. Deep to this will be the internal oblique, palpably running up and forward.
These abdominal obliques attach to the lower floating and abdominal ribs. We can move up from here using both the ribs themselves and the muscles between them. The lateral aspect of the rib cage is likewise crisscrossed with a similar pattern of myofasciae; the external intercostals running backward and up, the internal inercostals running forward and up. These muscles continue the same pattern all the way up the rib cage, under the overlying shoulder girdle and its associated muscles, up to the first ribs at the bottom of the neck (See Fig 5.6A9
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