THE MOUTH. 1109 



widens out, and terminates opposite a slight projection the labial tubercle on the 

 free edge of the upper lip. This tubercle is particularly well developed in children, 

 ind is chiefly responsible for the characteristic curve of the rima oris. The lower 

 iip is usually longer and more movable than the upper lip. 



For the manner in which the various muscles enter into the formation of the 

 .ip, see section on the Muscles (pages 450 to 451). 



The lips include within them the greater part of the orbicularis oris muscle, 

 which surrounds the aperture of the mouth, and in each lip the following series of 

 structures can be recognised from the external to the internal surface : (1) The 

 skin, which is closely beset with hairs, small and fine in the child and female, long 

 ind stout in the adult male. (2) A layer of fatty superficial fascia continuous with 

 the fascia of the face generally. (3) The orbicularis oris muscle, continuous at its 

 periphery with the various muscles converging towards the mouth. A number 

 3f its fibres, or those of the muscles joining it, pass through the superficial fascia 

 ind are attached to the skin, thus establishing a close connexion between the 

 skin and the muscle. (4) The submucous tissue, which is occupied by an almost 

 iontinuous layer of racemose glands the labial glands. These open into the vesti- 

 bule, and their secretion is said to be mucous. (5) The mucous membrane of the 

 mouth, covered by stratified squamous epithelium. . Between the orbicularis and 

 mucous membrane, but nearer to the former, that is, in the deeper part of the 

 submucosa, the labial artery is found, a short distance from the free margin of 

 the lip, running to meet its fellow of the opposite side. 



The free ma.rgin of the lip is covered with a dry and otherwise modified mucous^, membrane. 

 It begins where the integument changes colour at the outer edge of the lip, and ends posteriorly 

 just behind the line along which the two lips meet when closed, where it passes into the ordinary 

 moist mucous membrane of the vestibule. It presents numerous simple vascular papillge, and 

 its nerves terminate in special end organs, hence the acute sensitiveness of this part. In the 

 ' child, at birth, the margin of the lip is divided by a very pronounced groove or fissure into an 

 outer and an inner zone, differing considerably in their appearance. 



When the tongue is pressed firmly against the back of the lips and moved about, the labial 

 glands can be distinctly felt through the mucous membrane, giving the impression of a knobby 

 or irregular surface. The glands, which are about the size of hemp-seeds and can be readily 

 displayed by removing the mucous membrane, are more numerous in the lower than in the 

 upper lip. Stoppage of their ducts, with the resulting distension of the glands, gives rise to 

 " mucous cysts," a well-known pathological condition. 



Blood-vessels, Nerves, and Lymph-vessels. The lips receive a free blood supply, the 

 lower lip from the inferior labial, and the upper from the superior labial branches of the 

 external maxillary artery. 



The sensory nerve supply of the lips is derived from the trigeminal nerve, that of the upper 

 ' through the infra-orbital branch of the maxillary division, and that of the lower from the mental 

 ' branch of the inferior alveolar branch of the mandibular division, while the buccinator branch 

 of the mandibular division supplies the region of the angle. The lymph- vessels of the upper 

 'lip pass with the external maxillary artery to the submaxillary lymph-glands lying in the sub- 

 maxillary triangle, while those from the lower lip pass in part to the same glands, and in part 

 to the submental glands lying on the mylo-hyoid muscles, above the hyoid bone. 



Buccae. The cheeks resemble the lips in structure, being formed of corresponding 



layers, but the place of the orbicularis oris muscle is taken by the buccinator 



| muscle. They are covered externally by the skin and internally by the mucous 



'membrane. Under the skin lies the fatty superficial fascia of the face, 



through which the parotid duct (O.T. Stenson's duct) runs inwards to pierce the 



buccinator. Here too are placed some of the muscles of facial expression. Near 



the end of the duct are found four or five mucous glands, as large as hemp-seeds. 



These are known as the molar glands; their ducts pierce the cheek and open 



'into the vestibule. Beneath the superficial fascia lies the buccinator muscle, 



overed by the thin bucco-pharyngeal fascia. Deeper still is the submucosa^ 



which, like that of the lips, contains numerous racemose buccal glands. And 



finally the mucous membrane is reached (Fig. 876). 



An important constituent of the cheek of the infant is the corpus adiposum buccae (O.T. suck- 



l Pad), an encapsuled mass of fat, distinct from the surrounding superficial fascia, which lies on 



>uter side of the buccinator, and passes backwards into the large recess between that muscle 



the overlying anterior part of the masseter. This fatty mass, which is relatively more 



716 



