616 



THE DIGESTIVE SYSTEM 



has a secondary palate, but it is built more tenuously than that of the croco- 

 dilian-mammaHan group (fig. 289A-C). 



During secondary-palate formation in the mammal, the premaxillary, maxil- 

 lary, and palatine bones develop secondary plate-like growths which proceed 

 medially to fuse in the midline (fig. 289D-F). The secondary palate thus 

 forms the roof of the oral cavity — the air passageway from the outside to the 

 pharynx being restricted, when the mouth is closed, to the area above the 

 secondary palate. 



6) Formation of the Lips. Lips are ridge-like folds of tissue surrounding 

 the external orifice of the oral cavity. They are exceptionally well developed 

 in mammals, where they are present in the form of fleshy mobile structures. 

 They are absent in the prototherian mammal, Ornithorhynchus , as well as 

 in birds and turtles, where the horny edges of the beak displace the fleshy folds 

 at the oral margin. Lips are much reduced in sharks, where the toothed jaws 

 merge with the general epidermis of the skin, but are present in most fishes, 

 amphibia, and most reptiles. In general, lips are immobile or only slightly 

 mobile structures in the lower vertebrates, although in some fishes they possess 

 a mobility surpassed only in mammals. 



In the formation of the lips, a labial groove or insinking of a narrow ledge 



POISON GLAND 



DUCT OF PAROTID 



SUBMAXI LL ARY 

 GLAND 



DEEP PROCESS OF 

 SUBMAXILLARY GLAND Q, 



Fig. 290. Oral glands. (A) Poison and labial glands of the rattlesnake. Crotalus 

 horidus. (Redrawn from Kingsley, 1912, Comparative Anatomy of the Vertebrates, 

 Blakiston, Phila.) (B) Loci of origin of salivary glands in human embryo. (Redrawn 

 from Arey, 1946, Developmental Anatomy, Saunders. Phila.) (C) Position of mature 

 salivary glands in human. (Redrawn and modified from Morris, 1942, Human Anatomy, 

 Blakiston, Phila.) 



