948 THE DIGESTIVE SYSTEM. 
When the mouth is closed, and respiration is carried on through the nose, the cavum oris is 
reduced to a slit-like space, and practically obliterated by the tongue coming in_ contact 
with the palate above, and with the gums and teeth laterally and in front (Fig. 637). When the 
mouth is slightly open and the teeth nearly in contact, the tongue becomes somewhat concave 
or grooved along the middle line, and leaves a channel-like space between it and the palate, while 
it remains in contact with the roof and gums laterally. By depressing the hyoid bone with the 
root of the tongue, the cavum oris can be increased to a considerable size even when the teeth are 
in contact. Finally, by the simultaneous descent of the lower jaw and hyoid bone with the tongue, 
and the ascent of the soft palate, the cavit¥ is increased to its greatest dimensions (Hig. 638). 
Gums (gingivie).—This term is applied to the red firm tissue, continuous with 
the mucous membrane of the vestibule on the one hand, and with that of the 
palate or floor of the mouth on the other (Fig. 637), which covers the alveolar 
borders of the maxilla and mandible, and surrounds the necks of the teeth. 
The gums are composed of dense fibrous tissue, inseparably united to the periosteum 
and covered by mucous membrane. They are richly supphed with blood-vessels, — 
but sparsely with nerves, and are covered by stratified squamous epithelium. — 
Around the neck—or more correctly the base of the crown—of each tooth, the gum — 
forms a free overlapping collar, and at this part particularly it is closely beset with 
small papille, visible to the naked eye. In thickness it usually measures from 1 
to 2 mm. 
THE PALATE AND ISTHMUS FAUCIUM. 
The palate (palatum) is the term apphed to the strongly-arched structure which — 
forms the roof of the mouth, and projects posteriorly into the pharynx as a pliant — 
fold, imperfectly dividing that cavity into two (Figs. 639 and 641). Its anterior half 4 
or more has a foundation of bone, and separates the nasal fossee from the mouth. — 
This part is known as the hard palate. The posterior portion, which is free from 
hone, separates the naso-pharynx above, from the mouth and oral pharynx below, 
and is known as the soft palate. 
The hard palate (palatum durum, Fig. 659) occupies the space within the | 
upper dental arch, and is continuous with the gums in front and laterally, whilst_ 
behind it passes into the soft palate. It is formed by the palate processes of the ~ 
superior maxillary and palate bones (Fig. 652), covered by periosteum, and by a_ 
layer of firm mucous membrane. Beneath this mucous membrane, particularly at — 
the sides and in front, is found a considerable quantity of dense fibrous tissue, — 
firmly united to the periosteum on the one hand and to the mucous membrane on 
the other. This dense tissue forms an effective protection for the palate, and, in — 
addition to the palatine vessels and nerves, it contains in its posterior half a large 
number of racemose (palatine) glands. 
© ht ee 
Traversing the middle of the palate is seen a faint central ridge or raphe (Fig. 638), indicating 
its original development from two lateral halves. Behind, this raphe is continued along the soft 
palate to the base of the uvula, and in front it ends in a slight elevation, the incisive pad or 
papilla palatina. From the anterior end of the raphe a series of transverse ridges of mucous 
membrane, about six in number, run outwards, Just behind the incisor teeth ; they are known as 
the palatine rug, and are composed of dense fibrous tissue. Sometimes a small pit, which will — 
admit the point of a pin, is seen on each side immediately behind the central incisor teeth, and 
about 2 mm. from the middle line. These pits correspond to the lower openings of Stenson’s 
canals, with which they are occasionally continuous. 
The soft palate (palatum molle, or velum pendulum palati, Fig. 639) is a’ 
movable valve-like fold which runs obliquely downwards and backwards, like the 
“tongue” of a whistle, across the cavity of the pharynx almost as far as its 
posterior wall. Whilst it is attached to the hard palate in front, and blends with 
the pharyngeal walls laterally, its posterior border is free, and between it and the 
posterior wall of the pharynx is left a valvular passage—the isthmus of the 
pharynx—tbrough which the naso-pharynx and the oral pharynx communicate 
with one another. 
The soft palate is composed of two layers of mucous membrane between which 
are contained the palatine muscles, vessels, nerves, and connective tissue, the 
aponeurosis of the palate, and, in addition, a very large number of racemose glands. 
These glands are arranged in a thick continuous layer on the inferior surface beneath 
the mucous membrane, and form quite one-half of the mass of the soft palate 
