VESTIGIAL PARTS. | 
quences in another way. In the ordinary course of 
events the duct should shrivel as far as its attachment 
to the bowel ; occasionally the process of obliteration 
may involve the wall of the ileum and lead to the | 
formation of a septum, which gradually contracts and 
slowly causes death by obstruction of the bowel. In 
rarer cases the occlusion of 
the duct may extend to the 
ileum, and divide it com- 
pletely. Such a condition is of 
course incompatible with life. 
This duct is the source of 
other lighter troubles ; those 
described above are the most 
erious. 
The tongue contains a ves- / 
tigial duct of great interest. 
At a very early period in the 
life-history of the mamma- 
lian embryo a diverticulum 
arises from the ventral wall 
of the pharynx, and even- 
tually gives rise : 
ee to the Fic. 37-—A diagram of the alimen- 
middle portion of that very tary canal showing the yolk-sac 
puzzling organ—the thyroid and its ducts, ¢, caecum ; 4, lung. 
body. Fora time this duct retains its connection with 
the mouth; eventually the hyoid bone appears and 
divides the duct into two portions. The portion in 
relation with the mouth becomes surrounded by the 
developing tongue, and finally disappears, leaving 
nothing but a small depression on the surface of the 
