— 346 A Lesson in Comparative Histology. [June, 
there intervenes another kind of tissue, called connective, which 
also extends into the centre of each fold, separating the epithelium 
of the two sides. These three layers, the epithelium or ‘unica 
mucosa, the connective tissue or tunica submucosa, and the muscles 
or tunica muscularis, are the primary constituents of the digestive 
canal throughout the animal kingdom. It is always the epithe- 
lium which is the active agent of the secretion of the digestive 
juices as well as of the absorption of the food; hence it is 
important to make the surface of the epithelium which is to come. 
in contact with the food as large as possible, and it is by the 
formation of folds of various shapes that this is accomplished. 
The third and last segment of the alimentary tract consists of 
three divisions: first, the ileum, Plate , Fig. 1, which is as large 
in diameter as the stomach, for a part of which it was formerly 
mistaken; second, the very small colon, which bends forward and 
upwards (co/), and opens dorsally into the larger and horizontal 
rectum, Æ. The whole of this segment is traversed by six longi- 
tudinal folds of the epithelium, which are broad with flat surfaces 
in both the ileum and rectum, but narrow and irregular in the 
intervening colon. Each of the six folds ends at the front end of 
the ileum in two rounded protuberances, making twelve in all. 
They have hitherto escaped notice. I propose for them the 
name of gastro-ileal valves. When viewed from their inner sur- 
face they have the appearance indicated by Fig. 7, being rounded 
in front and gradually fading out posteriorly. They are strongly 
pigmented. In transverse section, Fig. 10, we recognize the three 
characteristic layers, the epithelium which alone contains the pig- 
ment, the very distinct coat of connective tissue, and externally 
the muscular fibres. Examined with a higher power, Fig. 6, the 
epithelial cells will be found filled with round granules of various 
sizes of brownish color; which are the pigment; moreover, there 
is a thin but quite resistent cuticula armed with minute spines. — 
In the ileum we find the cells modified in still another way, aS 
shown in Fig. 5, which is a surface view of part of the edge of | 
one of the folds. The cells in the middle are quite large, but 
their size diminishes towards the edge, until at the edge itself 
they are comparatively diminutive. Fig. 9 is a transverse section 
through one of the furrows between two folds. Æ is the furrow; 
Ep the epithelium, the cells of which are smallest in the furrow. 
The epithelium rests upon a layer of connective tissue, conn, 
| 
do OO tanta oe aes T 
s 
u a e o e a a a 
FS e eaoat 
EEN 
ae ties Ca e 
eg aa eee 
