THE DIGESTIVE SYSTEM 221 



the definitive mouth." According to this view, the neural tube was for- 

 merly a part of the digestive system, and its anterior embryonic external 

 opening, the neuropore, once functioned as a mouth. For a part of the 

 digestive system to become nervous in function is indeed a surprising 

 change, which is no greater, however, than others which have occurred in 

 phylogenesis. 



If we add to the three mouths mentioned by Delsman the hypophysial 

 "paleostoma " mentioned by Beard and Kupffer, then there have been four 

 mouths in the phylogenesis of vertebrates, the present mouth being the 

 fourth and last. 



Diagrams showing the position of the four mouths mentioned are 

 shown in Fig. 208. The objection to this idea that there have been a series 

 of mouths in the course of animal phylogenesis, on the ground that the 

 chances are against the appearance of more than one ingestive opening 

 into the enteron, loses much of its weight in view of the fact that many 

 openings into the aUmentary canal, such as the gill-slits, have made their 

 appearance in the course of phylogenesis. 



The phylogenesis of the vertebrate mouth remains, therefore, an 

 unsolved problem. That there have been at least two mouths in the 

 course of animal evolution, all morphologists agree. These are the 

 coelenterate mouth, w^hich is the blastopore, and the definitive vertebrate 

 mouth. Evidence is, however, not wanting that the embryonic neuropore 

 and the hypophysis may have served as mouths. But such assumptions 

 are considered to have a relatively insecure foundation. 



The Salivary Glands in Man 



As food enters the mouth, it is moistened by the secretion of a number 

 of salivary glands, in addition to which are lingual, labial, buccal, palatine 

 and molar mucus-secreting glands. Besides moistening the food, the 

 chief salivary glands contain serous cells which secrete the starch-splitting 

 enzyme ptyalin and the sugar-spUtting enzyme maltase. The sublingual 

 and submaxillary glands secrete mucus also. 



The largest of the salivary glands is the parotid, which lies below the 

 ear. It is a serous tubulo-acinous gland, and empties by Stenon's duct 

 into the vestibule of the mouth opposite the second upper molar tooth. 



The submaxillary is a mixed (mucous and serous) tubulo-acinous gland 

 located in the floor of the mouth near the angle of the lower jaw. Its 

 secretions are carried by Wharton's duct which opens near the frenulum at 

 the front margin of the tongue. 



The sublingual is also a mixed tubulo-acinous gland lying below the 

 tongue in the front of the mouth near the median line. Mucus and serous 

 cells are about evenly distributed. The openings of the sublingual ducts 

 lie in front of the tongue near those of Wharton's ducts. 



