VESTIGIAL PARTS. 



77 



I 



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 

 serious. 



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 to the t 



FIG. 37. A diagram of the ahmen- 

 middle portion of that very tary canal showing the yolk-sac 



puzzling organ-the thyroid and its ducw ' caecum : l ' lung> 

 body. For a 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, oLthe 



j^vS^> 



UNIVEKSITY 



