The =tongue= (Lingua) is situated on the floor of the mouth, between the
rami of the mandible, and is supported mainly in a sort of sling formed
by the mylo-hyoid muscles. Its posterior portion, the =root= (Radix
linguæ), is attached to the hyoid bone, soft palate, and pharynx. Only
the upper surface of this part is free, and slopes downward and
backward. The middle portion, the =body= (Corpus linguæ), has three free
surfaces. The upper surface or =dorsum= (Dorsum linguæ) is rounded. The
lateral surfaces are nearly flat for the most part, but anteriorly
become rounded and narrower. The lower surface is attached to the
mandible. The =apex= or tip (Apex linguæ) is free, spatula-shaped, and
presents superior and inferior surfaces and a rounded border.
[Illustration:
FIG. 241.—TONGUE OF HORSE, DORSAL ASPECT.
]
=Structure.=—The tongue consists of: (1) The mucous membrane; (2) the
glands; (3) the muscles.
The =mucous membrane= (Tunica mucosa linguæ) adheres intimately to the
subjacent tissue, except on the lower part of the lateral surfaces of
the body and the under surface of the tip. It varies considerably in
thickness. On the dorsum it is very thick and dense. Underneath this
portion there is a dense fibrous cord, which extends medially a distance
of five or six inches forward from the vallate papillæ. On the sides and
under surface of the tongue the membrane is much thinner and smooth, and
can more readily be dissected off the muscular tissue. From the under
surface of the free part of the tongue a fold of the mucous membrane
passes to the floor of the mouth, forming the =frenum linguæ=. This
contains the anterior edges of the genio-glossi muscles. Posteriorly a
fold passes on each side from the edge of the dorsum to join the soft
palate, forming the anterior pillars of the latter. A central
glosso-epiglottic fold (Plica glossoepiglottica) passes from the root to
the base of the epiglottis. The mucous membrane presents numerous
=papillæ=, which are of four kinds—filiform, fungiform, vallate, and
foliate. The =filiform papillæ= (Papillæ filiformes) are fine, pointed
projections. They cover the upper surface of the body and tip, to which
they give a distinct pile. The =fungiform papillæ= (Papillæ fungiformes)
are rounded at the free end, which is supported by a neck. They occur
principally on the lateral part of the tongue, but are also found
scattered over the dorsum and upper surface of the free portion. The
=vallate papillæ= (Papillæ vallatæ) are usually two or three in number.
The two constant ones have a diameter of about 7 mm., and are found on
the posterior part of the dorsum, one on each side of the median plane,
about an inch (ca. 3 cm.) apart. The third, when present, is behind
these, is centrally situated, and is always smaller. Rarely a fourth may
be seen. They are rounded, broader at their exposed than at their
attached surfaces, and are situated in a cup-shaped cavity. The =foliate
papillæ= (Papillæ foliatæ) are situated just in front of the anterior
pillars of the soft palate, where they form a rounded eminence about an
inch (ca. 2 to 3 cm.) in length, marked by transverse fissures. The last
three varieties are covered with microscopic secondary papillæ and are
furnished with taste buds. The mucous membrane of the root of the tongue
presents numerous folds and depressions. Into the latter open the ducts
from the =lingual glands= (Glandulæ linguales), which constitute a thick
layer in the loose submucous tissue. Mucous glands are found also on the
dorsum and sides of the tongue. In the submucosa of the root is found
also a large quantity of lymph follicles (Folliculi tonsillares) and
diffuse lymphoid tissue.
[Illustration: