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 layer. 



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 develop, it does not grow into a 

 taenia; but when the muscle entirely surrounds the tube, as in the human colon, two 

 lateral thickenings appear, one on each side of the bowel equidistant from the 

 mesentery, which become true taeniae (fig. 7). In the caecum, however, which is 

 much longer than in man, a third taenia is present. It extends from the ilio-caecal 

 junction to the tip of the caecum along the line of the meso-caecal attachment, corre- 



