38 



STUDIES ON THE LONGITUDINAL MUSCLE OF THE HUMAN COLON. 



appearance. In the younger stages, just following the formation of the circular 

 muscle, these aggregations of nerves may be wrongly interpreted as the cut ends of 

 the longitudinal fibers; but proper staining and careful study will reveal their true 



identity. 



The investigation proceeded with the aim of noting the first muscle-fibers 

 which could be unquestionably identified as such, and these were not found earlier 

 than the 40-mm. stage. In a fetus of this size they are seen in the lower end of the 

 intestine. In the extreme caudal end, near the anal canal, a complete layer, quite 

 thin but well defined, completely surrounds the tube. Johnson has shown how they 

 intermingle with the internal sphincter and levator ani muscles in older fetuses, a 

 condition which is seen as early as this 40-mm. stage. A 46-mm. fetus, No. 95 C. C. 

 (fig. 2), shows the whole muscle grown a little farther upward and its dorsal fibers 



Fig. 2. — Sagittal section of a human 

 fetus 46 mm. CR. length (No. 95, 

 Carnegie Collection), showing the 

 presence of longitudinal muscle- 

 fibers (the circular muscle is not 

 drawn). The ventral portion of 

 the muscle extends only as far as 

 the pelvic cavity, while the dorsal 

 fibers extend well into the abdom- 

 inal region. 



extending well into the sigmoid region; no well-defined fibers can be detected in any 

 of the regions higher up. In a 50-mm. fetus there is a distinct layer at the mesen- 

 teric attachment, the continuation of the dorsal fibers of the 46-mm. stage. The 

 layer is well defined and extends throughout the length of the colon. Lewis noted 

 this muscle and stated that in a 75-mm. fetus it was seen along the transverse colon, 

 although he made no mention of it in other parts of the bowel. 



Thus it is seen that the longitudinal fibers, as well as the circular, have their 

 origin in the extreme caudal end of the intestine and grow rapidly upward toward 

 the CEecal end. In the pelvis the muscle forms a continuous layer surrounding the 

 tube, but as the more open region of the sigmoid is reached it extends only along the 

 line of the mesentery attachment. Here it lies just over the plexus of nerves in a 

 crescentic shape with its horns directed laterally. The remainder of the circle 

 outside of the plexus is a zone of quite uniform thickness, where there are found cells 

 generally of an indefinite type; some are fibrous and resemble the cut ends of the 

 longitudinal muscle, but do not react as such to the Mallory stain. In a 52-mm. 



