1. Ribs 24 2. Sternum 1

D. _Hyoid_, 1 (at base of tongue).

II. APPENDICULAR SKELETON

A. _Shoulder girdle_ 4.

1. Clavicle, collarbone. 2 2. Scapula, shoulder blade 2

B. _Upper extremities_, 60.

1. Humerus 2 2. Radius 2 3. Ulna 2 4. Carpal, wrist bones 16 5. Metacarpal 10 6. Phalanges of fingers 28

C. _Pelvic girdle_, 2.

1. Osinnominatum 2

D. _Lower extremities_, 60.

1. Femur, thigh bone 2 2. Tibia, shin bone 2 3. Fibula 2 4. Patella, kneepan 2 5. Tarsal, ankle bones 14 6. Metatarsal, instep bones 10 7. Phalanges of toes 28

ARTICULATIONS

Any place in the body where two or more bones meet is called an articulation, or joint. At the place of meeting the bones are firmly attached to each other, thereby securing the necessary coherence of the skeleton. The large number of bones, and consequently of articulations, are necessary for the different movements of the body and also on account of the manner in which the skeleton develops, or grows. Articulations are cla.s.sed with reference to their freedom of motion, as _movable_, _slightly movable_, and _immovable_ articulations.

Most of the _immovable_ articulations are found in the skull. Here irregular, tooth-like projections from the different bones enable them to interlock with one another, while they are held firmly together by a thin layer of connective tissue. The wavy lines formed by articulations of this kind are called _sutures_ (Fig. 100).

The best examples of joints that are _slightly_, but not freely, _movable_ are found in the front of the spinal column. The cartilaginous pads between the vertebrae permit, by their elasticity, of a slight bending of the column in different directions. These movements are caused, not by one bone gliding over another, but by compressions and extensions of the cartilage. Between the vertebrae in the back of the spinal column, however, there is a slight movement of the bone surfaces upon one another.

*Structure of the Movable Joints.*-By far the most numerous and important of the joints are those that are freely movable. Such joints are strongly constructed and endure great strain without dislocation, and yet their parts move over each other easily and without friction. The ends of the bones are usually enlarged and have specially formed projections or depressions which fit into corresponding depressions or elevations on the bones with which they articulate. In addition to this the articular surfaces are quite smooth and dense, having no Haversian ca.n.a.ls, and they are covered with a layer of cartilage. Strong ligaments pa.s.s from one bone to the other to hold each in its place (_A, _Fig. 103). Some of these consist simply of bands, connecting the joint on its different sides, while others form continuous sheaths around the joint.

[Fig. 103]

Fig. 103-*Outside and inside view of knee joint.* 1. Tendons. 2.

Ligaments. 3. Cartilage. 4. s.p.a.ce containing synovial fluid. This s.p.a.ce is lined, except upon the articular surfaces, by the synovial membrane.

The interior of the joint, except where the bone surfaces rub upon each other, is covered with a serous lining, called the _synovial membrane_ (_B_, Fig. 103). This secretes a thick, viscid liquid, the _synovial fluid_, which prevents friction. The synovial membrane does not cover the ends of the bones, but pa.s.ses around the joint and connects with the bones at their edges so as to form a closed sac in which the fluid is retained.

*Kinds of Movable Joints.-*The different kinds of movable joints are the ball and socket joint, the hinge joint, the pivot joint, the condyloid joint, and the gliding joint. These are constructed and admit of motion, as follows:

1. In the _ball and socket_ joint the ball-shaped end of one bone fits into a cup-shaped cavity in another bone, called the socket. The best examples of such joints are found at the hips and shoulders. The ball and socket joint admits of motion in all directions.

2. In the _hinge_ joint the bones are grooved and fit together after the manner of a hinge. Hinge joints are found at the elbows and knees and also in the fingers. The hinge joint gives motion in but two directions-forward and backward.

3. A _pivot_ joint is formed by the fitting of a pivot-like projection of one bone into a ring-like receptacle of a second bone, so that one, or the other, is free to turn. A good example of the pivot joint is found at the elbow, where the radius turns upon the humerus. Another example is the articulation of the atlas with the axis vertebra as already noted. The pivot joint admits of motion around an axis.

4. The _condyloid_ joint is formed by the fitting of the ovoid (egg-shaped) end of one bone into an elliptical cavity of a second bone.

Examples of condyloid joints are found at the knuckles and where the wrist bones articulate with the radius and ulna. They move easily in two directions, like hinge joints, and slightly in other directions.

5. _Gliding_ joints are formed by the articulation of plain (almost flat) surfaces. Examples of gliding joints are found in the articulations between the bones of the wrist and those of the ankle. They are the simplest of the movable joints and are formed by one bone gliding, or slipping, upon the surface of another.

*The Machinery of the Body.*-A machine is a contrivance for directing energy in doing work. A sewing machine, for example, so directs the energy of the foot that it is made to sew. Through its construction the machine is able to produce just that form of motion needed for its work, and no other forms, so that energy is not wasted in the production of useless motion. The places in machines where parts rub or turn upon each other are called _bearings_, and extra precautions are taken in the construction and care of the bearings to prevent friction.

The body cannot properly be compared to any single machine, but must be looked upon as a complex organization which employs a number of different kinds of machines in carrying on its work. The majority of these machines are found in the skeleton. The bones are the parts that are moved, and the joints serve as bearings. Connected with the bones are the muscles that supply energy, and attached to the muscles are the nerves that control the motion. Other parts also are required for rendering the machines of the body effective in doing work. These are supplied by the tissues connected with the bones and the muscles.

HYGIENE OF THE SKELETON

Of chief concern in the hygiene of the skeleton is the proper _adjustment_ of its parts. The efficiency of any of the body machines is impaired by lack of proper adjustment. Not only this, but because of the fact that the skeleton forms the groundwork of the whole body-muscles, blood vessels, nerves, everything in fact, being arranged with reference to it-any lack of proper adjustment of the bones interferes generally with the arrangement and work of tissues and organs. The displaced bones may even compress blood vessels and nerves and interfere, in this way, with the nourishment and control of organs remote from the places where the displacements occur. For these reasons the proper adjustment of the different parts of the skeleton supplies one of the essential conditions for preserving the health.

*Hygienic Importance of the Spinal Column.*-What has been said about the adjustment of the skeleton in general applies with particular force to the spinal column. The spinal column serves both as the central axis of the body and as the container of the spinal cord. Thirty-one pairs of nerves pa.s.s between the vertebrae to connect the spinal cord with different parts of the body, and two important arteries (the vertebral) pa.s.s through a series of small openings in the bones of the neck to reach the brain.

Unnatural curves of the spine throw different parts of the body out of their natural positions, diminish the thoracic and abdominal cavities, and, according to the belief of certain physicians, compress the nerves that pa.s.s from the cord to other parts of the body. Slightly misplaced vertebrae in the neck, by compressing the vertebral arteries, may also interfere with the supply of blood

[Fig. 104]

Fig. 104-A tendency toward spinal curvature (after Mosher)

[Fig. 105]

Fig. 105-Effect on spinal column of improper position in writing. (From Pyle's _Personal Hygiene._)

*How the Skeleton becomes Deformed*-We are accustomed to look upon the skeleton as a rigid framework which can get out of its natural form only through severe strain or by violence. This view is far from being correct.

On account of their necessary freedom of motion, the bones, especially those of the spinal column, are easily slipped from their normal positions; and where improper att.i.tudes are frequently a.s.sumed, or continued through long periods of time, the skeleton gradually becomes deformed (Fig. 104). For example, the habit of always sleeping on the same side with a high pillow may develop a bad crook in the neck; and the ugly curves, a.s.sumed so frequently in writing (80) (Fig. 105), and also in standing, when the weight is shifted too much on one foot, may become permanent. Then the habit of reclining in a chair with the hips resting on the front of the seat often deforms the back and causes a drooping of the shoulders. In fact, slight displacements of the vertebrae come about so easily _through incorrect positions_, that they may almost be said to "occur of themselves" where active measures are not taken to preserve the natural form of the body. The very few people who have perfectly formed bodies show to what an extent has been overlooked an essential law of hygiene.

*Prevention of Skeletal Deformities.*-Those deformities of the skeleton that are acquired through improper positions are prevented by giving sufficient attention to the positions a.s.sumed in sitting, standing, and sleeping, and also to the posture in various kinds of work. In sitting the trunk should be erect and the hips should touch the back of the chair. One should not lounge in the ordinary chair. In standing the body should be erect, the shoulders back and down, the chest pushed slightly up and forward, and the chin slightly depressed, while the weight should, as a rule, rest about equally on the two feet. The habit of leaning against some object when standing (the pupil in reciting often leans on his desk) should be avoided. In sleeping the pillow should be of the right thickness to support the head on a level with the spinal column and should not be too soft. If one sleeps on his back, no pillow is required. It is best not to acquire the habit of sleeping always on the same side.

Where one is compelled by his work to a.s.sume harmful positions, these should be corrected by proper exercises, and by cultivating opposing positions during the leisure hours. Much is to be accomplished through those forms of physical exercise which develop the muscles whose work it is to keep the body in an upright position.

*School Furniture.*-It has long been observed that school children are more subject to curvature of the spine and other deformities of the skeleton than the children who do not attend school. While this is due largely to faulty positions a.s.sumed by the pupils at their work, it has been suggested that the school furniture may be in part to blame for these positions. Investigations of this problem have shown that most of the school desks and seats in use in our public schools are unhygienically constructed, in that they _force_ pupils into unnatural positions. School seats should support the pupil in a natural position, both in the use of his books and in writing, and there are many arguments in favor of the so-called "adjustable" school furniture. Fig. 106 shows the seat and desk designed by the Boston, Ma.s.s., Schoolhouse Commission after much study and experimenting and used in the Boston schools. This furniture, which provides a seat adjustable for height, having a back rest also adjustable for height, and a desk which is likewise provided with a vertical adjustment, supplies all essential hygienic requirements. It is to be hoped that school furniture of this character may in the near future come into general use.

[Fig. 106]

Fig. 106-Adjustable seat and desk used in schools of Boston, Ma.s.s.

*Correction of Skeletal Deformities.*-It is, of course, easier to prevent deformities of the skeleton by giving attention to proper positions, than to correct them after they have occurred. It should also be noted that severe deformities cannot be corrected by the individual for himself, but these must come under the treatment of specialists in this line of medical work. In mild cases of spinal curvature, drooping of the head, and round shoulders, the individual _can_ benefit his condition. By working to "subst.i.tute a correct att.i.tude for the faulty one,"(81) he can by persistence bring about marked improvements. It is better, however, to have the advice and aid of a physical director, where this is possible. It should also be borne in mind that the correction of skeletal deformities requires effort through a long period of time, especially where the deformities are p.r.o.nounced; and one lacking the will power to persist will not secure all the results which he seeks.