. Carnegie Institution of Washington publication. STUDIES ON THE LONGITUDINAL MUSCLE OF THE HUMAN COLON. 39 fetus, No. 448 C. C. (figs. 3 and 4), this remaining portion is occupied by definitely formed muscle-fibers which are, however, loosely scattered in the zone, whereas those at the mesenteric arc are compact. This may cause the erroneous conclusion that the mesenteric arc is the only part of the muscle present at this stage, but a careful search reveals the scattered fibers as just mentioned. This state of the muscle continues unaltered for some time, no marked changes being noted until n
. Carnegie Institution of Washington publication. STUDIES ON THE LONGITUDINAL MUSCLE OF THE HUMAN COLON. 39 fetus, No. 448 C. C. (figs. 3 and 4), this remaining portion is occupied by definitely formed muscle-fibers which are, however, loosely scattered in the zone, whereas those at the mesenteric arc are compact. This may cause the erroneous conclusion that the mesenteric arc is the only part of the muscle present at this stage, but a careful search reveals the scattered fibers as just mentioned. This state of the muscle continues unaltered for some time, no marked changes being noted until near the 90-mm. stage. At this time the entire circle of fibers becomes compact, with the mesenteric portion still prominent (fig. 5). This harmonizes with what Lewis stated concerning the arc, and also with Broman's statement about a con- tinuous Fig. 3.—Sagittal section of a human fetus 52 mm. CR. length (No. 448, Carnegie Collec- tion), showing both dorsal and ventral portions of the muscle extending into the abdominal region. The important development up to the 90-mm. stage is the appearance of the longitudinal muscle in the rectum, where it is a complete layer entirely surrounding the tube. By rapid growth it extends to the caecal end, first along the fine of the mesenteric attachment, then (closely following this) the rest of the circle becomes occupied by muscle-fibers which are at first loosely scattered but soon become condensed into a well-defined layer. The condition in the pig is quite similar with the exception of a few minor differences. Shortly after the muscle appears in the pelvis it is also seen in the csecal end with apparently no muscle in the bowel between these regions. Growth then proceeds from both ends and, as in the human intes- tine, the muscle rapidly fills the entire length of the colon. Another peculiarity is that the two bands in the pig's colon have a striking position. Although the mesenteric arc is the first part of the muscle to devel
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