1102 



DIGESTIVE SYSTEM 



The development of the oral cavity. — As stated in the section on Morphogenesis, the oral 

 cavit}' has its origin in a depression, the oral fossa, situated between the ventrally bent, devel- 

 oprag head and the region occupied by the developing heart. This fossa is bounded anteriorly 

 by the fronto-nasal process, and laterally by the maxillary and mandibular processes, portions 

 of the first branchial arches. The fossa is lined by ectoderm. Its floor is in apposition with 

 the cephaUc end of the archenteron, hned by entoderm, the ectoderm of the oral fossa and the 

 entoderm of the archenteron being in immediate contact and forming the pharyngeal mem- 

 brane. The oral fossa deepens with further development, and becomes the oral sinus. The 

 pharyngeal membrane becomes perforated in embryos about 2 mm. in length and disappears, 

 leaving a free communication between the oral sinus and archenteron. On each side of the 

 developing head and in a latero-ventral position there is early developed an area of thickened 

 ectoderm, known as the nasal area. These areas soon develop into depressions, the nasal fossae, 

 and assume a position, one on either side of the fronto-nasal process; on each side of the fronto- 

 nasal process there is developed a prominent protuberance, the globular process, each process 

 forming the median wall of a nasal fossa. The lateral wall of each nasal fossa also thickens 

 to form the lateral nasal process. With the further development, the ventral portion of each 

 lateral nasal process fuses with the corresponding globular process, the maxillary processes also 

 uniting with the globular processes, in this way separating the nasal fossae from the oral sinus. 

 With the further growth toward the median line of the maxillary processes the fronto-nasal 

 process becomes narrower, ultimately forming the nasal septum and a small median portion 

 of the upper jaw, the remainder of the upper jaw being formed by the maxillary processes, and 

 the lower jaw having its origin in the mandibular processes. 



Fig. 849. — Sagittal Section of the Lower Lip. (Lewis and Stohr. 



Sebaceous gland 



Oblique sec- 

 tion of palpillse 



Hair shafts and 

 sebaceous glands 



Sebaceous gland 

 Hair shaft 



Vein 



Artery 



Bulb of a hair 



Epithelium Tunica Submucosa Orbicular Mimetic Corium Epidermis 



propria muscle muscle 



Variations. — The mouth is rarely absent, due to failure of the stomatodeal invagination, 

 or imperforate, due to atresia of the pharyngeal membrane. Other variations will be mentioned 

 in connection with the various mouth organs. 



Comparative. — The phylogenotic origin of the mouth cavity from the integument is indi- 

 cated not only by the ectodermal orighi of its lining epithelium, but by its general structure 

 and its appendages. Among the latter may be noted the teeth (representing modified dermal 

 papillae), sebaceous glands, and (in some rodents) even hairs in the mucosa lining pouches in 

 the cheeks, 



THE LIPS AND CHEEKS 



The lips [labia oris] form the ant(?rior wall of the mouth cavity. The lower 

 lip [labium infcrius] is marked off from the chin by the sulcus mentolabialis. 

 The 7ippcr lip [labium superius] extends upward to the nose medially and the sul- 

 cus nasolabialis laterally. 'J'lie phiUrurn is a median groove on the upper lip 

 extendinf^ from the septum of the nose above to th(^ Inbidl tubercle [tuberculum 

 labii superioris] Ix'low, at the middle of the rima oris. On each side of the rima 

 oris the upper and the lower lips arc continuous at the angle of the mouth [angulus 

 oris], which is usually opposite the first premolar teeth. Laterally, the lips are 



