THE DIGESTIVE SYSTEM 3OI 



impossible, however, to compare the definitive mouth and anus of verte- 

 brates with the mouth and anus of Peripatus, since the former He outside 

 of and ventral to the ring of nervous tissue which forms the central nervous 

 system, while the latter lie within the nerve-ring. Only the neuropore 

 and neurenteric canal of vertebrates could be compared with the mouth 

 and anus of Peripatus. 



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, which 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. Such assumptions are 

 considered to have a relatively insecure foundation. 



The Lips. The lips are fleshy folds of skin surrounding the mouth of 

 tetrapods. They are immovable in amphibia and reptiles. In mammals, 

 however, slips of muscle derived from the sphincter colli migrate into 

 the lips, which as a result become movable, and useful in sucking and in 

 the prehension of food. Their increased size in mammals is correlated 

 with the habit of mastication; and they, together with the cheeks, restrict 

 the opening of the mouth. In man, the lips become an important aid 

 to articulate speech. 



The Salivary Glands in Man 



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

 of saHvary glands, in addition to which are hngual, labial, buccal, palatine 

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

 chief saHvary glands contain serous cells which secrete the starch-spHtting 

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

 and submaxillary glands secrete mucus also. 



The largest of the saHvary 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. 

 It is innervated by fibers from the otic gangHon and has nervous connexion 

 both with the fifth and ninth cranial nerves. Inflammation of the parotid 

 is the cause of mumps. 



The submaxillary is a mixed (mucous and serous) tubuloacinous gland 

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

 secretions are carried by Wharton's duct to the frenulum at the front 

 margin of the tongue near its median Hne. The cells of the gland are 

 mostly serous, mucus-secreting cells occurring irregularly and constituting 

 only a minor portion. The slender intercalated ducts present in the 

 parotid are absent in the submaxillary. 



