Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/110

 A special examination in dentistry now exists in connection with the Royal College of Surgeons of England for students training in that profession, a certain amount of information being required in various branches of medicine and surgery. A curriculum of study in these departments has been arranged; and candidates who can produce certificates of attendance on it are admitted for examination, and, if found fit, receive a certificate entitling them to practise as dentists.

In America this special system has for long been adopted and carried to a much greater extent. Colleges of dentistry are established in many of the leading cities there, each with what they designate a faculty of professors in the various departments of the art. In the Dental Cosmos, vol. xvii. No. 11, an American periodical, advertisements appear of seven different dental colleges, with seventy-eight professors, demonstrators, &c. The professorships in these institutions comprehend those of mechanical dentistry, operative dentistry, dental physiology, dental pathology, dental therapeutics, mechanical dentistry and metallurgy, institutes of dentistry, &c. In each a diploma in dentistry—“doctor of dental surgery” or of ”dental medicine,“ as the case may be—is conferred, the general fee for which seems to be $30, on the candidates having fulfilled the curriculum and passed the examination.

In the medical schools and examining boards in Scotland all this is different. No special or partial diploma is there given by the Royal College of Surgeons or other licensing body, while diseases of the teeth and adjacent structures are understood to be made subjects of lecture and examination in the same manner as other regional or special diseases occurring in the practice of medicine or surgery; and great as the improvements certainly are which such arrangements as those of England and America are on the old system, still it is to be hoped, and it is likely, that ere long practitioners devoting themselves to dental surgery will—like oculists, or aurists, or obstetricians, or other physicians or surgeons restricting themselves to or selecting one branch of practice in preference to another—be at the same time fully qualified medical men.

Number of Teeth. —The complement of teeth in the adult human subject amounts to 32—16 in the upper, and 16 in the lower jaw. These are divided into what are termed incisors, canines, bicuspids or small grinders, and molars or the large grinding teeth. The order in which these different forms of teeth are placed in each jaw is the following: —there are four incisors in front; immediately behind these on each side is placed the canine or eye tooth; next come the bicuspids, two on each side; and behind these again are placed on each side the three molar teeth, the last of which is sometimes termed the wisdom tooth, from its gene rally appearing so late as from eighteen to twenty-five.

In the infant or milk teeth, or, as they are more properly denominated, the temporary teeth, the number and class of these organs is different. Here only 20 members of the series exist, and are divided into four incisors, two canines, and four molar teeth, similarly placed—ten in each jaw. The four temporary molar teeth represent or rather precede the four bicuspid teeth of the adult set, while the six molars above and below of the adult are not represented in the temporary set at all. In other words, the true permanent molar teeth have no predecessors.

Dentition.—The temporary set appear, or are cut, as follows. The two lower central incisors appear between the sixth and eighth months of infant life—these are generally succeeded in a few weeks by those of the upper jaw; the two lateral incisors of the upper jaw next appear about the eighth or ninth month, and those of the lower jaw quickly follow; the anterior molars of the lower jaw are cut about the twelfth, fourteenth, or sixteenth month, and those of the upper jaw immediately after; the canines appear about the seventeenth or eighteenth month, generally those in the upper jaw first; and before the age of two and a half years the second milk molars have usually commenced to appear, thus completing the temporary set of teeth at the age of about three years.

The temporary set of teeth begin to be shed between the sixth and eighth years of life. Previous to this, however, the first permanent molars are cut, generally about the age of seven. These are followed by the central and then by the lateral incisors. Next come the anterior bicuspids about nine years old; the posterior about ten or eleven; the canines about twelve; the second molars at thirteen; and the last molars, or wisdom teeth, from the eighteenth to the twenty-fifth years of life. Deviations from the order and time of appearance of both sets occur, but the above may be regarded as the general rule in the evolution of the temporary and permanent teeth of the human subject.

Structure and Form of Teeth.—The structure of both sets may be said to be the same. The body of each tooth is composed of a dense bony substance termed dentine. This is invested on the crown by a cap of still more dense material termed enamel; while the root, or fang, is coated externally by a layer of a softer substance, closely resembling ordinary bone, and termed cement. In the centre of each fang, and extending into the body of the tooth, is a hollow canal termed the pulp cavity, for the passage of vessels and nerves.

In form the incisors of both jaws are single-fanged, as are also the canines. The bicuspids of the lower jaw are also single-fanged, while those of the upper jaw are occasionally double fanged, or have a single fang bifid at its extremity. The lower molars, both temporary and permanent, possess two fangs, one behind the other. These two fangs are widely separated in the temporary molars; while, on the other hand, in the posterior molars of the permanent set they are not uncommonly united into one. The upper molars of both sets possess three fangs—two external or cheek fangs placed one behind the other, and a third situated on that side of the tooth next the palate.

No such spaces exist between any of the teeth in the dental arch of man as occur in the lower animals. In this way, where the jaw is small, or where unusually rapid or simultaneous appearance of the members of the second, or persistence of those of the first set occurs, irregularity of the teeth results. This is sometimes increased by the evolution of supernumerary teeth, these being generally out of the line of the others; and occasionally matters are rendered worse by the natural teeth being themselves of unusually large size. Cases also occur in which the number of the teeth is defective, and some rare instances have been recorded where these organs never appeared at all.

The remedy in cases of dental irregularity is to remove