ale 
DEVELOPMENT OF SALIVARY GLANDS, PALATE, AND TONGUE. 963 
prominent border is covered only by the mucous membrane of the mouth, here raised up 
by the gland to form the plica sublingualis (Fig. 643). The anterior portion of the eland 
is much deeper and more bulky than the posterior half, and it meets its fellow in the 
middle line beneath the frenulum linguze. The posterior extremity grows gradually more 
slender, and ends near the posterior part of the mylohyoid ridge, where it lies above the 
deep process of the submaxillary gland. 
Its ducts, generally known as the ducts of Rivinus (ductus sublinguales minores), 
are numerous and of small size; they leave the upper part of the gland, and, after a 
short course, open on a series of papillie, visible to the naked eye, which are placed 
along the summit of the plica sublingualis. 
The gland is not énclosed in a distinct capsule, thus differing from the parotid and submaxil- 
lary glands ; but its numerous lobules, which are smaller than those of the glands just mentioned, 
are held together by fine connective tissue, loose ly, but still in such a manner as to make one 
more or less consolidated mass out of what w as, in the embryo, a number of separate glands. 
As a rule all the ducts open separately on the summit of the plica sublingualis, and appar- 
ently none of them join Wharton’s duct. Frequently some of those from the anterior and more 
bulky part of the gland are larger than the others, but the presence of a large duct running 
alongside of Wharton’s duet, and opening with or beside it (ductus major Rivini, duct of 
Bartholin), is very rare, and must be considered as an exceptional condition in man, ‘although 
normal in the ox, sheep, and goat. The same may also be said of ducts from the sublingual, 
which are described as opening into the duct of Wharton. 
Vessels and Nerves.—The arteries are derived from the sublingual branch of the lingual 
and from the submental branch of the facial. The nerves come from the lingual, the chorda 
tympani, and the sympathetic, through a branch of the submaxillary ganglion which joins the 
lingual, and is conveyed by it to the gland. 
The apical gland of the tongue (Nuhn’s) is described with the tongue, p. 956. 
DEVELORMENT OF THE SALIVARY GLANDS, PALATE, AND TONGUE. 
The general Meeelopniént of the lips, mouth, palate, and tongue is described on pages 
35 to 40, and reference will be made here only to a few special points connected with 
these parts. 
Several explanations of the formation of the philtrum or groove on the front of the 
upper lip have been put forward ; most probably it is produced by the union of the median 
fronto-nasal process with the two maxillary processes (see p. 38), the floor of the groove 
being formed by the fronto-nasal process, and the ridges bounding the groove at the sides 
corresponding to the line of meeting of the fronto- nasal with the maxillary processes. 
The salivary glands are deve sloped as solid outgrowths of the buccal epithelium, one 
each for the submaxillary (the first developed) and the parotid, several for the sublingual 
eland. The outgrowths are at first simple ; they subsequently divide, and finally develop 
alveolar enlargements on their extremities. By a separation of the lining cells, the ducts, 
and later on (about the 22nd week) the alveoli become hollowed out, and present a lumen 
as in the adult. 
The development of the palate is given at page 39; but it should be mentioned 
that, in order to account for the position which the fissure in cleft palate usually occupies, 
viz. between the central and lateral incisors, the theory has been advanced by Albrecht, 
that each premaxilla is made up of two separate segments, an inner (or endo-gnathion), 
containing the central incisor, and an outer (or meso-gnathion), containing the lateral 
incisor (the rest of the maxilla constituting the exo-gnathion). Between these two 
segments of the premaxilla (endo- and meso-gnathion) the cleft is said to run, and not 
between the premaxilla and maxilla as usually held. 
Tongue.—The tongue is developed in the embryo, not on the floor of the mouth, 
but in connexion with the enterior wall of the pharynx, and in two parts, which are at 
first distinct but soon unite. The anterior two-thirds of the organ is formed from the 
tuberculum impar, a single median elevation, developed on the ventral wall of the pharynx, 
immediately behind the first, or mandibular, visceral arch. Behind the tuberculum impar, 
at first, lies a prominent elevation—the fureula, from which the epiglottis is formed—the 
two being separated by a distinct sulcus, the sinus arcuatus (see p. 35). Soon, however, 
the ventral extremities of the second and third visceral arches, erowing downwarde: unite 
across the middle line, and separate the tuberculum from the fureula, thus dividing 
the middle portion of the sinus arcuatus into an anterior and a_ posterior part. 
The ventral ends of the two arches having fused, develop, after a little time, into a 
prominent semilunar ridge, the rudiment of the posterior third of the tongue. This ridge 
