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138 ANATOMY.
ment throughout its entire extent to the mvlo-hyoid muscle, which,
together with the base of the tongue, forms the true floor.
The anterior and two Uiteral walls of the mouth are formed by the
alveolar processes and the teeth of both jaws.
Posteriorly the oral cavity opens into the pharyngeal space.

CARTILAGE.
Cartilage is one of the three groups of connective tissues of the
body. It is made up of cells imbedded in a matrix, which yields, on
boiling, chondrin, the basement-substance. That this differs from other
connective tissue has of late been questioned, and the view that it is a
distinct chemical substance now appears to be undergoing a change.
By some it is believed to be a mixture of gelatin, mucin, and salts.
(See Prudden's Normal Histology, p. 53.) Cartilage forms the entire
skeleton of many of the cold-blooded (or lower order of) animals, and
of others it constitutes a varying proportion. In the highest verte-
brates only a small portion of cartilage exists at puberty, though it is
found after this period in the covering of the articulating surfaces of
bones and connecting the ribs with the sternum ; in the rings of the
trachea, walls of the bronchi, larynx, and other parts of the air-
passages ; in the grooves through which muscular tendons glide ; and
in interarticular discs situated between the articulating surfaces of bones,
where a decidedly firm though more yielding structure than bone is
required. The early embryonic life of the entire skeleton, with but
minor exceptions, is composed of cartilage, in which is gradually depos-
ited calciferous matter; it is then apparently absorbed and replaced by
bone-cells or osteoblasts, which first appear at the different points of
ossification and develop the entire bony structure. Temporary cartilage
is that which gradually develops into bone. Permanent cartilage is that
which remains cartilage throughout life, as the interarticular discs that
cover the articular extremities of bones, etc.
The principal function of cartilage in the higher vertebrates is its
physical property of elasticity. It yields to pressure or to muscular
force, but immediately resimies its normal position or shape when such
pressure or force is removed.
When placed between articulating extremities, as the proximate sur-
faces of tlie vertebne, the temporo-maxillary articulation, etc. etc., it
acts as a cushion, diminishing the force of concussion. In positions
where shock would be particularly harmful there is interposed within
the joint, in addition to the cartilage covering the articular surfaces of
bones, a cartilaginous disc or extra cushion. Were it not for the car-
tilage placed within the joints situated between the feet and the head,
the shock communicated to the brain in the simple act of walking
would probably be so great as to absolutely prevent its practice in man.
The cartilages connecting the ribs with the sternum permit the