THE SUPPORTING FRAMEWORK
Since protoplasm is so very soft and fragile it must be supported in all
animals and plants except the very tiniest. The nature of the supporting
framework has a great deal to do with both the form and the working of
the body, so it is desirable that we become familiar with it before
trying to go further in the examination of the living protoplasm itself.
A large heavy body like that of man requires an arrangement for support
that shall meet several conditions. In the first place there must be
strength and stiffness, combined with flexibility, so that the body as a
whole shall be firm, yet not rigid. The weight, also, must be kept as
small as possible. Then every single cell, and every grouping of cells
that we call an organ, must be supported in its place securely but
without hindering the free performance of its function. Not only must
the protoplasm be held in place, but on account of its fragility it has
also to be protected against injury; the vital parts require more
careful protection than those that are less immediately essential to
life. Finally, bodily motions of all sorts depend on the framework to
give purchase to the muscles, which are the actual organs of motion, and
so to make their movements effective. For support, for protection, and
for motion, then, the framework is important.
The material that does the real supporting is not, of course, alive, for
living protoplasm lacks the necessary qualities needed here. It is
manufactured and put in place, however, by living cells. They do this by
withdrawing the special materials needed from the body fluid which
surrounds them; in large part what they get from the fluid is not the
finished substance but material from which the living cells make the
finished substance. It is then passed outside their bodies and deposited
in the surrounding space. Of course this is a gradual process. Bit by
bit the structure, bone, cartilage, or connective tissue, as the case
may be, is built up by the combined activities of many cells.
Of the three kinds of supporting material mentioned above, bone is the
most familiar. No description of its appearance is necessary, for
everyone has seen it as it appears in meat animals and in poultry, and
it looks precisely the same in man. There are several things about bone,
however, that are worth describing. One is the arrangement by which the
very hard, compact material is deposited in large masses without cutting
off the cells which are doing the depositing from their contact with the
body fluid, and so destroying them and bringing their work to an end.
The way this is managed can be made out by examination of the figure,
showing the structure of bone. At the beginning the bone cells are lying
near one of the tiny blood vessels known as _capillaries_, which are the
exchange stations for material between blood and the stationary part of
the body fluid. Thus these cells are favorably located for obtaining
materials from which bone can be constructed. As they proceed with the
formation of bone they always leave tiny passages open between
themselves and the blood capillary. Finally the capillary may become
completely surrounded by bone, but all along it will be left the
passages through which fluid can make its way from the blood to where
the cells are imprisoned within the bony walls of their own
construction. The metabolism of bone cells is not on a very active
scale; the amount of bone substance that a single bone cell has to
produce in a day is only a fraction of the amount of saliva, for
instance, that a single cell of the salivary gland turns out in the same
time; so the bone cell can manage even though its supply of material has
to come to it through a few very tiny passages in the bone.
[Illustration: CROSS SECTION OF COMPACT BONE FROM THE SHAFT OF THE
HUMERUS
_A_, bone cells; _B_, blood capillaries. (From Martin’s “Human Body”)]
Another interesting feature of bone is the ease with which it can be
remodeled. We are apt to think of bone as permanent, after it has once
been formed, but as a matter of fact bone is about as subject to change
as any of the softer tissues. This is because there are in and around
the bones, in addition to the bone-forming cells, a great many cells of
different appearance which may be named bone-destroying cells. These
latter have the ability to dissolve out the hard material which the
bone-forming cells have deposited. Good examples of their work are seen
in the hollows of the long bones. We know, of course, that the bones in
a child’s leg are so much smaller than those in the leg of an adult that
they could almost be fitted into the hollows of the latter. Evidently
the bone substance has been moved bodily outward during the course of
growth. As the bone-forming cells add material to the outer surface of
the bone, the bone-destroying cells dissolve it away from the inner
surface. The same thing happens all over the body. A child’s face grows
by an increase in size of the bones. Again the inner surfaces are
dissolved away. Apparently one condition which makes the bone-destroying
cells active is constant pressure. A good example of this is seen in
what is known as a gumboil. If a tooth becomes ulcerated, gas and pus
are formed at its root, and cannot escape since this is completely
surrounded by bone. They accordingly press upon the surrounding bone,
and also upon the sensitive tissues, resulting in extreme pain. The
pressure upon the bone starts the bone-destroying cells into great
activity and in the course of a few days they will dissolve a hole right
through the bone, allowing the gas and pus to escape to the outside, and
relieving the pain.
Of recent years school authorities have had much to say about the
importance of adjusting school seats and desks so that they shall be at
the proper height for the particular children that are to occupy them.
This is because if the feet hang clear of the ground for hours at a
time, as they will if the seat is too high, or if the body must be
screwed around to enable the child to work at his desk, as happens when
the desk is too low, there is real danger that some of the bones may
become misshapen. Most of the stoop shoulders and many of the crooked
backs that we see are the result of the habitual taking of wrong
postures. Children, and adults as well, should form habits of standing
and sitting so straight that none of the bones are put under a pressure
that may tend to distort them.
After the teeth are lost the bony sockets in which they lie are
dissolved away, making the jaws much shallower than formerly, a fact
that accounts for the shortening of the distance between chin and nose
in aged people. An important result of this dissolving away of bone by
the bone-destroying cells is that the bones are kept as light as
possible, without undue sacrifice of strength.
A second kind of supporting material is cartilage. This is both softer
and more flexible than bone. It is found in places where flexibility is
more important than great strength, as in the ears, the parts of the
nose just below the bridge, the Adam’s apple and wind pipe. The chief
difference in make-up between bone and cartilage is that while in bone
about three-fourths of the nonliving substance consists of lime salts,
in cartilage there is almost none of this material, organic substances
making up the entire mass. There are no living cells in the body that
are more poorly located with respect to obtaining supplies from the body
fluids than the cartilage cells, for as these deposit the cartilage
around themselves they leave no definite passages through which fluid
may pass; the material incloses the cells completely. Although cartilage
looks as though it were altogether nonporous, there must be some degree
of sponginess present, since the cells do succeed in getting the
materials on which their life depends. Cartilage seems to be a more
primitive kind of supporting substance than bone. This is shown by the
fact that it makes up the entire skeleton in the lowest fishes, and also
by the fact that in the higher animals, including man, the bony skeleton
starts, in large part, as cartilage. In the parts in which this happens
a mass of cartilage is deposited in the place which is later to be
occupied by bone. Then at certain points the cartilage begins to be
dissolved away by cartilage-destroying cells, which are precisely like
bone-destroying cells, and the bone-forming cells come in and build up
the real bone as fast as the cartilage is removed. This process of
replacing cartilage by bone is practically completed at birth, except in
the long bones of legs and arms. These bones, which will about double in
length during the growth of the body to adult size, as well as the other
bones, which grow to some extent, retain plates of cartilage near each
end during all the growth period, and the increase in length is obtained
by a continuous formation of new cartilage, which is continuously
replaced by bone.
The third kind of supporting material is connective tissue. This is
composed of tiny threads or fibers, some of which are inelastic, others
are very elastic. The inelastic fibers are found in places where a
flexible, but unyielding support is required; the elastic fibers are
located where elasticity is particularly important. Either kind of fiber
may be grouped into sheets, or into loose networks, or into stout
cords. A good example of inelastic connective tissue in sheet form is in
the _mesentery_ which holds the organs of the abdominal cavity in place.
Just under the skin, anchoring it loosely to the underlying muscles, is
connective tissue in network form. The tendons by which most of the
muscles are attached to the bones upon which they pull are made up of
inelastic connective tissue in the form of cords. The best example of
elastic connective tissue is in the large arteries, which are just as
elastic as best quality rubber tubing. Another good example is the large
and strong elastic cord which passes along the back of the neck in
cattle and sheep, and helps to support the weight of the head.
Connective tissue fibers are deposited by living connective tissue
cells. Since connective tissue is of open and relatively loose
construction, there is no problem presented in supplying the cells with
material. The meshes among the fibers are filled with fluid, and this
fluid has ready connection, in turn, with the blood. Use is made of the
abundance of body fluid in the connective tissue spaces whenever a
subcutaneous injection is given, for what is done is to inject the
desired material into the fluid which fills the spaces in the connective
tissue just beneath the skin, trusting that it will pass from there to
the blood, which it does rather gradually, and so is distributed about
the whole body.
While we are on the topic of the supporting framework, something must be
said about the grouping of the bones into what we know as the skeleton.
Of course it is evident that the effectiveness of the bony part of the
framework depends almost altogether on the way in which the individual
bones are grouped together. If the whole skeleton were
[Illustration: THE BONY AND CARTILAGINOUS SKELETON
(From Martin’s “Human Body”)]
composed of one great bone, or of different bones anchored solidly
together, the body would be perfectly rigid; since motion is necessary
to life, flexible connections between some of the bones are absolutely
essential. Our movements are actually made by means of muscles, but
nearly all of them become effective through the motions of bones to
which the muscles are fastened. The bones are often very irregular in
shape; careful study shows that the irregularities are due either to
provision for the contact of one bone with another in the joints, a
contact that must allow in most cases for motion of one on the other, or
to provision of places to which muscles can be fastened in such a way as
to make their pull effective. It is, of course, out of the question for
us to examine the skeleton bone by bone. Figures are given of a number
of typical bones: all that we can do in addition is to mention some of
the interesting features of the skeleton.
The skeleton of the head is called the skull; its chief features are the
brain case, the eye sockets, and the parts about the nose and mouth. The
brain case is made up of eight bones firmly joined together by saw-tooth
margins to make up a roughly spherical box which holds the brain, and
protects this delicate and vitally important organ from all injury
except the most severe. There are a number of small openings out from
the brain case through which nerves pass, and one large opening below
and at the back through which the spinal cord merges into the brain. The
bones which make up the sides of the brain case are much thickened just
behind the ears. A hollow extends from each ear into the bone, and
within this hollow, securely protected from harm, is the actual organ of
hearing. There are extensions of the hollow backward which are not
occupied by any organs, and which communicate with the cavity of the
ear. These sometimes become infected from the ear, causing the condition
known as mastoiditis. Not only is this condition excruciatingly painful,
but on account of the thin layer of bone which separates it from the
brain itself it is highly dangerous. For this reason any ear trouble
should be carefully watched lest it develop into mastoid trouble.
[Illustration: A SIDE VIEW OF THE SKULL]
Of the bones that make up the eye sockets not much need be said, except
that they have a great deal to do with determining the shape of the
upper part of the face and so the appearance. There are bones within the
nostrils that are very irregular in outline. Their effect is to increase
greatly the surface over which the air that is breathed must pass,
enabling it to become both warm and moist before entering the lungs. The
jaw bones serve as receptacles for the teeth; the lower jaw, which is
the only movable bone of the head, except for the tiny bones within the
ears, has also the duty of operating as a mill in reducing the food to
suitable form for swallowing. To aid in this function the lower jaw is
hinged to the rest of the skull in such a way that it not only opens and
closes but can slide forward and back or from side to side. All these
motions are used in chewing. There are twenty-two bones altogether in
the skull, not counting the three tiny ones in each ear which will be
described later.
[Illustration: FRONT VIEW OF TRUNK AND LIMB ARCHES
_c_, collar bone; _s_, shoulder blade; _Oc_, innominate bone (From
Martin’s “Human Body”)]
[Illustration: SIDE VIEW OF THE SPINAL COLUMN
_C_ 1-7, cervical; _D_ 1-12, dorsal; _L_ 1-5, lumbar; _S_ 1, sacrum;
_Co_ 1-4, coccygeal. (From Martin’s “Human Body”)]
The body consists of trunk and limbs, and each part has its skeleton.
The skeleton of the trunk consists of the spinal column, the rib cage,
the shoulder girdle, and the hip girdle. The skeletons
[Illustration: Photo, A. N. Mirzaoff
A FRENCH METHOD OF MEASURING VERTICAL CONFORMATION, CONSIDERED IMPORTANT
FOR ATHLETES]
[Illustration: Photo, A. N. Mirzaoff
THE CURVE OF THE SPINE MEASURED AND DRAWN FOR STUDY OF AN ATHLETE’S
POSSIBILITIES]
of the limbs are all according to a single plan to be described in a
moment. The spinal column is a remarkable example of strength combined
with flexibility and elasticity. It is made up of thirty-three bones or
_vertebræ_; each of these has a disk-shaped part known as the _body_,
and these disks are placed in line as shown in the figure. Between each
disk and its neighbor is an elastic pad composed of a mixture of
cartilage and elastic connective tissue. There is a small amount of give
in each pad and this, taken along the whole length of the spinal column,
is enough to give it the great flexibility which it enjoys. During the
day the weight of the body packs these pads down hard, so that it is
said that a man may be as much as an inch shorter at night than in the
morning. Behind the disk each vertebra has an arch of bone, and beyond
and beside this arch most of them have projections. All the arches
together make up a bony canal which contains and protects the spinal
cord. The projections serve for the attachment of the ribs and the back
muscles by which the bending motions of the body are made.
The rib cage includes the breastbone and twelve pairs of ribs. It serves
two purposes: to protect the heart and lungs from injury; and to take
part in the movements of breathing. The latter function involves some
degree of motion of the rib cage. All the ribs are attached behind to
the vertebral column in a fashion that permits of a little motion up and
down. All except the last two are fastened in front, seven pairs to the
breastbone, three pairs each to the rib above it. In breathing the
breastbone and ribs are moved up and down by muscles attached to them.
The shoulder girdle is made up of the collar bones and shoulder blades.
Each collar bone is fastened at its inner end to the upper edge of the
breastbone; this is the only direct contact the shoulder girdle has with
any other part of the skeleton of the trunk; at the point where collar
bone and shoulder blade meet, there is a shallow cup into which the head
of the skeleton of the arm fits. The arrangement is favorable to great
freedom of movement of the arm. Not only is the shoulder joint very
flexible owing to the shallowness of the cup into which the arm bone
fits, but the shoulder blade itself is capable of a considerable range
of movement. This is because it is imbedded in and held in place by
muscles. If one watches a person with bare shoulders while he raises his
arms, it will be seen that the shoulder blades do not move much while
the arms are being lifted to the horizontal position, but as that point
is passed they begin to swing outward rapidly, so that when the arms are
high above the head the shoulder blades are in a quite different
position from that which they have when the arms are down.
The hip girdle consists of five bones of the vertebral column welded
firmly together to make up what is called the sacrum, and two other
large bones known as the innominate bones, each of which, in turn, is
made up of three bones tightly fused together. The innominate bones are
firmly joined to the sacrum at the back and they meet in front, also in
a firm joint. The hip girdle or _pelvis_ is rigid, suiting it to bear
the strains that come upon it on account of its position at the junction
of the legs with the trunk. At the outer side of each innominate bone is
a cup, much deeper than the corresponding cup of the shoulder girdle,
and into this fits the head of the skeleton of the leg. The arrangement
is a typical ball and socket, and has been much copied in machinery
where a flexible joint is required. In a good many people the union of
the innominate bones to the sacrum is not so firm but that it yields
somewhat when strains are put on it. Ordinary strains in these cases
produce severe backache. Heavy strains may cause an excessively painful
as well as disabling dislocation. In either case medical attention is
needed.
Each arm can be subdivided into upper arm, forearm, wrist, and hand. The
skeleton of the upper arm is a single long bone. The forearm has two
bones, one of which is hinged at the elbow to the bone of the upper arm
in a way to limit the movement to the single back and forth swing of
which the elbow is capable. The other bone of the forearm can be rolled
over the one which is fast at the elbow; this is what happens whenever
the hand is changed from the palm up to the palm down position. There
are eight bones in the wrist; these are irregular in shape, and are so
grouped as to permit of a very wide range of movement. The bones of the
hand and fingers make up five rows numbering four bones each for the
fingers and three for the thumb. The joints are all practically simple
hinges except for the one where the thumb joins the wrist, which is a
much more flexible joint; flexible enough, in fact, to allow the thumb
to be brought opposite any of the fingers. No animal except man enjoys
this degree of flexibility in the thumb, so no animal equals man in the
nicety of the grasp, particularly of small tools. When we recall how
constantly we take advantage of this property of our hands we can
realize how greatly our superiority over the lower animals has been
aided by this rather slight structural difference between our hands and
theirs.
[Illustration: THE BONES OF THE ARM
_a_, upper arm; _b_, forearm; _c_, wrist; _d_, hand
(From Martin’s “Human Body”)]
The leg subdivides along the same lines as the arm into upper leg, or
thigh, lower leg, or shin, and foot. The order of bones is, on the
whole, the same; one in the thigh; two in the lower leg. Instead of a
flexible wrist the corresponding bones of the foot are grouped into a
less flexible, but much stronger, heel and upper instep. Two of the
bones of this group are fused together into one, reducing the total
number from eight to seven. The bones of the lower instep and toes
correspond in number and arrangement to those of the hand and fingers,
but the great toe does not have superior flexibility as does the thumb.
There is one bone in the leg, the knee cap or _patella_, that does not
correspond to any bone in the arm, although it does correspond to a part
of a bone, namely, the projection, at the elbow, of the long bone of the
forearm. A feature of the skeleton of the foot that is worth a word is
the arching of the instep. This undoubtedly adds greatly to the ease of
walking. The natural position for the foot is, of course, with both the
heel and the ball of the foot on the ground. For some reason it has
become the universal custom among civilized people to raise the heel off
the ground by adding a heel to the shoe. This does not seem to make much
difference as long as the heel is not too high. In fact soldiers wearing
properly fitted heel shoes can march as far and fast as can be expected.
Excessively high heels throw the weight too much on the ball of the
foot, thus doing away with the benefits that come from the arching of
the instep. The effect on the gait is very apparent in any one who walks
in high-heeled shoes. The foot itself does not appear to be greatly
harmed by the wearing of high heels provided the shoes are otherwise
well fitting. Whether the heels are high or low, the fit of the shoe is
of utmost importance to the preservation of the feet. Crowding the feet
into shoes that are too small in any direction is a fruitful means of
bringing on foot trouble. Wearing shoes that are loose enough to allow
the foot to turn over inside the shoe is nearly as bad. If the shoes
are properly fitted in the beginning and then the heels are kept squared
up, so that the feet will always stand straight on the ground, there
will usually be little trouble with fallen arches or other foot
disturbances.
[Illustration: BONES OF THE LEG
_a_, thigh; _b_, shin; _c_, foot; _d_, knee cap (From Martin’s “Human
Body”)]
The bones are fastened together at the movable joints by stout sheets or
bands of connective tissue known as ligaments. These hold them in place
very securely and as additional support the muscles which surround
every joint help to prevent the bones from slipping out of place. At
nearly all the joints of the body the combined action of ligaments and
muscles is sufficient to guarantee the joint against dislocation; the
shoulder joint, and to a less extent the hip joint, is more likely to
suffer this accident. The reason is that in obtaining flexibility of
movement security of attachment is somewhat lessened. If the ligaments
at the shoulder were tight enough to prevent the joint from ever
becoming dislocated they would bind it to a serious degree. Most of the
ligaments are of inelastic connective tissue, but those that fasten the
separate vertebræ of the spinal column together are elastic, allowing of
the bending in every direction which makes our backs as flexible as they
are. The only movable joints which are bound by other means than
ligaments are the connections of the ribs with the breastbone. These are
of cartilage, but the movement here is so slight that the cartilage
yields enough to permit it.
This completes our account of the bony skeleton. We shall finish the
description of the supporting framework by a word about what may be
called the connective tissue skeleton. The bony skeleton serves to
support the body as a whole and to permit the muscles to do their work;
the individual organs and the cells which make them up are held in place
by sheets and bands of connective tissue. These are coarse and strong
when their purpose is to support a large and heavy organ like the
stomach; they become finer and finer as the parts to be supported become
smaller, and when the individual cells are reached the connective tissue
which surrounds them is almost inconceivably delicate. So completely
does connective tissue permeate the whole body that it has been said
that if everything else could be dissolved away, leaving only this
tissue in place, there would still remain a model of the body, complete
to the last detail.