704 



HANDBOOK OF PHYSIOLOGY. 



From the fore-gut are formed the pharynx, oesophagus, and stom- 

 ach ; from the hind-gut, the lower end of the colon and the rectum. 

 The mouth is developed by an involution of the epiblast between the 

 maxillary and mandibular processes, which becomes doeper and deeper 

 till it reaches the blind end of the fore-gut, and at length communicates 

 freely with the pharynx by the absorption of the partition between the 

 two. 



At the other end of the alimentary canal the anus is formed in a pre- 

 cisely similar way by an involution from the free surface, which at length 

 opens into the hind gut. When the depression from the free surface 

 does not reach the intestine, the condition known as imperforate anus 

 results. A similar condition may exist aj; the other end of the alimen- 

 tary canal from the failure of the involution which forms the mouth, to 



FIQ. 490. 



FIG. 491. 



FIG. 490. First appearance of the parotid gland in the embryo of a sheep. 

 FIG. 491. Lobules of the parotid, with the salivary ducts, in the embryo of the sheep, at a more 

 advanced stage. 



meet the fore-gut. The middle portion of the digestive canal becomes 

 more and more closed in till its originally wide communication with the 

 yelk-sac becomes narrowed down to a small duct (vitelline). This duct 

 usually completely disappears in the adult, but occasionally the proxi- 

 mal portion remains as a diverticulum from the intestine. Sometimes 

 a fibrous cord attaching some part of the intestine to the umbilicus, re- 

 mains to represent the vitelline duct. Such a cord has been known to 

 cause in after-life strangulation of the bowel and death. 



The alimentary canal lies in the form of a straight tube close beneath 

 the vertebral column, but it gradually becomes divided into its special 

 parts, stomach, small intestine, and large intestine (Fig. 489), and at the 



