CHAPTER III

THE SKELETON

_Experiment 1._ (At home.) =Is the Arch of the Foot Elastic?=--Wet the foot in a basin of water and, while sitting, place the foot flat upon a piece of paper. Draw the outline of the track. Repeat, but stand with your whole weight upon the foot. Draw track. Conclusion? (Take sketches to school. Which sketch shows the flattest foot?) Devise a method for measuring the length of the foot with and without the weight of the body upon it. What difference? Conclusion?

_Experiment 2._ =Composition of Bone.=--Place a bone in a hot fire and let it remain for three or four hours. It will keep its shape however long you burn it; but unless you handle it carefully when you take it out, it will crumble to pieces. If not thoroughly burned, the bone will be black from the carbon of the animal matter still left in it. _Experiment 3._ Obtain a slender bone like the rib of a hog or the leg bone of a fowl, and put the raw bone into a vessel containing strong vinegar or two ounces of muriatic acid and a pint of water. Leave it there for four days. When the bone is taken out, it can be tied into a knot. The acid may be washed off, and the bone preserved in a bottle of alcohol or glycerine.

_Experiment 4._ =The Forms of Joints.=--Obtain the disjointed bones of a fowl or small mammal and place them one at a time in their sockets and study the fit and motion of the joints.

_Experiment 5._ =Pivot Joints.=--Through what fraction of a circle do the pivot joints in the forearm and neck allow the hand and head to rotate?

=Review Questions.=--Where are the bone cells? How does nourishment reach them? How has the mineral part of the bones been deposited? How long may bone cells live? Name animals with outside skeletons. Inside skeletons. No skeleton.

=Forms and Uses of Bones.=--The three chief _uses of bones are protection, motion, and support_. In order to fulfill these purposes, the bones must have different sizes, shapes, and positions. The bones are classed by _shape_, as _long, flat, and irregular_. Those whose chief use is _to protect are broad and flat_. The bones which _furnish support are thick and solid_; those designed to _aid in motion are long and straight_. Including six small bones in the ear, there are two hundred and six bones in the adult skeleton.

=Gross Structure of Bones.=--The structure of a long bone is shown in Fig. 29. It has a long, _hollow shaft_ of hard, compact bone, and _enlarged ends_ composed of spongy bone. The hollow in the shaft is _filled with yellow marrow_, which is composed of blood vessels and fat, and aids in nourishing the bone. The long bones are found in the limbs (Fig. 28). The ribs and other flat bones and the irregular bones contain no yellow marrow; they are spongy inside, and hard and compact near the surface. There is a _red marrow_ in the cavities in the spongy parts of bones (Fig. 29). _New red blood cells are formed in this marrow._ The bones have a close-clinging, fibrous covering composed of connective tissue and blood vessels. It is called _periosteum_.

[Illustration: FIG. 29.--FEMUR, sawed lengthwise. The red blood cells are formed in the red marrow of the spongy part.]

[Illustration: FIG. 30.--FRONT VIEW OF RIGHT FEMUR.]

=Chemical Composition of Bone.=--Experiments (2 and 3) show that the bones contain a _mineral or earthy substance_, which makes them hard and stiff, and a certain amount of _animal matter, called gelatine_, which binds the mineral matter together and makes the bones tough and somewhat elastic. The fire burned out the animal matter of the first bone, and the acid dissolved out the mineral matter of the second bone. _The mineral matter is chiefly lime, and makes up about two thirds of the weight of the bone._ (Why is more mineral than animal matter needed?) The animal gelatine is a gristly substance. As the body grows old, the animal matter of the bones decreases, and they become lighter. They are more easily broken and do not heal so readily as the bones of young persons.

=The skeleton is subdivided= into the bones of the _head, trunk, and limbs_. The bones of the trunk are those of the spine, the chest, the shoulder blades, collar bone, and hip bones.

[Illustration: FIG. 31.--VERTEBRAL COLUMN. Side view.]

=The spinal or vertebral column= is made up of twenty-six bones (Fig. 31). It is the axis of the human skeleton, to which all other bones are directly or indirectly attached. Animals with inside skeletons have this column, and are called vertebrates. Fish, reptiles, birds, beasts, apes, and man are vertebrates. The spine, as this column is sometimes called, is not only the main connecting structure and support of the body, but it forms a channel through which passes the spinal cord.

Fig. 32 shows a =vertebra=, or one of the bones that compose the column. The three _projecting points or processes_ are for the attachment of ligaments and muscles. The _main body_ of each vertebra is for supporting the weight transmitted by the column above. Just behind this thick body is a _half ring_ (Fig. 32), which with the half rings on the other vertebrae form the channel for the spinal cord. Between the vertebrae are thick pads of gristle, or cartilage, which act as cushions to prevent jars, and by compression allow bending of the spinal column in all directions.

[Illustration: FIG. 32.--SIDE AND UNDER VIEW OF A VERTEBRA.]

=The Chest= (see Fig. 75).--The twelve pairs of ribs are attached to the spinal column behind, and extend around toward the front of the body, somewhat like hoops. The first seven pairs, called _true ribs_, are attached directly to the flat breastbone, or _sternum_. Each of the next three pairs, called _false ribs_, is attached to the pair above it. The last two pairs, called _floating ribs_, are free in front.

=The Shoulder Girdle.=--_The collar bones_ (Fig. 28) can be traced from the shoulders until they nearly meet on the breastbone at the top of the chest. The collar bone is shaped like the italic letter _f_; it helps to form the shoulder joint and holds the shoulder blade out from the chest that the motions of the arm may be free.

The flat, triangular _shoulder blade_ (Fig. 75) can be felt by reaching with the right hand over the left shoulder. It spreads over the ribs like a fan. Its edges can be made out, especially if the shoulder is moved while it is being felt. The high ridge which runs across the bone can be felt extending to the top of the shoulder.

=The Pelvic Girdle.=--The edges of the _hip bones_ can be felt at the sides of the hips (Fig. 28). The hip bones, with the base of the spine, form a kind of basin called the _pelvis_.

The =skull= (Fig. 33) rocks, or nods, on the top vertebra. It consists of the cranium, or brain case, and the bones of the face. The shapes and names of the bones of the skull are shown in Fig. 33.

[Illustration: FIG. 33.--HUMAN SKULL, disjointed.]

=Adaptations of the Skull for Protection.=--Its arched form is best for resisting pressure and turning aside blows. Like all flat bones, the skull has a spongy layer of bone between the layers of compact bone forming the outer and inner surfaces; hence it is elastic and not easily cracked. The nose, brow, and cheek bones project around the eye for its protection. The delicate portions of the ear are embedded in the strongest portion of the skull. The branches of the nerves of smell end in the lining of the bony nasal chambers. The spinal cord rests securely in the spinal canal.

The =arms and legs= have bones that closely correspond to each other. The Latin names of these bones, as well as of all the other bones, are given in Fig. 28. There are 30 bones in each arm and 30 in each leg (Fig. 34). Here is a list of the bones of the arm, followed by the names in brackets of the corresponding leg bones: upper arm bone [thigh bone], 2 forearm bones [shin bone and splint bone], 8 wrist bones [7 ankle bones], 5 palm bones [5 bones of instep], 14 finger bones [14 toe bones]. The shin bone is the larger bone between knee and ankle. The long, slender splint bone and the shin bone are bound side by side.

[Illustration: FIG. 34.--BONES OF ARM AND LEG.]

[Illustration: FIG. 35.--SUTURES OF SKULL.]

=Differences between Arm and Leg.=--There is a saucer-like bone, called the _kneecap_, embedded in the large ligament which passes over each knee. There is no such bone in the elbow. There is one less bone in the ankle than in the wrist, hence there are the same number of bones in the arm and leg. The shoulder joint is more freely movable than the hip joint. The fingers are longer and more movable than the toes; the thumb moves far more freely than the big toe. The instep is much stronger than the palm; for each instep must support, unaided, the weight of the whole body at each step, with any other weight that the person may be carrying. The palm is nearly flat, but the instep is arched to prevent jars. When the weight of the body is thrown on the foot at each step, the top of the arch is pressed downward, making the foot longer than before. The arch springs up when the weight is removed (Exp. 1).

ILLUSTRATED STUDY. =The Shapes of Bones.=--Write _L_, _F_, or _I_ after these names (see Fig. 28, etc.), according as the bones are long, flat, or irregular: face, cranium, vertebra, hip, rib, breastbone, collar bone, shoulder blade, upper arm bone, lower arm bones, wrist, palm, fingers, thigh bone, shin bone, splint bone, ankle, instep, toes, kneecap.

=Structure of Joints.=--The meeting of two bones forms a joint (Exp. 4). Some of the joints are immovable. The skull bones join in zigzag lines called _sutures_, formed by the interlocking of sawlike projections (Fig. 35). These _immovable_ joints are necessary for the protection of the brain, which is the most delicate of the organs. The brain attains almost its full size by the seventh year of life; its bony case needs to grow very little after that. The joints of the pelvis are also immovable. All _movable joints have two cartilages_, and as the bones turn, one cartilage slips over the other. There is an intermediate class of joints found between the vertebræ and where the ribs join the breastbone. These joints depend for their motion upon the flexibility and compressibility of their cartilages. They are called mixed, or _elastic_, joints, and allow slight motion. _Such a joint has only one cartilage._

=Kinds of Movable Joints.=--The movable joints are found chiefly in the limbs. When one end of the bone is rounded and fits into a cuplike hollow, the joint allows motion in all directions, and is known as a _ball-and-socket_ joint. The hip joints and shoulder joints are examples. A _hinge joint_ allows motion in only two (opposite) directions; for example, the to-and-fro motion of the elbow. A _pivot_ joint allows a rotary motion; examples, the first vertebra on the second, one bone of forearm upon the other. A gliding joint consists of several bones that slide upon one another, as at the wrists and ankles.

=The Four Features presented by a Movable Joint= (Fig. 36).--If not held in place, the bones would slip out of their sockets, hence there are _ligaments_, or tough bands, to bind the bones together. Sudden jolts would jar the bones and injure them; shocks are prevented by a layer of elastic _cartilage_ over the end of each bone. The moving of one bone over another in bending a joint would wear the bone with friction unless the cartilages were very smooth and lubricated with a fluid called the _synovial fluid_. The synovial fluid would be constantly escaping into the surrounding tissues except for the collarlike ligament called the _capsule_, which surrounds the joint and is attached to each bone entirely around the joint (Fig. 36).

[Illustration: FIG. 36.--DIAGRAM OF A JOINT.]

THOUGHT QUESTIONS. =The Kinds of Joints.=--Write _B_, _H_, _G_, _E_, _P_, or _I_ after these names according to the kind of joint (ball-and-socket, hinge, gliding, elastic, pivot, immovable): between bones of skull, head nodding, head turning, vertebræ, lower jaw, ribs to breastbone (Fig. 75), shoulder, elbow, wrist, fingers, hip, knee, ankle, toes.

=Growth of Bones.=--The blood vessels pass into the bones from the periosteum. _If the periosteum is removed, the larger blood vessels are taken away and the bone beneath it perishes._ If the underlying bone is removed and the periosteum left, the bone will be replaced. A curious proof of the active circulation in the bone is furnished when madder is mixed with the food of pigs. In a few hours the bones become a darker pink than usual; and if the madder is fed to the pigs for a few days, their bones become red. A child grows in height chiefly during three or four months in spring and summer; but its body broadens and becomes heavier during autumn.

=Health of the Bones.=--It is plain that _a strong and free circulation of pure blood contributes to the health and strength of the bones_; good food and pure air make pure blood. Cases of “delayed union,” or slow mending of broken bones, occur more often with intemperate than with sober people. This is because the vitality of the bone cells has been weakened by the use of alcohol. Many surgeons dislike to operate on an old drunkard.

=Posterior Curvature of the Spine.=--The spine (see Figs. 28, 31) has two backward curves (opposite chest and hips) and two forward curves (at loins and neck). The deformity called posterior curvature is chiefly an exaggeration of the upper posterior curve. Round shoulders is the slightest, and hunchback the most marked, degree of this deformity. Causes: 1, _bending over the work_ while either standing or sitting; 2, _slipping down in the seat_, as in Figure 51; 3, working habitually with the _work low in front_, as reading and writing at too low a desk (Fig. 49), or bending over while hoeing, sitting on the floor (Japanese and Chinese); 4, _weak muscles_ in the back; 5, wearing shoes with _high heels_; 6, binding the ribs down with _tight clothing_; 7, walking with the _head drooped forward_ or the chest flat; 8, wearing suspenders without a pulley, or lever, at the back; 9, carrying the hands in the pockets. (Swing the arms to keep the hands out of the pockets and break the habit); 10, wearing a coat or vest that is tight at the back of the neck. This deformity is brought about by _stretching the ligaments_ at the back side of the spine, and by _compressing the cartilages until they become wedge-shaped_, with the thin part of the wedge in front. The flexibility of the spine is a great advantage, but it increases the risk of deformity. One of the most serious evils of posterior curvature is a flat chest and restricted breathing.

[Illustration: FIG. 37.--INCORRECT POSTURE.]

[Illustration: FIG. 38.--CORRECT POSTURE, but strained and stiff.]

=Lateral Curvature of the Spine.=--A perfect spine curves to neither side (Fig. 47), but is perfectly erect. The least habitual lateral curvature is deformity. Causes: 1, writing at a _desk that is too high_; 2, habitually carrying a book, satchel, or other _weight in the same hand_; 3, carrying the _head on one side_ (Fig. 46); 4, habitually standing with the weight on the same foot; 5, a certain defect of vision (astigmatism, Chap. IX).

=To overcome Spinal Deformities.=--The work, or the manner of doing the work, should be so changed as _to give extra labor to the neglected muscles_. Avoid the habits mentioned above as causing deformity. Sit and stand in the manner described in the next paragraph. Sleeping on the back upon a hard mattress without a pillow tends to cure posterior curvature and flat chest.

=The correct position= in standing is: _chest forward, chin in, hips back_ (Figs. 38, 39). To sit correctly, _sit far back in the chair_ (Figs. 49, 50, 51) with the body erect and balanced. In youth the bones are soft and growing; they will readily grow into perfect shape, and will almost as readily grow deformed.

=Sprains.=--_Immerse the part in hot water_ for half an hour, then _bandage_ to keep the part at rest. _Use the limb as little as possible._ It may be necessary for a physician to apply a plaster dressing to a very bad sprain where the ligament is torn from the bone.

=Broken Bones.=--To prevent bone from cutting flesh and skin, do not move the person until a temporary splint has been provided by tying sticks or umbrellas around the limb with handkerchiefs.

PRACTICAL QUESTIONS. =The Skeleton.=--=1.= What kind of a chair back causes one to slide forward in the seat? =2.= What fault in sitting is made necessary by using a chair with so large a seat that the front edge strikes the occupant behind the knee? =3.= Why is the shoulder more often dislocated than the hip? =4.= High pillows may cause what deformity? =5.= Find three bones in the body not attached to other bones. Find twenty-five bones attached to other bones by one end only (Figs. 28 and 39). =6.= What deformities may result from urging a young child to stand or walk? =7.= Which bone is most often broken by falling upon the shoulder? =8.= Where in bones is fat stored for future use? =9.= Ligaments grow very slowly. Why is recovery from a sprain often tedious?

[Illustration: FIG. 39.--THE HUMAN SKELETON IN ACTION.]