Chapter 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, in general, of two compact plates, with a spongy layer between. These are in several pieces, the outer ones being joined by notched edges, sutures (su'tyurs,), in the way carpenters term dovetailing. (See Fig. 4.)

FIG. 4.

The peculiar structure and form of the skull afford a perfect shelter for the brain--an organ so delicate that, if unprotected, an ordinary blow would destroy it. Its oval or egg shape adapts it to resist pressure. The smaller and stronger end is in front, where the danger is greatest. Projections before and behind shield the less protected parts. The hard plates are not easy to penetrate. [Footnote: Instances have been known where bullets, striking against the skull, have glanced off, been flattened, or even split into halves. In the Peninsular Campaign, the author saw a man who had been struck in the forehead by a bullet which, instead of penetrating the brain, had followed the skull around to the back of the head, and there passed out.] The spongy packing deadens every blow. [Footnote: An experiment resembling the familiar one of the balls in Natural Philosophy ("Steele's Popular Physics," Fig. 6, p. 26), beautifully illustrates this point. Several balls of ivory are suspended by cords, as in Fig. 5. If A be raised and then let fall, it will transmit the force to B, and that to C, and so on until F is reached, which will fly off with the impulse. If now a ball of spongy bone be substituted for an ivory one anywhere in the line, the force will be checked, and the last ball will not stir.] The separate pieces with their curious joinings disperse any jar which one may receive, and also prevent fractures from spreading.

FIG. 5.

The frequent openings in this strong bone box afford safe avenues for the passage of numerous nerves and vessels which communicate between the brain and the rest of the body.

FIG. 6.

2 THE TRUNK.

THE TRUNK has two important cavities. The upper part, or _chest_, contains the heart and the lungs, and the lower part, or _abdomen_, holds the stomach, liver, kidneys, and other organs (Fig. 31). The principal bones are those of the _spine_, the _ribs_, and the _hips_.

THE SPINE consists of twenty-four bones, between which are placed pads of cartilage. [Footnote: These pads vary in thickness from one fourth to one half an inch. They become condensed by the weight they bear during the day, so that we are somewhat shorter at evening than in the morning. Their elasticity causes them to resume their usual size during the night, or when we lie down for a time.] A canal is hollowed out of the column for the safe passage of the spinal cord. (See Fig. 50.) Projections (processes) at the back and on either side are abundant for the attachment of the muscles. The packing acts as a cushion to prevent any jar from reaching the brain when we jump or run, while the double curve of the spine also tends to disperse the force of a fall. Thus on every side the utmost caution is taken to guard that precious gem in its casket.

THE PERFECTION OF THE SPINE surpasses all human contrivances. Its various uses seem a bundle of contradictions. A chain of twenty-four bones is made so stiff that it will bear a heavy burden, and so flexible that it will bend like rubber; yet, all the while, it transmits no shock, and even hides a delicate nerve within that would thrill with the slightest touch. Resting upon it, the brain is borne without a tremor; and, clinging to it, the vital organs are carried without fear of harm.

FIG. 7.

THE SKULL ARTICULATES with (is jointed to) the spine in a peculiar manner. On the top of the upper vertebra (atlas [Footnote: Thus called because, as, in ancient fable, the god Atlas supported the world on his shoulders, so in the body this bone bears the head.]) are two little hollows (_a_, _b_, Fig. 7), nicely packed and lined with the synovial membrane, into which fit the corresponding projections on the lower part of the skull, and thus the head can rock to and fro. The second vertebra (axis) has a peg, _e_, which projects through a hole, _c_, in the first.

FIG. 8.

The surfaces of both vertebræ are so smooth that they easily glide on each other, and thus, when we move the head side wise, the atlas turns around the peg, _e_, of the axis.

THE RIBS, also twenty-four in number, are arranged in pairs on each side of the chest. At the back, they are all attached to the spine. In front, the upper seven pairs are tied by cartilages to the breastbone (sternum); three are fastened to each other and to the cartilage above, and two, the floating ribs, are loose.

The natural form of the chest is that of a cone diminishing upward. But, owing to the tightness of the clothing commonly worn, the reverse is often the case. The long, slender ribs give lightness, [Footnote: If the chest wall were in one bone thick enough to resist a blow, it would be unwieldy and heavy As it is, the separate bones bound by cartilages yield gradually, and diffuse the force among them all, and so are rarely broken.] the arched form confers strength, and the cartilages impart elasticity,--properties essential to the protection of the delicate organs within, and to freedom of motion in respiration. (See note, p. 80.)

FIG. 9.

THE HIP BONES, called by anatomists the innominata, or nameless bones, form an irregular basin styled the _pelvis_ (_pelvis_, a basin). In the upper part, is the foot of the spinal column--a wedge-shaped bone termed the _sacrum_ [Footnote: So called because it was anciently offered in sacrifice.] (sacred), firmly planted here between the widespreading and solid bones of the pelvis, like the keystone to an arch, and giving a steady support to the heavy burden above.

3. THE LIMBS.

TWO SETS OF LIMBS branch from the trunk, viz.: the upper, and the lower. They closely resemble each other. The arm corresponds to the thigh; the forearm, to the leg; the wrist, to the ankle; the fingers, to the toes. The fingers and the toes are so much alike that they receive the same name, _digits_, while the several bones of both have also the common appellation, _phalanges_. The differences which exist grow out of their varying uses. The foot is characterized by strength; the hand, by mobility.

FIG. 10.

1. THE UPPER LIMBS.--THE SHOULDER.--The bones of the shoulder are the collar bone (clavicle), and the shoulder blade (scapula). The _clavicle_ (_clavis_, a key) is a long, slender bone, shaped like the Italic _f_. It is fastened at one end to the breastbone and the first rib, and, at the other, to the shoulder blade. (See Fig. 1.) It thus holds the shoulder joint out from the chest, and gives the arm greater play. If it be removed or broken, the head of the arm bone will fall, and the motions of the arm be greatly restricted. [Footnote: Animals which use the forelegs only for support (as the horse, ox, etc.), do not possess this bone. "It is found in those that dig, fly, climb and seize."]

THE SHOULDER BLADE is a thin, flat, triangular bone, fitted to the top and back of the chest, and designed to give a foundation for the muscles of the shoulder.

THE SHOULDER JOINT.--The arm bone, or _humerus_, articulates with the shoulder blade by a ball-and-socket joint. This consists of a cup-like cavity in the latter bone, and a rounded head in the former, to fit it,-- thus affording a free rotary motion. The shallowness of the socket accounts for the frequent dislocation of this joint, but a deeper one would diminish the easy swing of the arm.

FIG. 11.

THE ELBOW.--At the elbow, the humerus articulates with the _ulna_--a slender bone on the inner side of the forearm--by a hinge joint which admits of motion in only two directions, _i. e._, backward and forward. The ulna is small at its lower end; the _radius_, or large bone of the forearm, on the contrary, is small at its upper end, while it is large at its lower end, where it forms the wrist joint. At the elbow, the head of the radius is convex and fits into a shallow cavity in the ulna, while at the wrist the ulna plays in a similar socket in the radius. Thus the radius may roll over and even cross the ulna.

THE WRIST, or _carpus_, consists of two rows of very irregular bones, one of which articulates with the forearm; the other, with the hand. They are placed side to side, and so firmly fastened as to admit of only a gliding motion. This gives little play, but great strength, elasticity, and power of resisting shocks.

THE HAND.--The _metacarpal_ (_meta_, beyond; _karpos_, wrist), or bones of the palm, support each a thumb or a finger. Each finger has three bones, while the thumb has only two. The first bone of the thumb, standing apart from the rest, enjoys a special freedom of motion, and adds greatly to the usefulness of the hand.

FIG. 12.

The first bone (Figs. 11, 12) of each finger is so attached to the corresponding metacarpal bone as to move in several directions upon it, but the other phalanges form hinge joints.

The fingers are named in order: the thumb, the index, the middle, the ring, and the little finger. Their different lengths cause them to fit the hollow of the hand when it is closed, and probably enable us more easily to grasp objects of varying size. If the hand clasps a ball, the tips of the fingers will be in a straight line.

The hand in its perfection belongs only to man. Its elegance of outline, delicacy of mold, and beauty of color have made it the study of artists; while its exquisite mobility and adaptation as a perfect instrument have led many philosophers to attribute man's superiority even more to the hand than to the mind. [Footnote: How constantly the hand aids us in explaining or enforcing a thought! We affirm a fact by placing the hand as if we would rest it firmly on a body; we deny by a gesture putting the false or erroneous proposition away from us; we express doubt by holding the hand suspended, as if hesitating whether to take or reject. When we part from dear friends, or greet them again after long absence, the hand extends toward them as if to retain, or to bring them sooner to us. If a recital or a proposition is revolting, we reject it energetically in gesture as in thought. In a friendly adieu we wave our good wishes to him who is their object; but when it expresses enmity, by a brusque movement we sever every tie. The open hand is carried backward to express fear or horror, as well as to avoid contact; it goes forward to meet the hand of friendship; it is raised suppliantly in prayer toward Him from whom we hope for help; it caresses lovingly the downy cheek of the infant, and rests on its head invoking the blessing of Heaven,--_Wonders of the Human Body_.]

FIG. 13.

2. THE LOWER LIMBS.--THE HIP--The thigh bone, or _femur_, is the largest and necessarily the strongest in the skeleton, since at every step it has to bear the weight of the whole body. It articulates with the hip bone by a ball-and-socket joint. Unlike the shoulder joint, the cup here is deep, thus affording less play, but greater strength. It fits so tightly that the pressure of the air largely aids in keeping the bones in place. [Footnote: In order to test this, a hole was bored through a hip bone, so as to admit air into the socket, the thigh bone at once fell out as far as the ligaments would permit. An experiment was also devised whereby a suitably prepared hip joint was placed under the receiver of an air pump. On exhausting the air, the weight of the femur caused it to drop out of the socket, while the readmission of the air raised it to its place. Without this arrangement, the adjacent muscles would have been compelled to bear the additional weight of the thighbone every time it was raised. Now the pressure of the air rids them of this unnecessary burden, and hence they are less easily fatigued--WEBER] Indeed, when the muscles are cut away, great force is required to detach the limbs.

THE KNEE is strengthened by the patella_, or kneepan (_patella_, little dish), a chestnut-shaped bone firmly fastened over the joint.

The shin bone, or _tibia_, the large, triangular bone on the inner side of the leg, articulates both with the femur and the foot by hinge joints. The kneejoint is so made, however, as to admit of a slight rotary motion when the limb is not extended.

The _fibula_ (_fibula_, a clasp), the small, outside bone of the leg, is firmly bound at each end to the tibia. (See Fig. 1.) It is immovable, and, as the tibia bears the principal weight of the body, the chief use of this second bone seems to be to give more surface to which the muscles may be attached. [Footnote: A young man in the hospital at Limoges had lost the middle part of his tibia. The lost bone was not reproduced, but the fibula, the naturally weak and slender part of the leg, became thick and strong enough to support the whole body.--STANLEY'S _Lectures_.]

THE FOOT.--The general arrangement of the foot is strikingly like that of the hand (Fig. 1). The several parts are the _tarsus_, the _metatarsus_, and the _phalanges_. The graceful arch of the foot, and the numerous bones joined by cartilages, give an elasticity to the step that could never be attained by a single, flat bone. [Footnote: The foot consists of an arch, the base of which is more extended in front than behind, and the whole weight of the body is made to fall on this arch by means of a variety of joints. These joints further enable the foot to be applied, without inconvenience, to rough and uneven surfaces.--HINTON.] The toes naturally lie straight forward in the line of the foot. Few persons in civilized nations, however, have naturally formed feet. The big toe is crowded upon the others, while crossed toes, nails grown-in, enormous joints, corns, and bunions abound.

THE CAUSE OF THESE DEFORMITIES is found in the shape and size of fashionable boots and shoes. The sole ought to be large enough for full play of motion, the uppers should not crowd the toes, and the heels should be low, flat, and broad. As it is, there is a constant warfare between Nature and our shoemakers, [Footnote: When we are measured for boots or shoes, we should stand on a sheet of paper, and have the shoemaker mark with a pencil the exact outline of our feet as they bear our whole weight. When the shoe is made, the sole should exactly cover this outline.] and we are the victims. The narrow point in front pinches our toes, and compels them to override one another; the narrow sole compresses the arch; while the high heel, by throwing all the weight forward on the toes, strains the ankle, and, by sending the pressure where Nature did not design it to fall, causes that joint to become enlarged. The body bends forward to meet the demand of this new motion, and thus loses its uprightness and beauty, making our gait stiff and ungraceful. (See p. 271.)

DISEASES, ETC.--l. _Rickets_, a disease of early life, is caused by a lack of mineral matter in the bones, rendering them soft and pliable, so that they bend under the weight of the body. They thus become permanently distorted, and of course are weaker than if they were straight, [Footnote: Just here appears an exceedingly beautiful provision. As soon as the disproportion of animal matter ceases, a larger supply of mineral is sent to the weak points, and the bones actually become thicker, denser, harder, and consequently stronger at the very concave part where the stress of pressure is greatest.--WATSON'S _Lectures_. We shall often have occasion to refer to similar wise and providential arrangements whereby the body is enabled to remedy defects, and to prepare for accidents.] Rickets is most common among children who have inherited a feeble constitution and who are ill fed, or who live in damp, ill-ventilated houses. "Rickety" children should have plenty of fresh air and sunlight, nourishing food, comfortable clothing, and, in short, the best of hygienic care.

2. _A Felon_ is a swelling of the finger or thumb, usually of the last joint. It is marked by an accumulation beneath the periosteum and next the bone. The physician will merely cut through the periosteum, and let out the effete matter.

3. _Bowlegs_ are caused by children standing on their feet before the bones of the lower limbs are strong enough to bear their weight. The custom of encouraging young children to stand by means of a chair or the support of the hand, while the bones are yet soft and pliable, is a cruel one, and liable to produce permanent deformity. Nature will set the child on its feet when the proper time comes.

4. _Curvature of the Spine_.--When the spine is bent, the packing between the vertebræ becomes compressed on one side into a wedge-like shape. After a time, it will lose its elasticity, and the spine will become distorted. This often occurs in the case of students who bend forward to bring their eyes nearer their books, instead of lifting their books nearer their eyes, or who raise their right shoulder above their left when writing at a desk which is too high. Round shoulders, small, weak lungs, and, frequently, diseases of the spine are the consequences. An erect posture in reading or writing conduces not alone to beauty of form, but also to health of body. We shall learn hereafter that the action of the muscles bears an important part in preserving the symmetry of the spine. Muscular strength comes from bodily activity; hence, one of the best preventives of spinal curvature is daily exercise in the open air.

5. _Sprains_ are produced when the ligaments which bind the bones of a joint are strained, twisted, or torn from their attachments. They are quite as serious as a broken bone, and require careful attention lest they lead to a crippling for life. By premature use a sprained limb may be permanently impaired. Hence, the joint should be kept quiet, even after the immediate pain is gone.

6. _A Dislocation_ is the forcible displacement of a bone from its socket. It is, generally, the result of a fall or a violent blow. The tissues of the joint are often ruptured, while the contraction of the muscles prevents the easy return of the bone to its place. A dislocation should be reduced as soon as possible after the injury, before inflammation supervenes.

PRACTICAL QUESTIONS.

1. Why does not a fall hurt a child as much as it does a grown person?

2. Should a young child ever be urged to stand or walk?

3. What is meant by "breaking one's neck"?

4. Should chairs or benches have straight backs?

5. Should a child's feet be allowed to dangle from a high seat?

6. Why can we tell whether a fowl is young by pressing on the point of the breastbone?

7. What is the use of the marrow in the bones?

8. Why is the shoulder so often put out of joint?

9. How can you tie a knot in a bone?

10. Why are high pillows injurious?

11. Is a stooping posture a healthful position?

12. Should a boot have a heel piece?

13. Why should one always sit and walk erect?

14. Why does a young child creep rather than walk?

15. What is the natural direction of the big toe?

16. What is the difference between a sprain and a fracture? A dislocation?

17. Does the general health of the system affect the strength of the bones?

18. Is living bone sensitive? _Ans_.--Scrape a bone, and its vessels bleed; cut or bore a bone, and its granulations sprout up; break a bone, and it will heal; cut a piece away, and more bone will readily be produced; hurt it in any way, and it inflames; burn it, and it dies. Take any proof of sensibility but the mere feeling of pain, and it will answer to the proof.--BELL'S _Anatomy_. Animal sensibility would be inconvenient; it is therefore not to be found except in diseased bone, where it sometimes exhibits itself too acutely.--TODD'S _Cyclopedia of Anatomy_.

19. Is the constitution of bone the same in animals as in man? _Ans_.--The bones of quadrupeds do not differ much from those of man. In general they are of a coarser texture, and in some, as in those of the elephant's head, we find extensive air cells.--TODD'S _Anatomy_.

II.

THE MUSCLES.

"Behold the outward moving frame, Its living marbles jointed strong With glistening band and silvery thong, And link'd to reason's guiding reins By myriad rings in trembling chains, Each graven with the threaded zone Which claims it as the Master's own."

HOLMES.

ANALYSIS OF THE MUSCLES.

_ | 1. The Use of the Muscles. | 2. Contractility of the Muscles. _ | 3. Arrangement of the Muscles. | 1. THE USE, STRUCTURE | 4. The two Kinds of Muscles. | AND ACTION OF THE | 5. The Structure of the Muscles. | MUSCLES. | 6. The Tendons for Fastening Muscles. | | 7. The Muscles and Bones as Levers. | | 8. The Effect of Big Joints. | | 9. Action of the Muscles in Walking. | |_10. Action of the Muscles in Walking. | | 2. THE MUSCULAR SENSE. | _ | 3. HYGIENE OF THE | 1. Necessity of Exercise. | MUSCLES. | 2. Time for Exercise. | |_ 3. Kinds of Exercise. | | 4. WONDERS OF THE MUSCLES. | _ | | 1. St. Vitus's Dance. | | 2. Convulstions. | | 3. Locked-jaw. |_5. DISEASES. | 4. Gout. | 5. Rheumatism. | 6. Lumbago. |_ 7. A Ganglion.

FIG. 14.

THE MUSCLES.

THE USE OF THE MUSCLES.--The skeleton is the image of death. Its unsightly appearance instinctively repels us. We have seen, however, what uses it subserves in the body, and how the ugly-looking bones abound in nice contrivances and ingenious workmanship. In life, the framework is hidden by the flesh. This covering is a mass of muscles, which by their arrangement and their properties not only give form and symmetry to the body, but also produce its varied movements.

In Fig. 14, we see the large exterior muscles. Beneath these are many others; while deeply hidden within are tiny, delicate ones, too small to be seen with the naked eye. There are, in all, about five hundred, each having its special use, and all working in exquisite harmony and perfection.

CONTRACTILITY.--The peculiar property of the muscles is their power of contraction, whereby they decrease in length and increase in thickness. [Footnote: The maximum force of this contraction has been estimated as high as from eighty-five to one hundred and fourteen pounds per square inch.] This may be caused by an effort of the will, by cold, by a sharp blow, etc. It does not cease at death, but, in certain cold-blooded animals, a contraction of the muscles is often noticed long after the head has been cut off.

ARRANGEMENT OF THE MUSCLES. [Footnote: "Could we behold properly the muscular fibers in operation, nothing, as a mere mechanical exhibition, can be conceived more superb than the intricate and combined actions that must take place during our most common movements. Look at a person running or leaping, or watch the motions of the eye. How rapid, how delicate, how complicated, and yet how accurate, are the motions required! Think of the endurance of such a muscle as the heart, that can contract, with a force equal to sixty pounds, seventy-five times every minute, for eighty years together, without being weary."]--The muscles are nearly all arranged in pairs, each with its antagonist, so that, as they contract and expand alternately, the bone to which they are attached is moved to and fro. (See p. 275.)

If you grasp the arm tightly with your hand just above the elbow joint, and bend the forearm, you will feel the muscle on the inside (biceps, _a_, Fig. 14) swell, and become hard and prominent, while the outside muscle (triceps, _f_) will be relaxed. Now straighten the arm, and the swelling and hardness of the inside muscle will vanish, while the outside one will, in turn, become rigid. So, also, if you clasp the arm just below the elbow, and then open and shut the fingers, you can feel the alternate expanding and relaxing of the muscles on opposite sides of the arms.

If the muscles on one side of the face become palsied, those on the other side will draw the mouth that way. Squinting is caused by one of the straight muscles of the eye (Fig. 17) contracting more strongly than its antagonist.

KINDS OF MUSCLES.--There are two kinds of muscles, the _voluntary_, which are under the control of our will, and the _involuntary_, which are not. Thus our limbs stiffen or relax as we please, but the heart beats on by day and by night. The eyelid, however, is both voluntary and involuntary, so that while we wink constantly without effort, we can, to a certain extent, restrain or control the motion.

STRUCTURE OF THE MUSCLES.--If we take a piece of lean beef and wash out the red color, we can easily detect the fine fibers of which the meat is composed. In boiling corned beef for the table, the fibers often separate, owing to the dissolving of the delicate tissue which bound them together. By means of the microscope, we find that these fibers are made up of minute filaments (_fibrils_), and that each fibril is composed of a row of small cells arranged like a string of beads. This gives the muscles a peculiar striped (striated) appearance. [Footnote: The involuntary muscles consist generally of smooth, fibrous tissue, and form sheets or membranes in the walls of hollow organs. By their contraction they change the size of cavities which they inclose. Some functions, however, like the action of the heart, or the movements of deglutition (swallowing), require the rapid, vigorous contraction, characteristic of the voluntary muscular tissue--FLINT.] (See p. 276.) The cells are filled with a fluid or semifluid mass of living (protoplasmic) matter.

FIG. 15.

The binding of so many threads into one bundle [Footnote: We shall learn hereafter how these fibers are firmly tied together by a mesh of fine connective tissue which dissolves in boiling, as just described] confers great strength, according to a mechanical principle that we see exemplified in suspension bridges, where the weight is sustained, not by bars of iron, but by small wires twisted into massive ropes.

FIG. 16.

THE TENDONS.--The ends of the muscles are generally attached to the bone by strong, flexible, but inelastic tendons. [Footnote: The tendons may be easily seen in the leg of a turkey as it comes on our table; so we may study Physiology while we pick the bones.] The muscular fibers spring from the sides of the tendon, so that more of them can act upon the bone than if they went directly to it. Besides, the small, insensible tendon can better bear the exposure of passing over a joint, and be more easily lodged in some protecting groove, than the broad, sensitive muscle. This mode of attachment gives to the limbs strength, and elegance of form. Thus, for example, if the large muscles of the arm extended to the hand, they would make it bulky and clumsy. The tendons, however, reach only to the wrist, whence fine cords pass to the fingers (Fig. 16).

Here we notice two other admirable arrangements. 1. If the long tendons at the wrist on contracting should rise, projections would be made and thus the beauty of the slender joint be marred. To prevent this, a stout band or bracelet of ligament holds them down to their place. 2. In order to allow the tendon which moves the last joint of the finger to pass through, the tendon which moves the second joint divides at its attachment to the bone (Fig. 16). This is the most economical mode of packing the muscles, as any other practicable arrangement would increase the bulk of the slender finger.

FIG. 17.

Since the tendon can not always pull in the direction of the desired motion, some contrivance is necessary to meet the want. The tendon (B) belonging to one of the muscles of the eye, for example, passes through a ring of cartilage, and thus a rotary motion is secured.

FIG. 18.

THE LEVERS OF THE BODY. [Footnote: A _lever_ is a stiff bar resting on a point of support, called the _fulcrum_ (_F_), and having connected with it a _weight_ (_W_) to be lifted, and a _power_ (_P_) to move it. There are three classes of levers according to the arrangement of the power, weight, and fulcrum. In the first class, the _F_ is between the _P_ and _W_; in the second, the _W_ is between the _P_ and _F_; and in the third, the _P_ is between the _W_ and _F_ (Fig. 18). A pump handle is an example of the first; a lemon squeezer, of the second; and a pair of fire tongs, of the third. See "Popular Physics," pp. 81-83, for a full description of this subject, and for many illustrations.]--In producing the motions of the body, the muscles use the bones as levers. We see an illustration of the _first class_ of levers in the movements of the head. The back or front of the head is the weight to be lifted, the backbone is the fulcrum on which the lever turns, and the muscles at the back or front of the neck exert the power by which we toss or bow the head.

FIG. 19.

When we raise the body on tiptoe, we have an instance of the _second class_. Here, our toes resting on the ground form the fulcrum the muscles of the calf (gas-troc-ne'-mi-us, _j_ and so-le'-us, Fig. 14), acting through the tendon of the heel, [Footnote: This is called the Tendon of Achilles (_k_, Fig. 14) and is so named because, as the fable runs, when Achilles was an infant his mother held him by the heel while she dipped him in the River Styx, whose water had the power of rendering one invulnerable to any weapon. His heel, not being wet, was his weak point, to which Paris directed the fatal arrow--"This tendon," says Mapother, "will bear one thousand pounds weight before it will break." The horse is said to be "hamstrung," and is rendered useless, when the Tendon of Achilles is cut. (see p. 284.)] are the power and the weight is borne by the ankle joint.

An illustration of the _third class_ is found in lifting the hand from the elbow. The hand is the weight, the elbow the fulcrum, and the power is applied by the biceps muscle at its attachment to the radius (A, Fig. 19.) In this form of the lever there is great loss of force, because it is applied at such a distance from the weight, but there is a gain of velocity, since the hand moves so far by such a slight contraction of the muscle. The hand is required to perform quick motions, and therefore this mode of attachment is desirable.

The nearer the power is applied to the resistance, the more easily the work is done. In the lower jaw, for example, the jaw is the weight, the fulcrum is the hinge joint at the back, and the muscles (temporal, _d_, and the mas'-se'ter, _e_, Fig. 14) on each side are the power. [Footnote: We may feel the contraction of the masseter by placing our hand on the face when we work the jaw, while the temporal can be readily detected by putting the fingers on the temple while we are chewing. The tendon of the muscle (digastric)--one of those which open the jaw--passes through a pulley (_c_, Fig. 14) somewhat like the one in the eye.] They act much closer to the resistance than those in the hand, since here we desire force, and there, speed.

FIG. 20.

THE ENLARGEMENT OF THE BONES AT THE JOINTS not only affords greater surface for the attachment of the muscles, as we have seen, but also enables them to work to better advantage. Thus, in Fig. 20 it is evident that a muscle acting in the line _f b_ would not bend the lower limb so easily as if it were acting in the line _f k_, since in the former case its force would be about all spent in drawing the bones more closely together, while in the latter it would pull more nearly at a right angle. Thus the tendon _f_, by passing over the patella, which is itself pushed out by the protuberance _b_ of the thigh bone, pulls at a larger angle, [Footnote: The chief use of the processes of the spine (Fig. 6) and other bones is, in the same way, to throw out the point on which the power acts as far from the fulcrum as possible. The projections of the ulna ("funny bone") behind the elbow, and that of the heel bone to which the Tendon of Achilles is attached, are excellent illustrations (Fig. 1).] and so the leg is thrown forward with ease in walking and with great force in kicking.

HOW WE STAND ERECT.--The joints play so easily, and the center of gravity in the body is so far above the foot, that the skeleton can not of itself hold our bodies upright. Thus it requires the action of many muscles to maintain this position. The head so rests upon the spine as to tend to fall in front, but the muscles of the neck steady it in its place. [Footnote: In animals the jaws are so heavy, and the place where the head and spine join is so far back, that there can be no balance as there is in man. There are therefore large muscles in their necks. We readily find that we have none if we get on "all fours" and try to hold up the head. On the other hand, gorillas and apes can not stand erect like man, for the reason that their head, trunk, legs, etc., are not balanced by muscles, so as to be in line with one another.] The hips incline forward, but are held erect by the strong muscles of the back. The trunk is nicely balanced on the head of the thigh bones. The great muscles of the thigh acting over the kneepan tend to bend the body forward, but the muscles of the calf neutralize this action. The ankle, the knee, and the hip lie in nearly the same line, and thus the weight of the body rests directly on the keystone of the arch of the foot. So perfectly do these muscles act that we never think of them until science calls our attention to the subject, and yet to acquire the necessary skill to use them in our infancy needed patient lessons, much time, and many hard knocks.

FIG. 21.

HOW WE WALK.--Walking is as complex an act as standing. It is really a perilous performance, which has become safe only because of constant practice. We see how violent it is when we run against a post in the dark, and find with what headlong force we were hurling ourselves forward. Holmes has well defined walking as a perpetual falling with a constant self-recovery. Standing on one foot, we let the body fall forward, while we swing the other leg ahead like a pendulum. Planting that foot on the ground, to save the body from falling farther, we then swing the first foot forward again to repeat the same operation. [Footnote: It is a curious fact that one side of the body tends to outwalk the other; and so, when a man is lost in the woods, he often goes in a circle, and at last comes round to the spot whence he started.]

The shorter the pendulum, the more rapidly it vibrates; and so short- legged people take quicker and shorter steps than long-legged ones. [Footnote: In this respect, Tom Thumb was to Magrath, whose skeleton, eight and one half feet high, is now in the Dublin Museum, what a little fast-ticking, French mantel clock is to a big, old-fashioned, upright, corner timepiece.] We are shorter when walking than when standing still, because of this falling forward to take a step in advance. [Footnote: Women find that a gown that will swing clear of the ground when they are standing still, will drag the street when they are walking. The length of the step may be increased by muscular effort, as when a line of soldiers keep step in spite of their having legs of different lengths. Such a mode of walking is necessarily fatiguing. (See p. 280.)]

In running, we incline the body more, and so, as it were, fall faster. When we walk, one foot is on the ground all the time, and there is an instant when both feet are planted upon it; but in running there is an interval of time in each step when both feet are off the ground, and the body is wholly unsupported. As we step alternately with the feet, we are inclined to turn the body first to one side and then to the other. This movement is sometimes counterbalanced by swinging the hand on the opposite side. [Footnote: In ordinary walking the speed is nearly four miles an hour, and can be kept up for a long period. But exercise and a special aptitude for it enable some men to walk great distances in a relatively short space of time. Trained walkers have gone seventy-five miles in twenty hours, and walked the distance of thirty-seven miles at the rate of five miles an hour. The mountaineers of the Alps are generally good walkers, and some of them are not less remarkable for endurance than for speed. Jacques Balmat, who was the first to reach the summit of Mont Blanc, at sixteen years of age could walk from the hamlet of the Pélerins to the mountain of La Côte in two hours,--a distance which the best- trained travelers required from five to six hours to get over. At the time of his last attempt to reach the top of Mont Blanc, this same guide, then twenty years old, passed six days and four nights without sleeping or reposing a single moment. One of his sons, Édouard Balmat, left Paris to join his regiment at Genoa; he reached Chamouni the fifth day at evening, having walked three hundred and forty miles. After resting two days, he set off again for Genoa, where he arrived in two days. Several years afterward, this same man left the baths at Louèche at two o'clock in the morning, and reached Chamouni at nine in the evening, having walked a distance equal to about seventy-five miles in nineteen hours. In 1844, an old guide of De Saussure, eighty years old, left the hamlet of Prats, in the valley of Chamouni, in the afternoon, and reached the Grand-Mulets at ten in the evening; then, after resting some hours, he climbed the glacier to the vicinity of the Grand Plateau, which has an altitude of about thirteen thousand feet, and then returned to his village without stopping.--_Wonders of the Body_.]

THE MUSCULAR SENSE.--When we lift an object, we feel a sensation of weight, which we can compare with that experienced in lifting another body. [Footnote: If a small ivory ball be allowed to roll down the cheek toward the lips, it will appear to increase in weight. In general, the more sensitive parts of the body recognize smaller differences in weight, and the right hand is more accurate than the left. We are very apt, however, to judge of the weight of a body from previous conceptions. Thus, shortly after Sir Humphrey Davy discovered the metal potassium, he placed a piece of it in the hand of Dr. Pierson, who exclaimed: "Bless me! How heavy it is!" Really, however, potassium is so light that it will float on water like cork.] By balancing it in the hand. The muscular sense is useful to us in many ways. It guides us in standing or moving. We gratify it when we walk erect and with an elastic step, and by dancing, jumping, skating, and gymnastic exercises.

NECESSITY OF EXERCISE.--The effect of exercise upon a muscle is very marked. [Footnote: The greater size of the breast (pectoral muscle) of a pigeon, as compared with that of a duck, shows how muscle increases with use. The breast of a chicken is white because it is not used for flight, and therefore gets little blood.] By use it grows larger, and becomes hard, compact, and darker-colored; by disuse it decreases in size, and becomes soft, flabby, and pale.

Violent exercise, however, is injurious, since we then tear down faster than nature can build up. Feats of strength are not only hurtful, but dangerous. Often the muscles are strained or ruptured, and blood vessels burst in the effort to outdo one's companions. [Footnote: Instances have been known of children falling dead from having carried to excess so pleasant and healthful an amusement as jumping the rope, and of persons rupturing the Tendon of Achilles in dancing. The competitive lifting of heavy weights is unwise, sometimes fatal.] (See p. 278.)

Two thousand years ago, Isocrates, the Greek rhetorician, said: "Exercise for health, not for strength." The cultivation of muscle for its own sake is a return to barbarism, while it enfeebles the mind, and ultimately the body. The ancient gymnasts are said to have become prematurely old, and the trained performers of our own day soon suffer from the strain they put upon their muscular system. Few men have sufficient vigor to become both athletes and scholars. Exercise should, therefore, merely supplement the deficiency of our usual employment. _A sedentary life needs daily, moderate exercise, which always stops short of fatigue_. This is a law of health. (See p. 280.)

No education is complete which fails to provide for the development of the muscles. Recesses should be as strictly devoted to play as study hours are to work. Were gymnastics or calisthenics as regular an exercise as grammar or arithmetic, fewer pupils would be compelled to leave school on account of ill health; while spinal curvatures, weak backs, and ungraceful gaits would no longer characterize so many of our best institutions.

TIME FOR EXERCISE.--We should not exercise after long abstinence from food, nor immediately after a meal, unless the meal or the exercise be very light. There is an old-fashioned prejudice in favor of exercise before breakfast--an hour suited to the strong and healthy, but entirely unfitted to the weak and delicate. On first rising in the morning, the pulse is low, the skin relaxed, and the system susceptible to cold. Feeble persons, therefore, need to be braced with food before they brave the outdoor air.

WHAT KIND OF EXCERCISE TO TAKE.--For children, games are unequaled. Walking, the universal exercise, [Footnote: The custom of walking, so prevalent in England, has doubtless much to do with the superior physique of its people. It is considered nothing for a woman to take a walk of eight or ten miles, and long pedestrian excursions are made to all parts of the country. The benefits which accrue from such an open-air life are sadly needed by the women of our own land. A walk of half a dozen miles should be a pleasant recreation for any healthy person.] is beneficial, as it takes one into the open air and sunlight. Running is better, since it employs more muscles, but it must not be pushed to excess, as it taxes the heart, and may lead to disease of that organ. Rowing is more effectual in its general development of the system. Swimming employs the muscles of the whole body, and is a valuable acquirement, as it may be the means of saving life. Horseback riding is a fine accomplishment, and refreshes both mind and body. Gymnastic or calisthenic exercises bring into play all the muscles of the body, and when carefully selected, and not immoderately employed, are preferable to any other mode of indoor exercise. [Footnote: The employment of the muscles in exercise not only benefits their especial structure, but it acts on the whole system. When the muscles are put in action, the capillary blood vessels with which they are supplied become more rapidly charged with blood, and active changes take place, not only in the muscles, but in all the surrounding tissues. The heart is required to supply more blood, and accordingly beats more rapidly in order to meet the demand. A larger quantity of blood is sent through the lungs, and larger supplies of oxygen are taken in and carried to the various tissues. The oxygen, by combining with the carbon of the blood and the tissues, engenders a larger quantity of heat, which produces an action on the skin, in order that the superfluous warmth may be disposed of. The skin is thus exercised, as it were, and the sudoriparous and sebaceous glands are set at work. The lungs and skin are brought into operation, and the lungs throw off large quantities of carbonic acid, and the skin large quantities of water, containing in solution matters which, if retained, would produce disease in the body. Wherever the blood is sent, changes of a healthful character occur. The brain and the rest of the nervous system are invigorated, the stomach has its powers of digestion improved, and the liver, pancreas, and other organs perform their functions with more vigor. By want of exercise, the constituents of the food which pass into the blood are not oxidized, and products which produce disease are engendered. The introduction of fresh supplies of oxygen induced by exercise oxidizes these products, and renders them harmless. As a rule, those who exercise most in the open air will live the longest.--LANKESTER.] (See p. 280.)

THE WONDERS OF THE MUSCLES.--The grace, ease, and rapidity with which the muscles contract are astonishing. By practice, they acquire a facility which we call mechanical. The voice may utter one thousand five hundred letters in a minute, yet each requires a distinct position of the vocal organs. We train the muscles of the fingers till they glide over the keys of the piano, executing the most exquisite and difficult harmony. In writing, each letter is formed by its peculiar motions, yet we make them so unconsciously that a skillful penman will describe beautiful curves while thinking only of the idea that the sentence is to express. The mind of the violinist is upon the music which his right hand is executing, while his left determines the length of the string and the character of each note so carefully that not a false sound is heard, although the variation of a hair's breadth would cause a discord. In the arm of a blacksmith, the biceps muscle may grow into the solidity almost of a club; the hand of a prize fighter will strike a blow like a sledge hammer; while the engraver traces lines invisible to the naked eye, and the fingers of the blind acquire a delicacy that almost supplies the place of the missing sense.

DISEASES, ETC.--l. _St. Vitus's Dance_ is a disease of the voluntary muscles, whereby they are in frequent, irregular, and spasmodic motion beyond the control of the will. All causes of excitement, and especially of fear, should be avoided, and the general health of the patient invigorated, as this disease is closely connected with a derangement of the nervous system.

2. _Convulsions_ are an involuntary contraction of the muscles. Consciousness is wanting, while the limbs may be stiff or in spasmodic action. (See p. 261.)

3. _Locked-jaw_ is a disease in which there are spasms and a contraction of the muscles, usually beginning in the lower jaw. It is serious, often fatal, yet it sometimes follows as trivial an injury as the stroke of a whip lash, the lodgment of a bone in the throat, a fishhook in the finger, or a tack in the sole of the foot.

4. _Gout_ is characterized by an acute pain located chiefly in the small joints of the foot, especially those of the great toe, which become swollen and extremely sensitive. It is generally accompanied by an excess of uric acid in the blood, and a deposit of urate of soda about the affected joint. Gout is often the result of high living, and of too much animal food. It is frequently inherited.

5. _Rheumatism_ affects mainly the connective, white, fibrous tissue of the larger joints. While gout is the punishment of the rich who live luxuriously, rheumatism afflicts alike the poor and the rich. There are two common forms of rheumatism--the inflammatory or acute, and the chronic. The latter is of long continuance; the former terminates more speedily. The acute form is probably a disease of the blood, which carries with it some poisonous matter that is deposited where the fibrous tissue is most abundant. The disease flies capriciously from one joint to another, and the pain caused by even the slightest motion deprives the sufferer of the use of the disabled part and its muscles. Its chief danger lies in the possibility of its affecting the vital organs. Chronic rheumatism--the result of repeated attacks of the acute--leads to great suffering, and oftentimes to disorganization of the joints and an interference with the movements of the heart.

6. _Lumbago_ is an inflammation of the lumbar muscles and fascia. [Footnote: Lumbago is really a form of myalgia, a disease which, has its seat in the muscles, and may thus affect any part of the body. Doubtless much of what is commonly called "liver" or "kidney complaint" is only, in one case, myalgia of the chest or abdominal walls near the liver, or, in the other, of the back and loins near the kidneys. Chronic liver disease is comparatively rare in the Northern States, and pain in the side is not a prominent symptom; while certain diseases of the kidneys, which are as surely fatal as pulmonary consumption, are not attended by pain in the back opposite these organs.--WEY.] It may be so moderate as to produce only a "lame back," or so severe as to disable, as in the case of what is popularly termed a "crick in the back." Strong swimmers who sometimes drown without apparent cause are supposed to be seized in this way.

7. _A Ganglion_, or what is vulgarly called a "weak" or "weeping" sinew, is the swelling of a bursa. [Footnote: A bursa is a small sack containing a lubricating fluid to prevent friction where tendons play over hard surfaces. There is one shaped like an hourglass on the wrist, just at the edge of the palm. By pressing back the liquid it contains, this bursa may be clearly seen.] It sometimes becomes so distended by fluid as to be mistaken for bone. If on binding something hard upon it for a few days it does not disappear, a physician will remove the liquid by means of a hypodermic syringe, or perhaps cause it to be absorbed by an external application of iodine.

PRACTICAL QUESTIONS.

1. What class of lever is the foot when we lift a weight on the toes?

2. Explain the movement of the body backward and forward, when resting upon the thigh bone as a fulcrum.

3. What class of lever do we use when we lift the foot while sitting down?

4. Explain the swing of the arm from the shoulder.

5. What class of lever is used in bending our fingers?

6. What class of lever is our foot when we tap the ground with our toes?

7. What class of lever do we use when we raise ourselves from a stooping position?

8. What class of lever is the foot when we walk?

9. Why can we raise a heavier weight with our hand when lifting from the elbow than from the shoulder?

10. What class of lever do we employ when we are hopping, the thigh bone being bent up toward the body and not used?

11. Describe the motions of the bones when we are using a gimlet.

12. Why do we tire when we stand erect?

13. Why does it rest us to change our work?

14. Why and when is dancing a beneficial exercise?

15. Why can we exert greater force with the back teeth than with the front ones?

16. Why do we lean forward when we wish to rise from a chair?

17. Why does the projection of the heel bone make walking easier?

18. Does a horse travel with less fatigue over a flat than a hilly country?

19. Can you move your upper jaw?

20. Are people naturally right or left-handed?

21. Why can so few persons move their ears by the muscles?

22. Is the blacksmith's right arm healthier than the left?

23. Boys often, though foolishly, thrust a pin into the flesh just above the knee. Why is it not painful?

24. Will ten minutes' practice in a gymnasium answer for a day's exercise?

25. Why would an elastic tendon be unfitted to transmit the motion of a muscle?

26. When one is struck violently on the head, why does he instantly fall?

27. What is the cause of the difference between light and dark meat in a fowl?

III.

THE SKIN.

A protection from the outer world, it is our only means of communicating with it. Insensible itself, it is the organ of touch. It feels the pressure of a hair, yet bears the weight of the body. It yields to every motion of that which it wraps and holds in place. It hides from view the delicate organs within, yet the faintest tint of a thought shines through, while the soul paints upon it, as on a canvas, the richest and rarest of colors.

ANALYSIS OF THE SKIN.

_ _ | 1. The Cutis; its Composition and Character. | 1. THE STRUCTURE | 2. The Cuticle; its Composition and Character. | OF THE SKIN. | 3. The Value of the Cuticle. | |_4. The Complexion. | _ | | a. _Description._ | _ | b. _Method of Growth._ | | 1. The Hair.....| c. _As an Instrument of | | | Feeling._ | 2. THE HAIR AND | | d. _Indestructibility of | THE NAILS. | |_ the Hair._ | | _ | |_2. The Nails....| a. _Uses._ | |_b. _Method of Growth._ | _ | 3. THE MUCOUS | 1. The Structure. | MEMBRANE | 2. Connective Tissue. | |_3. Fat. | _ | | 1. Number and Kinds of Teeth. | | _ | | 1. The Two Sets.| 1. _The Milk Teeth._ | | |_2. _The Permanent Teeth._ | | | 4. THE TEETH. | 2. Structure of the Teeth. | | 3. The Setting of the Tooth in the Jaw. | | 4. The Decay of the Teeth. | |_5. The Preservation of the Teeth. | _ _ | | 1. The Two Kinds.| 1. _Oil Glands._ | | |_2. _Perpiratory Glands._ | | | 5. THE GLANDS | 2. The Perspiration. | | 3. The Absorbing Power of the Skin. (See | |_ Lymphatics.) | _ | | 1. About Washing and Bathing. | | 2. The Reaction. | | 3. Sea Bathing. _ | 6. HYGIENE | | a. _General Principles._ | | | b. _Linen._ | | | c. _Cotton._ | |_4. Clothing.......| d. _Woolen._ | | e. _Flannel._ | | f. _Color of Clothing._ | | g. _Structure of | | Clothing._ | | h. _Insufficient | _ |_ Clothing._ | | 1. Erysipelas. | | 2. Salt Rheum. |_7. DISEASES. | 3. Corns. | 4. Ingrowing Nails. | 5. Warts. |_6. Chilblains.

THE SKIN.

THE SKIN is a tough, thin, close-fitting garment for the protection of the tender flesh. Its perfect elasticity beautifully adapts it to every motion of the body. We shall learn hereafter that it is more than a mere covering, being an active organ, which does its part in the work of keeping in order the house in which we live. It oils itself to preserve its smoothness and delicacy, replaces itself as fast as it wears out, and is at once the perfection of use and beauty.

1. STRUCTURE OF THE SKIN.

CUTIS AND CUTICLE.--What we commonly call the skin--viz., the part raised by a blister--is only the cuticle [Footnote: _Cuticula_, little skin. It is often styled the scarfskin, and also the epidermis (_epi_, upon; and _derma_, skin).] or covering of the cutis or true skin. The latter is full of nerves and blood vessels, while the former neither bleeds [Footnote: We notice this in shaving; for if a razor goes below the cuticle, it is followed by pain and blood. So insensible is this outer layer that we can run a pin through the thick mass at the roots of the nails without discomfort.] nor gives rise to pain, neither suffers from heat nor feels the cold.

The cuticle is composed of small, flat cells or scales. These are constantly shed from the surface in the form of scurf, dandruff, etc., but are as constantly renewed from the cutis [Footnote: We see how rapidly this change goes on by noticing how soon a stain of any kind disappears from the skin. A snake throws off its cuticle entire, and at regular intervals.] below.

Under the microscope, we can see the round cells of the cuticle, and how they are flattened and hardened as they are forced to the surface. The immense number of these cells surpasses comprehension. In one square inch of the cuticle, counting only those in a single layer, there are over a billion horny scales, each complete in itself.--HARTING.

FIG. 22.

VALUE OF THE CUTICLE.--In the palm of the hand, the sole of the foot, and other parts especially liable to injury, the cuticle is very thick. This is a most admirable provision for their protection. [Footnote: We can hold the hand in strong brine with impunity, but the smart will quickly tell us when there is even a scratch in the skin. Vaccine matter must be inserted beneath the cuticle to take effect. This membrane doubtless prevents many poisonous substances from entering the system.] By use, it becomes callous and horny. The boy who goes out barefoot for the first time, "treading as if on eggs," can soon run where he pleases among thistles and over stones. The blacksmith handles hot iron without pain, while the mason lays stones and works in lime, without scratching or corroding his flesh.

THE COMPLEXION.--In the freshly made cells on the lower side of the cuticle, is a pigment composed of tiny grains. [Footnote: These grains are about 1/2000 of an inch in diameter, and, curiously enough, do not appear opaque, but transparent and nearly colorless.--MARSHALL.] In the varying tint of this coloring matter, lies the difference of hue between the blonde and the brunette, the European and the African. In the purest complexion, there is some of this pigment, which, however, disappears as the fresh, round, soft cells next the cutis change into the old, flat, horny scales at the surface.

Scars are white, because this part of the cuticle is not restored. The sun has a powerful effect upon the coloring matter, and so we readily "tan" on exposure to its rays. If the color gathers in spots, it forms freckles. [Footnote: This action of the sun on the pigment of the skin is very marked. Even among the Africans, the skin is observed to lose its intense black color in those who live for many months in the shades of the forest. It is said that Asiatic and African women confined within the walls of the harem, and thus secluded from the sun, are as fair as Europeans. Among the Jews who have settled in Northern Europe, are many of light complexion, while those who live in India are as dark as the Hindoos. Intense heat also increases this coloring matter, and thus a furnace-man's skin, even where protected by clothing, becomes completely bronzed. The black pigment has been known to disappear during severe illness, and a lighter color to be developed in its place. Among the negroes, are sometimes found people who have no complexion, _i. e._, there is no coloring matter in their skin, hair, or the iris of their eyes. These persons are called Albinos.]