How is this shown by experiment? 207. Upon what do the different intonations of sound or mechanical employments depend? Why are the first efforts in educating the muscles indifferent or irregular? 208.

Why is repet.i.tion of muscular action necessary? Why is it important that correct movements be adopted in the first efforts of muscular education?

_Ill.u.s.tration._ If a boy, while learning to mow, is allowed to swing his scythe in a stooping position, twisting his body at every sweep of the scythe, he will never become an easy, efficient mower. Proper instruction is as necessary in many of the agricultural branches as in the varied mechanical employments.

[Ill.u.s.tration: Fig. 52. An improper, but not an unusual position, when writing.]

[Ill.u.s.tration: Fig. 53. A proper position, when writing.]

209. _Good penmanship requires properly trained muscles._ To a deficient a.n.a.lysis of the movements of the arm, hand, and fingers, on the part of teachers and pupils in penmanship, together with an improper position in sitting, is to be ascribed the great want of success in acquiring this art. The pen should be held loosely, and when the proper position is attained, the scholar should make an effort to imitate some definite copy as nearly as possible. The movements of the fingers, hand, and arm, necessary to accomplish this, should be made with ease and rapidity, striving, at each effort, to imitate the copy more nearly.

How is this ill.u.s.trated? 209. Why have so many pupils failed in acquiring good penmanship?

210. When the arm, hand, and fingers are rigid, the large muscles, that bend and extend these parts, are called into too intense action.

This requires of the small muscles, that produce the lateral movements, which are essential to rapidity in writing, an effort which they cannot make, or can with difficulty accomplish.

_Experiment._ Vigorously extend the fingers by a violent and rigid contraction of the muscles upon the lower part of the arm, and the lateral movement which is seen in their separation cannot be made. But gently extend the fingers, and their oblique movements are made with freedom.

211. An individual who is acquainted with the laws of health, whose muscles are well trained, will perform a certain amount of labor with less fatigue and waste to the system, than one who is ignorant of the principles of hygiene, and whose muscles are imperfectly trained.

Hence the laboring poor have a deep interest in acquiring a knowledge of practical physiology, as well as skill in their trade or vocation.

It is emphatically true to those who earn their bread by the "sweat of their brow," that "knowledge is power."

210. What is said of the lateral and oblique movements of the arm, hand, and fingers in writing? How is this shown by experiment? 211.

Why is the study of physiology and hygiene of utility to the laborer?

CHAPTER XII.

THE TEETH.

212. The teeth, in composition, nutrition, and growth, are different from other bones of the body. They vary in number at different periods of life, and, unlike other bones, they are exposed to the immediate action of atmospheric air and foreign substances. The bones of the system, generally, when fractured, unite; but there is never a permanent union of a tooth when broken.

ANATOMY OF THE TEETH.

213. The TEETH are attached to the upper and lower jaw-bone, by means of bony sockets, called _alve-o-lar_ processes. These give great solidity to the attachment of the teeth, and frequently render their extraction difficult. The gums, by their fibrous, fleshy structure, serve to fix the teeth more firmly in the jaw.

_Observation._ When a _permanent_ tooth is extracted, these bony processes are gradually absorbed, so that in advanced age there remains only the jaw-bone covered by the lining membrane of the gum.

This accounts for the narrow jaw and falling in of the lips in old age. Frequently, a piece of the alveolar process comes out with the tooth when extracted, and the dentist has then the credit of "breaking the jaw." No great injury results from the removal of the process in this manner.

212. What is said of the teeth? In what respect do they differ from other bones of the body? 213-218. _Give the anatomy of the teeth._ 213. What confines the teeth in the jaw-bone? What becomes of the socket when a tooth is removed? What effect has this absorption upon the jaw and lips?

214. The teeth are formed in the interior of the jaws, and within _dental capsules_, (membranous pouches,) which are enclosed within the substance of the bone, and present in their interior a fleshy bud, or granule, from the surface of which exudes the ivory, or the bony part of the tooth. In proportion as the tooth is formed, it rises in the socket, which is developed simultaneously with the tooth, and pa.s.ses through the gum, and shows itself without.

[Ill.u.s.tration: Fig. 54. 1, The body of the lower jaw. 2, Ramus, or branch of the jaw, to which the muscles that move it are attached. 3, 3, The processes which unite the lower jaw with the head. _i_, The middle and lateral incisor tooth of one side. _b_, The bicuspid teeth. _c_, The cuspids, or eye teeth. m, The three molar teeth. A, shows the relation of the permanent to the temporary teeth.]

215. The first set, which appears in infancy, is called _tempo-ra-ry_, or milk teeth. They are twenty in number; ten in each jaw. Between six and fourteen years of age, the temporary teeth are removed, and the second set appears, called _perma-nent_ teeth. They number thirty-two, sixteen in each jaw.

214. Where and how are the teeth formed? Explain fig. 54. 215. What are the first set called? How many in each jaw? The second set? How many in number?

216. The four front teeth in each jaw are called _in-cisors_, (cutting teeth;) the next tooth in each side, the _cuspid_, (eye tooth;) the next two, _bi-cuspids_, (small grinders;) the next two, _molars_, (grinders.) The last one on each side of the jaw is called a _wisdom tooth_, because it does not appear until a person is about twenty years old. The incisors, cuspids, and bicuspids, have each but one root. The molars of the upper jaw have three roots, while those of the lower jaw have but two.

[Ill.u.s.tration: Fig. 55. The permanent teeth of the upper and lower jaw.

_a_, _b_, The incisors. _c_, The cuspids. _d_, _e_, The bicuspids. _f_, _g_, The molars, (double teeth.) _h_, The wisdom teeth.]

_Observation._ The shape of the teeth in different species of animals is adapted to the kind of food on which they subsist. Those animals that feed exclusively on flesh, as the lion, have the cuspids, or canine teeth, largely developed, and the molars have sharp cutting points. Those animals that feed on gra.s.s and grain, as the horse and the sheep, have their molar teeth more rounded and flat on the crown.

The human teeth are adapted to feed on fruits, grain, or flesh, as they are less pointed than those of the cat, and more pointed than those of the sheep.

216. Give the names of the permanent teeth. What teeth have but one root, or "fang"? How many roots have the molars of the upper jaw? Of the lower jaw? What is said of the shape of the teeth in different species of animals?

217. The teeth are composed princ.i.p.ally of two substances--the _ivo-ry_ and the _en-amel_. The internal part of the tooth or the ivory, is harder and more enduring than bone, and forms the body of the tooth. The enamel is remarkable for its hardness, and varies somewhat in color with the age, temperament, habits, and manner of living of different individuals. When any part of the enamel is destroyed, it is never regenerated.

[Ill.u.s.tration: Fig. 56. A side view of the body and enamel of a front tooth.]

[Ill.u.s.tration: Fig. 57. A side view of a molar tooth. 1, The enamel. 2, The body of the tooth. 3, The cavity in the crown of the tooth that contains the pulp. 4, A nerve that spreads in the pulp of the tooth. 5, An artery that ramifies in the pulp of the tooth.]

218. Each tooth is divided into two parts, namely, _crown_ and _root_.

The crown is that part which protrudes from the jaw-bone and gum, and is covered by the highly polished enamel. The root, or "fang," is placed in the sockets of the jaw, and consists of bony matter. Through this bony substance several small vessels pa.s.s, to aid in the growth and also in the removal of the tooth. There are, beside these vessels, small white cords pa.s.sing to each tooth, called _nerves_.

(See fig. 57.) When these nerves are diseased, we have the toothache.

217. Give the structure of the teeth. What is said of the enamel? 218.

Into how many parts are the teeth divided? Describe the crown. The root. What vessels pa.s.s through the bony matter? What is their use?