STRUCTURE OF THE INTESTINE. 
217 
collect around the nucleus, Fig. 43, and as they accumulate they extend 
through all the rest of the cell, except the upper part underneath the 
cuticula, which portion always remains clear, as is seen in Fig. 44. Viewed 
from above the epithelial cells appear as polygonal pigmented fields, 
each separated from its neighbors by a clear line. Posteriorly the cells 
become less and less pigmented, and pass by gradual changes into the 
epithelium of the ileum. 
Ileum. — The ileum is traversed by six longitudinal folds, with inter- 
vening furrows. Outside each furrow is a longitudinal muscular band. 
Viewed from the inner surface, the epithelium is seen to have an unusual 
character. The cells in the middle of each of the flat folds are quite large 
(Fig. 50 A), polygonal in outline, with large, round, granular nuclei, which 
stain very darkly with haematoxiline. Toward the furrows the cells be- 
come very much smaller, those at the edge of the furrow being not more 
than one-sixth the size of those in the middle (Fig. 50 B). Underneath 
the furrow, the longitudinal muscles (Fig. 50 L) are seen shining through. 
A transverse section (Fig. 51) shows that the walls are double ; the 
inner leaf is composed of epithelium, Ep., and connective tissue, the outer 
leaf, of the circular muscles, muc. C. The furrows are indicated by the 
six bands of longitudinal muscles, L L. It is only opposite these bands 
that the two leaves are united, as is shown more plainly in Fig. 52. 
The epithelium, Ep., rests directly upon and is intimately united with the 
connective tissue, so forming a single leaf, which then bends down, mak- 
ing afurrow, F, opposite the longitudinal muscle, L L, where it is united 
with the circular muscular layer, mice C. The consequence of this ar- 
rangement is that underneath each fold there is a very large longitudi- 
nal cavity between the propria and the muscularis. 
The cuticula (Fig. 52), cu n is thin, but probably chitiuous ; it resembles 
that on the gastro-ileal folds, except that there are no spines, but it is 
not in the least like the ventricular cuticula. It extends equally over 
the folds and the furrows. 
The epithelium has round nuclei ; the size both of the cells and of the 
nuclei diminishes rapidly towards the bottom of the furrows (Fig. 52), F. 
The bases of the cells are somewhat dome-shaped. The nuclei are sur- 
charged with granules, and have a less distinct outline than the nuclei 
from other parts of the body. 
The circular muscles are moderately developed. Each longitudinal 
muscular band consists of 10 to 15 single bundles. The fibres are 
striped. 
Colon. — In the colon the six longitudinal folds of the ileum are con- 
tinued, but their surface, instead of being smooth as in the ileum, is 
thrown up in numerous irregular curved and zigzag secondary folds, as 
is imperfectly indicated in Fig. 2, col. The cells of the epithelium are 
of uniform size, and contain, especially at the summits of the secondary 
folds, pigment granules like those in the cells of the gastro-ileal valve. 
The epithelium is covered by a highly refringent cuticula without spines, 
