A Practical Physiology | Page 9

see

Chapter XV.
[NOTE. Inasmuch as most of the experimental work of this chapter
depends upon the use of the microscope and also necessarily assumes a
knowledge of facts which are discussed later, it would be well to
postpone experiments in histology until they can be more satisfactorily
handled in connection with kindred topics as they are met with in the
succeeding chapters.]

Chapter II.
The Bones.

27. The Skeleton. Most animals have some kind of framework to
support and protect the soft and fleshy parts of their bodies. This
framework consists chiefly of a large number of bones, and is called the
skeleton. It is like the keel and ribs of a vessel or the frame of a house,
the foundation upon which the bodies are securely built.
There are in the adult human body 200 distinct bones, of many sizes
and shapes. This number does not, however, include several small
bones found in the tendons of muscles and in the ear. The teeth are not
usually reckoned as separate bones, being a part of the structure of the
skin.
The number of distinct bones varies at different periods of life. It is
greater in childhood than in adults, for many bones which are then
separate, to allow growth, afterwards become gradually united. In early
adult life, for instance, the skull contains 22 naturally separate bones,
but in infancy the number is much greater, and in old age far less.
The bones of the body thus arranged give firmness, strength, and
protection to the soft tissues and vital organs, and also form levers for
the muscles to act upon.
28. Chemical Composition of Bone. The bones, thus forming the
framework of the body, are hard, tough, and elastic. They are twice as
strong as oak; one cubic inch of compact bone will support a weight of
5000 pounds. Bone is composed of earthy or mineral matter (chiefly in
the form of lime salts), and of animal matter (principally gelatine), in
the proportion of two-thirds of the former to one-third of the latter.
[Illustration: Fig. 10.--The Skeleton.]
The proportion of earthy to animal matter varies with age. In infancy
the bones are composed almost wholly of animal matter. Hence, an
infant's bones are rarely broken, but its legs may soon become

misshapen if walking is allowed too early. In childhood, the bones still
contain a larger percentage of animal matter than in more advanced life,
and are therefore more liable to bend than to break; while in old age,
they contain a greater percentage of mineral matter, and are brittle and
easily broken.
Experiment 3. To show the mineral matter in bone. Weigh a large soup
bone; put it on a hot, clear fire until it is at a red heat. At first it
becomes black from the carbon of its organic matter, but at last it turns
white. Let it cool and weigh again. The animal matter has been burnt
out, leaving the mineral or earthy part, a white, brittle substance of
exactly the same shape, but weighing only about two-thirds as much as
the bone originally weighed.
Experiment 4. To show the animal matter in bone. Add a teaspoonful of
muriatic acid to a pint of water, and place the mixture in a shallow
earthen dish. Scrape and clean a chicken's leg bone, part of a sheep's rib,
or any other small, thin bone. Soak the bone in the acid mixture for a
few days. The earthy or mineral matter is slowly dissolved, and the
bone, although retaining its original form, loses its rigidity, and
becomes pliable, and so soft as to be readily cut. If the experiment be
carefully performed, a long, thin bone may even be tied into a knot.
[Illustration: Fig. 11.--The fibula tied into a knot, after the hard mineral
matter has been dissolved by acid.]
29. Physical Properties of Bone. If we take a leg bone of a sheep, or a
large end of beef shin bone, and saw it lengthwise in halves, we see two
distinct structures. There is a hard and compact tissue, like ivory,
forming the outside shell, and a spongy tissue inside having the
appearance of a beautiful lattice work. Hence this is called cancellous
tissue, and the gradual transition from one to the other is apparent.
It will also be seen that the shaft is a hollow cylinder, formed of
compact tissue, enclosing a cavity called the medullary canal, which is
filled with a pulpy, yellow fat called marrow. The marrow is richly
supplied with blood-vessels, which enter the cavity through small
openings in the compact tissue. In fact, all over the surface of bone are
minute canals leading into the substance. One of these, especially
constant and large in many bones, is called the nutrient foramen, and