THE ALIMENTARY CANAL AND.ITS APPENDAGES 165 
mentioned here. Attention may, however, be drawn to the saccus 
czecus, which is, as it were, indicative of the commencement of a 
process of chambering in the stomach, the antrum pyloricum, and 
a constriction (c’., Fig. 98) which but very rarely occurs + near the 
middle of the pyloric region. 
. The cesophageal mucous membrane, which after birth is 
covered with a dense stratified epithelium, is in the embryo 
beset by a columnar -ciliated epithelium, and thus recalls very 
primitive conditions. In Amphioxus and the young Lamprey 
(Ammocoetes), for example, nearly the whole intestine is still 
lined with a similar ciated epithelium. In the adult Lamprey 
it 1s somewhat more limited, and it is still to be found at various 
parts at least of the intestine, in a large number of the Anamnia. 
Cihated epithelium is also frequent in the cesophagus of Reptiles, 
and it has even been proved to exist in the intestinal canal of 
some Mammals, at least over small areas. 
[A similar replacement of ciliated by stratified non-ciliated epithelium 
may take place over localised areas of the mammalian trachea. In the Dog 
and Cat, for example, this change is effected over areas of attrition, resulting 
from a folding over of the tracheal wall ; and this and other allied considera- 
tions have led to the application of the term “frictional” to stratified 
squamous epithelium (cf. Haycraft and Carlier, Qu. Jowr. Mise. Scv., vol. xxx. 
p- 519).] 
Muscle bundles often occur between the posterior wall of the 
windpipe and the cesophagus, at the point where the left bron- 
chial tube crosses the latter, and at other parts of the intestinal 
canal, eg. the duodenum. Their significance is undetermined ; 
but their inconstancy, variability, and feeble development suggest 
that they may be among those organs which are being gradually 
lost by Man. 
The comparative anatomy of the stomach, and of the course 
and ultimate distribution of the vagus nerve, prove that the 
former, like some other organs of the viscera (¢.g. the heart, the 
thyroid, and the thymus glands), originally lay farther forward, 
i.e. nearer the head, and that it has secondarily shifted back 
(cf. ante, p. 38 and Fig. 31). 
It not infrequently happens that a blind diverticulum 
(diverticulum ilei or diverticulum of Meckel) arises from the 
‘ I noticed this constriction twice during the ordinary dissecting course in this 
University in the winter of 1892 and 1893 ; and careful dissection showed that there 
was at the constricted part a ring-like specialisation of the circular musculature. 
