Hygienic Physiology | Page 6

ends of the bone, thus giving greater size to form a strong joint,
while the solid portion increases near the middle, where strength alone
is needed. Each fiber of this bulky material diminishes the shock of a
sudden blow, and also acts as a beam to brace the exterior wall. The
recumbent position of the alligator protects him from falls, and
therefore his bones contain very little spongy substance.
In the body, bones are not the dry, dead, blanched things they
commonly seem to be, but are moist, living, pinkish structures, covered
with a tough membrane, called the per-i-os'-te-um [Footnote: The
relations of the periosteum to the bone are very interesting. Instances
are on record where the bone has been removed, leaving the periosteum,
from which the entire bone was afterward renewed.] (peri, around, and
osteon, a bone), while the hollow is filled with marrow, rich in fat, and
full of blood vessels. If we examine a thin slice with the microscope,
we shall see black spots with lines running in all directions, and
looking very like minute insects. These are really little cavities, called
la-cu'-næ [Footnote: When the bone is dry, the lacunæ are filled with
air, which refracts the light, so that none of it reaches the eye, and
hence the cavities appear black.] from which radiate tiny tubes. The
lacunæ are arranged in circles around larger tubes, termed from their
discoverer, Haversian canals, which serve as passages for the blood
vessels that nourish the bone.
GROWTH OF THE BONES.--By means of this system of canals, the
blood circulates as freely through the bones as through any part of the
body, The whole structure is constantly but slowly changing, [Footnote:
Bone is sometimes produced with surprising rapidity. The great Irish
Elk is calculated by Prof. Owen to have cast off and renewed, annually
in its antlers eighty pounds of bone.] old material being taken out and
new put in. A curious illustration is seen in the fact that if madder be
mixed with the food of pigs, it will tinge their bones red.

REPAIR OF THE BONES.--When a bone is broken, the blood at once
oozes out of the fractured ends. This soon gives place to a watery fluid,
which in a fortnight thickens to a gristly substance, strong enough to
hold them in place. Bone matter is then slowly deposited, which in five
or six weeks will unite the broken parts. Nature, at first, apparently
endeavors to remedy the weakness of the material by excess in the
quantity, and so the new portion is larger than the old. But the extra
matter will be gradually absorbed, sometimes so perfectly as to leave
no trace of the injury. (See p. 271.)
A broken limb should be held in place by splints, or a plaster cast, to
enable this process to go on uninterruptedly, and also lest a sudden jar
might rupture the partially mended break. For a long time, the new
portion consists largely of animal matter, and so is tender and pliable.
The utmost care is therefore necessary to prevent a malformation.
THE JOINTS are packed with a soft, smooth cartilage, or gristle, which
fits so perfectly as to be airtight. Upon convex surfaces, it is thickest at
the middle, and upon concave surfaces, it is thickest at the edge, or
where the wear is greatest. In addition, the ends of the bones are
covered with a thin membrane, the synovial (sun, with; ovum, an egg),
which secretes a viscid fluid, not unlike the white of an egg. This
lubricates the joints, and prevents the noise and wear of friction. The
body is the only machine that oils itself.
The bones which form the joint are tied with stout ligaments (ligo, I
bind), or bands, of a smooth, silvery white tissue, [Footnote: The
general term tissue is applied to the various textures of which the
organs are composed. For example, the osseous tissue forms the bones;
the fibrous tissue, the skin, tendons, and ligaments.] so strong that the
bones are sometimes broken without injuring the fastenings.
II. CLASSIFICATION OF THE BONES.
For convenience, the bones of the skeleton are considered in three
divisions: the head, the trunk, and the limbs.
1. THE HEAD.

THE BONES OF THE SKULL AND THE FACE form a cavity for the
protection of the brain and the four organs of sense, viz.: sight, smell,
taste, and hearing. All these bones are immovable except the lower jaw,
which is hinged [Footnote: A ring of cartilage is inserted in its joints,
something after the manner of a washer in machinery. This follows the
movements of the jaw, and admits of freer motion, while it guards
against dislocation.] at the back so as to allow for the opening and
shutting of the mouth.
THE SKULL is composed,