132 CAVITY OF THE MOUTH. 



lage, and the others join the inferior constrictor. . The internal layer, on 

 the other hand, is formed of circular fibres, which are continuous with 

 those of the inferior constrictor. The structure of the oesophagus is de- 

 scribed more fully in the dissection of the thorax. 



SECTION XII. 



CAVITY OF THE MOUTH. 



THE cheeks, the lips, and the teeth are to be examined with the mouth, 

 as all may be considered accessory parts. 



THE MOUTH. The cavity of the mouth is situate below that of the 

 nose, and extends from the lips in front to the isthmus of the fauces be- 

 hind. Its boundaries are partly osseous and partly muscular, and its size 

 depends upon the position of the lower jaw bone. When the lower jaw 

 is moderately removed from the upper, the mouth is an oval cavity with 

 the following boundaries. The roof, concave, is constituted by the hard 

 and soft palate, and is limited anteriorly by the arch of the teeth. In the 

 floor is the tongue, bounded by the arch of the lower teeth ; and beneath 

 the tip of that body is the fraenum linguae, with the sublingual gland on 

 each side. Each lateral boundary consists of the cheek and the ramus of 

 the lower jaw ; and in it, near the second molar tooth in the upper jaw, 

 is the opening of the parotid duct. The anterior opening of the mouth is 

 bounded by the lips ; and the posterior corresponds with the anterior pil- 

 lars of the soft palate. 



The mucous membrane is less sensitive on the hard than the soft parts 

 bounding the mouth ; it lines the interior of the cavity, and is reflected 

 over the tongue. Anteriorly it is continuous with the tegument, and pos- 

 teriorly with the lining of the pharynx. The epithelium covering the 

 membrane is of the scaly variety. 



Between each lip and the front of the corresponding jaw the membrane 

 forms a small ibid fraenulum. On the bony part of the roof it blends 

 with the dense tissue (gums) covering the vessels and nerves. On the 

 soft palate it is smooth and thinner, and along the middle of the palate is 

 a ridge which ends in front in a small papilla. In the floor of the mouth 

 the membrane forms the fnenum linguae beneath the tip of the tongue, and 

 sends tubes into the openings of the Whartonian and sublingual ducts ; 

 whilst on each side of the frajnum it is raised into a ridge by the subjacent 

 sublingual gland. On the interior of the cheek and lips the mucous lining 

 is smooth, and is separated from the muscles by small buccal and labial 

 glands. 



Over the whole cavity, but especially on the lips, are papillae for the 

 purpose of touch. 



The CHEEK extends from the commissure of the lips to the ramus of the 

 lower jaw, and is attached above and below to the alveolar process of the 

 jaw on the outer aspect. The chief constituent of the cheek is the fleshy 

 part of the buccinator muscle ; on the inner surface of this is the mucous 

 membrane ; and on the outer the integuments, with some muscles, vessels, 

 and nerves. The parotid duct perforates the cheek near the second molar 

 tooth of the upper jaw. 



