THE ALIMENTARY CANAL AND ITS APPENDAGES 165 



mentioned here. Attention may, however, be drawn to the saccus 

 csecus, 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 l near the 

 middle of the pyloric region. 



The O3sophageal 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 ciliated epithelium. In the adult Lamprey 

 it is 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. 

 Ciliated epithelium is also frequent in the resophagus of Eeptiles, 

 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. Jour. Misc. Sci., vol. xxx. 

 p. 519).] 



Muscle bundles often occur between the posterior wall of the t 

 windpipe and the oasophagus, at the point where the left bron- ] 

 chial tube crosses the latter, and at other parts of the intestinal ! 

 canal, e.g. 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 (e.g. the heart, the 

 thyroid, and the thymus glands), originally lay farther forward, 

 i.e. nearer the head, and that it has secondarily shiftecf back 

 (cf. ante, p. 38 and Fig. 31). 



It not infrequently happens that a blind diverticulum 

 (diverticulum ilei or diverticulum ofMeckel) arises from the 



1 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. 



