946 THE DIGESTIVE SYSTEM. 
The anterior border of the masseter can be distinctly felt with the finger, on the outer wall 
of the vestibule, when the muscle is thrown into a state of contraction. Still further back, the 
front of the coronoid process, bearing the lower part of the imsertion of the temporal muscle, can 
be easily made out. Whilst the pterygo- -maxillary ligament, which corresponds to, and is felt 
along with, the anterior border of the internal pterygoid, is distinguishable as a pliant ridge 
when the finger is carried from the front of the coronoid process inwards behind the wisdom 
teeth to the cavity of the mouth. 
In addition to Stenson’s duct, the ducts of numerous small glands which are embedded in the 
lips and cheeks open into the vestibule. 
Under normal conditions, as pointed out above, the lips and cheeks he against the teeth and 
gums, obliterating the cavity of the vestibule, and helping, with the aid of the tongue, to keep 
the food between the grinding surfaces of the molar teeth during mastication. In facial palsy, 
however, owing to the paralysis of their muscles, the lips and cheeks fall aw ay from the dental 
arches, and allow the food to pass out from between the teeth and to accumulate in the vestibule. 
Lips (labia oris, Fig. 659).—These are the two movable folds, covered superficially 
by skin, and on their deep surface by mucous membrane, which surround the rima 
oris. Laterally the two meet at the angles of the mouth, and beyond this are 
Antrum of 
Highmore 
Palatine glands 
Superior 
lingualis 
Buecinator 
Vestibule of 
mouth 
__2nd molar 
tooth 
\ ‘ ‘\ . : ui iy) = BS 
yo S17 @ NS ae 
Genio- 
ad Inferior 
hyoglossus 
lingualis 
Wharton's 
: Sublingual 
duct 
gland 
oY Genio-hyoid 
Inferior 
dental canal 
Digastric Mylo-hyoid 
Platysma ——— 
Fic. 637. CORONAL SECTION THROUGH THE CLOSED MourtuH. | 
The slit-like character of the vestibule, the manner in which the tongue fills up the mouth cavity, the 
close apposition of the teeth, the relations of the roots of the upper molars to the antrum of High- 
more, the plica sublingualis over the sublingual gland, and the position of the ranine artery should 
be noted. 
prolonged into the cheeks, with which they are continuous. The upper lip 
presents on its superficial surface a well-marked vertical groove, the philtrum, — 
bounded by two distinct ridges descending from the columella nasi (Fig. 643) ; 
inferiorly the groove widens ‘out, and terminates opposite a sheght projection— — 
the labial tuberele—on the free edge of the upper lip. This tubercle is particularly 
well developed in children, and is “chiefly responsible for the characteristic curve of — 
the rima oris. The lower is usually longer and more movable than the upper lip. 
In passing from before backwards the folowing structures are found in the 
lips :—(1) The skin, which is closely beset with hairs, small and fine in the child 
and female, long and stout in the adult male. (2) A lay er 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 of its fibres, or those of the muscles joining it, pass through the super- 
ficial fascia and are attached to the skin, thus establishing a close connexion ! 
between the latter and the muscle. (4) The submucous tissue, which is occupied 
by an almost continuous layer of racemose glands—the labial glands (glandule 
labiales). These open into the vestibule, 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 
