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bones and muscles become developed, and the obstacles to progression are removed, he succeeds in walking alone; but it is some time before he is capable of running or leaping. Perhaps the average period, at which the infant begins to walk, is about twelve months; but we see great difference in this respect.

When once the infant is fairly on his legs, the whole of his waking hours is spent in incessant activity and amusement. His functions of expression are commensurate with his intellectual development, which we have seen to be great in this period. Sleep, which is now more interrupted, is still imperiously and frequently demanded, the nervous system being devoid of that strength, which it subsequently possesses, and therefore requiring repose.

One of the most important changes going on at this age concerns the function of digestion. This is the process of dentition, which usually commences about the seventh month, and continues till the end of the second year at least. Prior to the appearance of the teeth, mastication is of course impracticable; and the food, best adapted for the delicate powers of the infant, is that afforded by the maternal breast, or a substitute which resembles it as closely as possible. The appearance, however, of teeth Would seem to indicate that the infant is about to be adapted for more solid aliment. As early as the second month of utero-gestation, if the jaws be carefully examined, the germs of the teeth are perceptible in their substance, under the form of membranous follicles of an oval shape, attached by their deep-seated extremity to a vascular and nervous pedicle, and by their superficial extremity to the gum. The cavity of these follicles, according to Beclard, is at first filled with a colourless, limpid fluid; but a kind of vascular and nervous papilla or pulp soon forms in it, which commences at the deep-seated portion of the follicle, proceeds towards the other extremity, and ultimately fills it,--the fluid diminishing in proportion to the increase of the pulp. About the termination of the third month, ossification begins, and a little sooner in the lower than in the upper jaw. This consists, at first, in a deposition of ivory matter on the surface of the pulp and at its top; which goes on increasing in width until it covers the whole of the dental pulp with a shell of bone. It augments also in thickness at the expense of the dental pulp, which becomes gradually less and less. When the bony shell has extended as far as the neck of the tooth, the external membrane or sac of the tooth, --for the follicle consists of two membranes—attaches itself closely, but not by adhesion to the part. The inner membrane becomes much more vascular, and the enamel is secreted by it. A thickish fluid is observed to be poured out from the inner surface which is soon consolidated into a dark, chalky substance, and afterwards becomes white and hard.

At birth, the coronæ of the incisors are formed; those of the canine are not completed; whilst the molares have only their tubercles. The root or fang is formed last of all. As ossification proceeds, the corona of the tooth presses upon the gum, a portion

of the follicle being interposed, which is gradually absorbed, as well as the gum, and the tooth issues.

The age, at which the teeth make their appearance, varies. Occasionally, children have been born with them, whilst in other cases they have not pierced the gum until after the period we are considering. Generally, however, the middle incisors of the lower jaws appear about the seventh month, and, subsequently, those of the upper jaw; next the inferior and superior lateral incisors in succession; then the first lower molares, and the first upper; next the inferior and superior canine teeth, successively; and, lastly, the second molares of each jaws.

The approximate times of their appearance are thus estimated by Mr. Thomas Bell.

From five to eight months, the four central incisors.
From seven to ten, the four lateral incisors.
From twelve to sixteen, the four anterior molares.
From fourteen to twenty, the four canine.
From eighteen to thirty-six, the four posterior molares.

Fig. 160.

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1. Front view of the temporary teeth.

2. The separate temporary teeth of each jaw. a. The central incisor.–6. The lateral incisor.-c. The canine.-d. The first molaris.-e. The second molaris,

Dentition is necessarily a physiological process, but it is apt to be a cause of numerous diseases. The whole period of its continuance is one of great nervous susceptibility,--so that the surgeon never operates during it unless when compelled, and we can understand, that the pressure, exerted by the tooth on the gum, and the consequent inflammation and irritation, may lay the foundation of numerous diseases. More are doubtless ascribed to the process than it is entitled to, but still they are sufficiently numerous; and all require, in their management, the free division of the distended gum, so as to set the presenting part of the tooth at liberty.

Whilst the teeth are appearing, the muscular structure of the body generally is acquiring strength, and the salivary organs are described by anatomists as becoming much more developed. The food of the child is now diversified, and it begins to participate in the ordinary diet of the table. The excrementitious matters are consequently altered in their character, particularly the alvine, which become firmer, and acquire the ordinary fæcal smell; the urea is still, however, in the generality of cases, in less proportion than in the adult. The other functions require no particular mention.

The mortality, during this period, is great. The bills of mortality of London show, that the deaths under two years of age are nearly thirty per cent. of the whole number. In Philadelphia, during a period of twenty years ending with 1826, the proportion was rather less than a third. The cholera of infants is the great scourge of our cities during the summer months, whilst in country situations it is comparatively rare; and it is always found to prevail most in crowded alleys, and in the filthiest and impurest habitations. There is something in the confined and deteriorated atmosphere of a town, which seems to act in a manner directly unfavourable to human life, and of the young especially This is not confined to man. It is applicable also to the animal. Experiments were instituted by Jenner, and since him by Dr. Baron, which show that a privation of free air and of their natural nourishment has a tendency to produce disorganization and death. Dr. Baron placed a family of young rabbits in a confined situation, and fed them with coarse green food, such as cabbage and grass. They were perfectly healthy when put up. In about a month one of them died,--the primary step of disorganization being evinced by a number of transparent vesicles in the external surface of the liver. In another, which died nine days after, the disease had advanced to the formation of tubercles in the liver. The liver of a third, which died four days later, had nearly lost its true structure, so completely was it pervaded by tubercles. Two days afterwards a fourth died: a number of hydatids was attached to the lower surface of the liver. At this time, Dr. Baron removed three young rabbits from the place where their com

panions had died to another situation, dry and clean, and to their proper accustomed food. The lives of these were obviously saved by the change. He obtained similar results from experiments of the same nature performed on other animals.

3. Third period of infancy.--This requires no distinct consideration; the growth of the child and the activity of the functions going on as in the preceding period, but gradually acquiring more and more energy. Within this period a third molaris appears, which is not, however, a temporary tooth, but belongs to the permanent set.

During the whole of infancy, the dermoid texture—both skin and mucous membranes—is extremely liable to be morbidly affected; hence the frequency of eruptive diseases, and of diarrhœa, aphthæ, croup, bronchitis, &c., many of which are of very fatal tendency. Owing, also, to the susceptibility of the nervous system, convulsions, hydrocephalus, and other head affections are by no means infrequent.

Sect. II.- Childhood.

Childhood may be considered to extend from the seventh to the fifteenth year, or to the period of puberty; and it is particularly marked by the shedding of the first set of teeth, and the appearance of the second. It is manifest, that in the growth of the jaws with the rest of the body, the teeth, which, for a time, may have been sufficient in magnitude and number, must soon cease to be so; hence the necessity of a fresh set which may remain permanently. The process for the formation of the permanent teeth is similar to that of the milk or temporary teeth; yet it presents some remarkable points of difference; and it affords us another surprising instance of the wonderful adaptation of means to definite objects, of which we have so many in the human body.

This process has been well described by Mr. Thomas Bell, in his recent work on the "Anatomy, Physiology, and Diseases of the Teeth,"—an individual whose opportunities for observation have been unusually numerous, and to whose zeal and ability in his profession, as well as in the prosecution of natural science, it affords us pleasure to testify.

The rudiments of the permanent teeth are not original and independent like those of the temporary. They are derived from the latter, and continue, for a considerable time, attached to, and intimately connected with, them.

At an early period in the formation of the temporary teeth, the investing sac gives off a small process or bud, containing a portion of the essential rudiments, namely, the pulp, covered by its proper membrane. This constitutes the rudiment of the permanent tooth Vol. II.

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It commences in a small thickening on one side of the parent sac,

which gradually becomes more and Fig. 161.

more circumscribed, and at length b

assumes a distinct form, though still connected with it by a pedicle. For a time the new rudiment is contained within the same alveolus as its generator, which is excavated by

the absorbents for its reception. It 2.–Permanent rudiment given off from the is not, according to Mr. Bell, in 6.-Permanent rudiment given off from the consequence of the pressure of the temporary in a molaris.

new rudiment upon the bone, that this absorption is occasioned, but by a true process of anticipation; for he states that he has seen in the human subject, and still more evidently in the foal, the commencement of the excavation before the new sac was formed, and consequently before any pressure could have taken place on the parietes of the socket. The absorption does not indeed begin in the smooth surface of the socket, but in the cancelli of bone immediately behind it. By degrees, a small recess is thus formed in the parietes of the alveolus, in which the new rudiment is lodged, and this excavation continues to increase with the increasing size of the rudiment, whilst, at the same time, the maxillary bone becomes enlarged, and the

temporary tooth, advancing in its formation, rises in Fig. 162. the socket. The new cell is thus gradually separated

from the other, both by being itself more and more deeply excavated in the substance of the bone, and also by the formation of a bony partition between them, as seen in the marginal figure, 162, which exhibits the connexion between the temporary tooth and the permanent rudiment, as it exists after the former has passed through the gum. As the temporary tooth grows and rises in the jaw, the connecting cord or pedicle elon

gates, and although the sac, from which it is derived, is gradually absorbed, it still remains attached to the neck of the Fig. 163.

temporary tooth. The situation of each permanent rudiment, when its corresponding temporary tooth has made its appearance through the gum, is deeper in the jaw and a little behind the latter, as represented in the marginal illustrations of the upper and lower jaw after the whole of the temporary teeth have passed

through the gum. Figs. 163 and 164. From these it will be understood, how the upper part of the sac of

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