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.

[Illustration]

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.

[Illustration: _Microscopic view of a Muscle, showing, at one end, the fibrillæ; and, at the other, the disks, or cells, of the fiber._]

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.

[Illustration: _Tendons of the Hand._]

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.