THE DIGESTIVE TRACT. J4jj 



The oral mucous membrane is thickly beset with small mucous 

 racemose glands in nearly all parts. These are especially well 

 marked on the lips, the cheeks, the under surface of the tongue, and 

 the soft palate, constituting, respectively, the labial, the buccal, the 

 lingual, and the palatine glands ; on the gums and the hard palate 

 such structures are absent or present in very limited numbers. The 

 acini are situated within the deeper layers of the mucosa, while the 

 ducts pierce the superficial layers to open on the free surface. The 

 squamous epithelium of the latter is continued within the duct usually 

 as far as its first division. Small lateral isolated groups of acini, 

 constituting accessory mucous glands, sometimes open into the long 

 narrow excretory duct of the main glandular mass during its journey 

 to the free surface. 



The larger blood-vessels supplying the oral mucous membrane 

 lie within the submucous tissue and give off branches which extend 

 through the deeper layers of the mucosa to the superficial portions 

 of the connective-tissue stratum ; on reaching the outer boundary 

 of the latter the arteries break up into rich subepithelial capillary 

 net-works, or, where papillae are present, enter the minute elevations 

 to supply their apices with terminal capillary loops. The capillaries 

 likewise enclose the acini of the oral glands. 



The lymphatics begin in the irregular net-work of interfascicular 

 spaces between the connective-tissue bundles of the tunica propria ; 

 these spaces unite to form definite lymphatics in the deepest layers 

 of the mucosa, which in turn are taken up by the larger lymph- 

 vessels of the submucous tissues. 



Nerve-fibres, largely of the medullated variety, accompany the 

 blood-vessels, to end in a subepithelial plexus ; special terminations 

 the end-bulbs are found in the apices of some of the papillae, 

 while additional numerous tactile corpuscles occur on the lips. 



THE TEETH. 



In principle, and among many of the lower animals in fact as well, 

 the teeth may be regarded as hardened papillae of the oral mucous 

 membrane. 



The teeth are firmly retained within their appropriate sockets by 

 the close attachment afforded by the alveolar periosteum which 

 holds together the alveolus and the root of the tooth. The" perios- 

 teum lining the alveolus is composed of dense fibrous tissue, whose 

 fibres have a general transverse disposition : elastic tissue is almost 

 wanting, nerves and blood-vessels being, however, numerous. At 

 its neck the tooth is especially embraced by the thickened perios- 

 teum, which then becomes continuous with the periosteum covering 

 the alveolar process of the jaw and with the gum. 



