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THE SKIN. 141

Cells are often differently arranged in the same kind of cartilage, this
depending npon the depth of the cartilage and the connection it has
with other tissues. Where cartilage is joined to a synovial membrane
and an articulating capsule, the cartilage-cells are more or less branched,
and pass insensibly into the branched connective-tissue cells of the mem-
brane. In the hyaline cartilage of the foetus are many spindle or
Fig. 70.

: J!':!ii'i;i:: Ti':3;ii iii:ii;!i 'im 'M :iKi! saiie: liiil mw
1 «!i!'MWm iii::uii;iiiiiiiii;i! i!i.;iiiii: i; «':
Plan of the Multiplication of Cells of Cartilage : A, cell in its capsule : B, divided into two, each with
a capsule; C, primary capsule disajipeared, secondary capules coherent with matrix; i>, tertiary
division ; E, secondary capsules disappeared, tertiary coherent with matrix.
branched cells. The cells in the cartilage which separates an apophysis
from a diaphysis of long bones are arranged uniformly in vertical rows.
The Penchondrium covers cartilage as periosteum covers bone : it is
a vascular, fibro-connective tissue envelope containing a few elastic
fibres. It is furnished with blood-vessels, lymphatics, and nerves, and
is important, as it furnishes protection to the blood-vessels that supply
the cartilage. It covers all cartilage excepting that on the articulating
surfaces of bone and in the lines of ossification. Through the blood-
vessels of the perichondrium the adjacent cartilage receives nourishment
—not by the passage of the blood itself into the cartilage, but from the
plasma of the blood in the perichondrium, which permeates the cartilage
through numberless minute tubes that open into the lacunse or capsules
of the cartilage by one end, while by the other they open into larger
tubes, freely communicating with the perichondrium.
The distance between the substance of the cartilage and its source
of pabulum or nourishment accounts for its slow repair after injury.
When cartilage has been injured the wound at first fills up with connec-
tive tissue. This connective tissue at times remains permanent, but is
occasionally transformed into hyaline cartilage. When the lesions are
deep the margins of the wound, being situated nearer to the perichon-
drium, are more likely to heal than the deeper portions.

THE SKIN.
The skin is the superficial covering of the body, extending over its
entire surface and into the openings of its mucous canals to varying
depths until it joins their mucous membrane. It is flexible, elastic,
and extensible. It is loosely attached to the parts directly beneath,