On the musculature of the gastro-intestinal tract of the guinea-pig

Summary

The small intestine, caecum, colon and part of the stomach of guinea-pigs were studied by light microscopy, using semithin sections of plastic embedded specimens. The muscular coat is thicker in the duodenum than in the rest of the small intestine. The ratio between longitudinal and circular muscle is 1:4.6 in the duodenum and it increases regularly along the small intestine, to reach 1:2 in the terminal ileum. In the caecum, shape and sectional area of the muscle tissue were analyzed along the full length of the taeniae. In the caecal circular muscle there is a characteristic change in the arrangement of the muscle bundles from the regions in the centre of the haustrations to the regions of the grooves between haustrations or to those lying beneath a taenia. The functional significance of the taeniae is discussed in terms of an arrangement allowing reduction of the lumen of the organ (which at the level of the grooves between haustrations acquires a triangular outline) more efficiently than if the longitudinal musculature were spread over the entire surface of the organ. Haustrations are present also on one side of the wall of the ascending colon where there is no longitudinal muscle layer. In the descending colon the structure of the wall is examined in different functional states, namely in the regions between fecal pellets (constricted regions) and in the regions around a fecal pellet (moderately distended regions). The musculature increases considerably in thickness in the constricted regions (both muscle layers being actively contracted), while the mucosa and submucosa are thrown into prominent longitudinal folds. These folds produce occlusion of the lumen when the circular muscle has shortened by about 50%.