STUDIES ON THE LONGITUDINAL MUSCLE OF THE HUMAN COLON. 43 



pass just beneath the serous layer, around to the vicinity of the bands, before pene- 

 trating deeper (fig. 7). Usually, on the mesenteric side of each taenia and very close 

 to it, there can be seen numerous branches of the nerves and blood-vessels passing 

 in and out. If the proposition of an active growth for the production of taenia? be 

 tenable, then, instead of a passive adjustment by a splitting process, it is essential 

 to look for some causative factor back of this growth. The breaking up of mesen- 

 teric vessels into capillary plexuses in the region where the bands will later develop 

 is fundamental. Numerous instances of this phenomenon were noted, but such a 

 line of investigation requires further and more careful attention. 



The effect of these segmental contractions is to accentuate the taeniae, causing 

 them to stand out in marked distinctness, especially when the contractions are most 

 active (fig. 8) . Previous to the appearance of the contractions the taeniae are poorly 

 outlined to the unaided eye, but by their action the bands become sharply defined. 

 This condition is noted in the adult as well as in the fetus. There are portions of the 

 adult colon, when directly examined, in which there seem to be no taeniae and the 

 content of which appears to have no effect upon this state of the wall; but where 

 contractions and saccules are in an active state the taeniae are marked. 



In the consideration of this fundamental factor in taenia? production another 

 aspect of the bands proper presents itself — that of an added function. In addition 

 to holding the colon in position and shortening it, the bands functionate as fixed 

 and firm longitudinal cables between which the circular muscle-fibers are stretched 

 and against which they may pull in segmental contraction. When one considers 

 the more solid content of the colon, with its retarded progress and the necessity of 

 more force in moving it, some such adjustment for only partial circular-muscle con- 

 traction would seem logical. This can be accomplished only by the development of 

 strong, fixed, longitudinal bands, against which the segments of the circular muscle 

 may pull. By such an adjustmentt here are three rows of counteracting segments 

 which, working alternately, tend to gradually move the massive content along without 



violence to the wall. 



SUMMARY. 



An attempt has been made to give a more detailed account of the appearance, 

 development, and nature of the taeniae than is found in the literature. In so doing a 

 few features are noted in addition to and at variance with accounts already given. 



1. The longitudinal muscle begins its growth in the caudal end of the intestine 

 and rapidly extends to the caecal end; first, as a layer at the mesenteric attachment, 

 followed shortly by the whole circle of the tube becoming incased in a complete 

 layer of longitudinal muscle-fibers before taeniae are formed. 



2. The mesenteric thickening becomes the first taenia, and the other two are 

 developed in the already existing layer of muscle. 



3. The production of taeniae, especially of the adult state, is due to an added 

 factor, the segmental contraction of the circular muscle between the bands. These 

 contractions are in turn dependent upon the taeniae, which functionate as cables 

 against which the circular muscle may pull. 



4. The whole aspect of taeniae production is based on active growth and func- 

 tional factors rather than being the passive results of foreign elements. 



