300 A?inals Efitomological Society of America [Vol. XI, 



fibrillar, staining light pink. These areas are of al:»(:»ut equal 

 width. This condition (which is doubtless related to the absorp- 

 tion of food, since such a differentiation is not apparent unless 

 there is food in the intestine) is most prominent proximally, but 

 becomes less and less distinct, although it is still plainly dis- 

 cernible through the whole course of the ileum. The nuclei, 

 w^hich are ciuite large, may be either round or oval: they are 

 densely chromatic, especially around the perifery and in the 

 center. The chromatin is present in the form of coarse granules. 



In some sections the epithelium appears only slightly, if at 

 all, folded, but usually it is conspicuously and strongly thrown 

 into tooth-like projections, typically six in number. 



The distal portion of the ileum is shown in figure 21. 



Basement membrane. The basement membrane is thin, but 

 clearly distinguishable. 



Loiigittidinal muscles. The longitudinal muscle fibres are 

 continuous anteriorly with those of the proximal portion of the 

 ileum, and are inserted posteriorly on the intima at the extreme 

 end of this region. The longitudinal muscles are continuous 

 nearly the whole length of the alimentary canal ; they arise at the 

 extreme anterior end of the pharynx and lie inside of the circular 

 muscle sheath in the fore-intestine; they pass outside this 

 sheath, and are external in the mid-intestine; they pass inside 

 again at the beginning of the hind-intestine, so that they are 

 internal, and are finally inserted on intima at the posterior end 

 of the ileum. 



Circular muscles. The striated circular muscle fibres are 

 well developed. They lie in a connective tissue sheath contin- 

 uous anteriorly with that of the proximal portion of the ileum, 

 and posteriorly with that of the colon. 



THE COLON. 



Transition. The epithelium changes gradually in type 

 between this region and the preceding. The intima, basement 

 membrane, and circular muscles are also continuous. The 

 longitudinal muscles of the ileum disappear, as they are inserted 

 on the intima at the end of the ileum, and a new layer of much 

 larger longitudinal muscle fibres, external and not internal, 

 appears, inserted on the intima at the beginning of this region. 



