THE MOUTH. 945 
the abdominal cavity, immediately beneath the diaphragm, and its secretion— 
the bile—is conveyed into the duodenum by the bile duct. The aang 
next in size, lies across the front of the vertebral column, with its right end « 
head resting in the concavity of the duodenum, into which its secretion ae 
through the: pancreatic duct. The salivary glands, of which there are three chief 
pairs—parotid, submaxillary, and sublingual—are placed about the face, and their 
duets, which convey the saliva, open into the mouth. Whilst the secretion of 
these latter undoubtedly possesses some digestive action, and they must conse- 
quently be classed as digestive elands, nevertheless the saliva is to be looked upon 
as a mechanical lubricant, which facilitates sw allowing and the movements of the 
tongue in speaking and masticating, rather than as an aid to the digestive process. 
Accessory Parts.—Under this heading we group the teeth, tongue, gums, 
palate, and tonsils. The teeth, 32 in number in the adult, are embedded j in the 
jaws and surrounded by the gums. The tongue is a muscular organ, useful alike 
in masticating, sw allowing, s speaking, and in the exercise of he sense of taste, 
which specially resides in its modified epithelium ; it occupies the greater portion 
of the floor of the mouth, whilst the roof of that cavity 1s formed by the hard 
palate anteriorly, and by the soft palate behind. Finally, the tonsils (Fi ig. 638) 
are two large masses of lymphoid tissue, found on the side- walls of the oral portion 
of the phary nx, Just behind the mouth. 
THE MOUTH 
The mouth is the expanded upper portion of the digestive tube, specially 
modified for the reception and mastication of the food. In it we distinguish : : the 
aperture of the mouth placed between the lips; the vestibule, the slit-like space 
which intervenes between the teeth and gums internally, and the lips and 
cheeks externally (Fig. 637); and the cavity ‘of the mouth, which lies within the 
round of the dental arches, and opens behind into the pharynx through the isthmus 
of the fauces (Fig. 638). 
The aperture of the mouth (rima oris) is the upper or anterior opening of the 
alimentary canal, and is bounded above and below by the corresponding lips, 
which, by their junction at the sides, form the angles of the mouth (commissuree 
labiorum). In a state of rest, with the lips in apposition, the rima appears as a 
shehtly curved line, corresponding in length to the interval between the first 
premolar teeth, and in level to a line drawn across just below the middle of the 
upper incisor crowns. The shape of the rima varies with every movement of the 
lips, from the resting linear form, curved like the conventional bow, to a circular 
or oval shape when the mouth is widely open, or the “ pursed up” condition pro- 
duced by the contraction of the orbicularis oris. 
The vestibule (vestibulum oris, Fig. 637) lies immediately within the aperture 
of the mouth. In the normal resting condition its cavity is practically obliterated 
by the meeting of its walls, which reduces it to a slit-like space. It is limited on 
the one hand by the deep surface of the lips and cheeks, and on the other by the 
dental arches and gums. Its narrow roof and floor are formed respectively by the 
reflection of the mucous membrane, from the deep surface of the lips and cheeks to 
the corresponding gum. ‘This reflection is interrupted in the middle line by a small 
but prominent fold of the mucous membrane, the frenulum, which connects the 
back of each lip to the front of the gum. The upper frenulum is the better 
developed, and is readily brought into view by everting the lip. 
On the outer wall of the vestibule, opposite the crown of the middle upper 
molar, is seen, upon a variably developed eminence (Fig. 647), the small opening of 
Stenson’s duct, which conveys the saliva from the parotid gland to the mouth. 
When the teeth are in contact the vestibule communicates with the cavity 
of the mouth, only through the small and irregular spaces left between the oppos- 
ing teeth, and posteriorly by a wider but variable aperture behind the last molars. 
Advantage is sometimes taken of the presence of this aperture, which lies between the 
wisdom teeth and the ramus of the j jaw, for the introduction of liquid food in certain cases— 
trismus, anchylosis, ete—in which the mouth is rigidly closed. 
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