THE DIGESTIVE CANAL AS A WHOLE. 
219 
cells lying at the bases of the high columnar ciliated epithelium. It is, 
however, uncertain whether these small cells lie between the others, or 
form a sub epithelial endothelium, similar to that described by Debove. 313 
Underneath the epithelium appear the round nuclei of the tunica 
propria, and the very much elongated nuclei of the tracheal ramifica- 
tions. 
In a transverse section, Fig. 42, it is seen that each gland is a low flat 
fold of the epithelium ; each fold is separated from its neighbor on 
either side by a deep but narrow furrow, F, F, and is covered internally 
by a cuticula, which is quite resistent, highly refringent, and very 
slightly tinged with yellow. The epithelium, Fig. 41, is, as was to be 
expected from the presence of the two sets of nuclei, composed of two 
kinds of cells ; 1st, cylindrical cells corresponding to the oval nuclei ; 
in sections these nuclei appear round and are seen to lie in the basal por- 
tion of the cells; 2d, cells corresponding to the superficial nuclei ; each 
of these nuclei is surrounded by a clear space, as indicated in Fig. 41, 
but this space has not a sharp outline as there represented ; the shape 
of these cells I have been unable to determine. 
The epithelium rests upon a layer of connective tissue, in which there 
are round granular nuclei, as before stated. Outside of the connective 
tissue there is a thin layer of circular muscular fibres, Fig. 42, muc. 
The tracheae, with their distinctive nuclei, ramify throughout all parts 
of these two layers. Opposite each furrow there is a longitudinal mus- 
cular band, Fig. 42, L D ', composed of some twenty or more striated 
bundles. Attached to the outer walls are found large tracheal trunks, 
Tr., and Malpighian vessels, M. v. 
At the points where the epithelium of the folds descends to form the 
intervening furrows, there is a little accumulation of pigment granules. 
From the above description it will be seen that the rectal folds do 
not offer the least appearance of glandular structure; neither is any evi- 
dence deducible from their microscopic anatomy to indicate that their 
function is that of absorption. Neither does it appear to me that Chun, 
in his memoir, has elucidated their function in other insects, and the 
opinions he expresses with apparent confidence I cannot regard as any- 
thing more than speculative. 
SUMMARY OF OBSERVATIONS ON THE DIGESTIVE CANAL. 
* 
If we now glance back at the descriptions above given of the histo- 
logical peculiarities of the various divisions of the digestive canal, 
there are certain general features which deserve especial attention. In 
the first place it will be recognized that the digestive tract is composed 
of three main divisions: 1, the oesophagus, crop, and proventriculus ; 
2, the ventricle and diverticula ; 3, the ileum, colon, and rectum. 
In the first division there are two coats of muscles, an internal longi- 
313 Debove : Memoire sur la couche endothelial sous-6pith61iale des membranes muqneuses. Arch, do 
Physiol., 1874, p. 19. 
