he fibrous membrane that invests the bones is called
_per-i-os'te-um_; that which covers the cartilages is called
_per-i-chon'dri-um_. When this membrane invests the skull, it is
called _per-i-cra'ni-um_.

[Illustration: Fig. 6. A section of the knee-joint. The lower part of the
femur, (thigh-bone,) and upper part of the tibia, (leg-bone,) are seen
ossified at 1, 1. The cartilaginous extremities of the two bones are seen
at _d_, _d_. The points of ossification of the extremities, are seen at
2, 2. The patella, or knee-pan, is seen at _c_. 3, A point, or centre of
ossification.]

73. The PERIOSTEUM is a firm membrane immediately investing the bones,
except where they are tipped with cartilage, and the crowns of the
teeth, which are protected by enamel. This membrane has minute nerves,
and when healthy, possesses but little sensibility. It is the
nutrient membrane of the bone, endowing its exterior with vitality; it
also gives insertion to the tendons and connecting ligaments of the
joints.

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72. What is the membrane called that invests the bones? That covers
the cartilage? That invests the skull? Explain fig. 6. 73. Describe
the periosteum.

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74. There are two hundred and eight[3] bones in the human body,
beside the teeth. These, for convenience, are divided into four
parts: 1st. The bones of the _Head_. 2d. The bones of the _Trunk_.
3d. The bones of the _Upper Extremities_. 4th. The bones of the
_Lower Extremities_.

[3] Some anatomists reckon more than this number, others less, for
the reason that, at different periods of life, the number of
pieces of which one bone is formed, varies. _Example._ The
breast-bone, in infancy, has _eight_ pieces; in youth, _three_;
in old age, but _one_.

75. The bones of the HEAD are divided into those of the _Skull_,
_Ear_, and _Face_.

76. The SKULL is composed of eight bones. They are formed of two
plates, or tablets of bony matter, united by a porous portion of bone.
The external tablet is fibrous and tough; the internal plate is dense
and hard, and is called the _vit're-ous_, or glassy table. These
tough, hard plates are adapted to resist the penetration of sharp
instruments, while the different degrees of density possessed by the
two tablets, and the intervening spongy bone, serve to diminish the
vibrations that would occur in falls or blows.

77. The skull is convex externally, and