sheets of bacteria in various situations on the enamel surface. During investigations carried on concerning the flora of carious teeth I have constantly met with a series of bacteria which are characterised by a curious facility for forming extremely tough gelatinous colonies, not by any means due to the presence of carbohydrate as in the mucinous fermentation of sugar and molasses, but occurring on media free from any carbohydrate whatever; one organism in particular, a coccus, is frequently present. This organism is frequently to be met with upon the enamel surface of teeth, particularly the white opaque patches of softened enamel to which Williams has drawn attention (for description see p. 172).

I found no difficulty in reproducing the plaque-like layer upon sterilised teeth suspended in a cultivation of one of these bacteria, and moreover when another organism capable of acid fermentation was mixed with the plaque-forming organism, and carbohydrate media used, under the plaques formed upon the enamel surface by the two bacteria superficial disintegration of the enamel was observed to occur in a week to ten days. Such an experiment is no doubt largely in favour of the organisms; there is no cleansing due to mastication or movement of tongue or saliva, and no great and constant dilution of the acids formed, which are at liberty to attack the tooth under the bacterial sheet. Nevertheless the colonies, which are often formed even upon the surface of the glass in the culture tube, are remarkably adherent and resist removal, and it is not unfair to suppose that such a condition obtains in the mouth. An important coincidence to this supposition is afforded by the fine teeth of many native races, many of which, particularly the Zulus and Kaffirs, are particularly assiduous in cleansing their teeth. Amongst the former it is the common practice for the Zulu mother to carefully cleanse her child's mouth after every meal until it is old enough to do so for itself; the finger is generally used, and some ashes (wood) from the fire are employed. The majority of the adult natives in the beds of the Seamen's Hospitals are especially careful of their teeth, at times refusing to eat unless first supplied with water with which to wash their mouths after the meal.

In making cultivations from the mouths of natives with good dentition, and also from the mouths of some of the monkeys at the Zoological Gardens, I have been struck with the number of putrefactive rather than acid-forming bacteria present in the mouths.

Certain of these bacteria will also form a definite layer upon the surface of enamel when a sterilised tooth is suspended in a broth culture.

It is a frequently observed clinical fact that individuals applying for treatment at dental hospitals may possess peculiarly dirty mouths, with marginal inflammation of the gums, and yet exhibit extremely small evidence of caries; the appearance of such mouths is strikingly similar to several of the monkeys I examined. Putrefaction was evidently the ascendant process and therefore an accompanying alkaline reaction, such carbohydrate food as was taken probably happening to be of a species only fermented with difficulty. The coincidence is interesting, and if open to several explanations is not opposed to the general principles we have discussed.

I have already referred to the question of roller-flour, and to the increased amount of acid it is said to engender. Acid may be present in the mouth in certain pathological and physiological conditions, in pyrosis, the vomiting of pregnancy; in diabetes mellitus an acid saliva is frequently present. Acid contained in medicine has also to be mentioned in the same category.

It is possible that small quantities of acid frequently applied to the teeth may produce microscopical irregularities, or what is more important, solution of the interprismatic substance of the enamel prisms, or of the axial portions of those prisms, assisting in the adherence of organisms and forming microscopical points of entrance from which portals the process may extend.

On the other hand we must not forget that some people are in the habit of consuming acid foods, and eating strongly acid fruits, as is the case with the Sicilians1, and it is conceivable that the acid may act as a protective by preventing the development of acidforming organisms, or dissolving away the outer layers of the enamel and with it the contained bacteria, secondary dentine occluding the pulp chamber before the process had threatened that cavity. Such an obliteration is common in old skulls with great denudation of the dentine.

So far we have not considered the relation of pathological malformations of the teeth in their relation to caries.

Cosmos, 1898. Dr. Leon Williams tells me that the Sicilians, who are particularly free from caries, are large consumers of lemons.

Leon Williams has pointed out that pits, grooves and fissures, pigmentation, granular and amorphous enamel are to be found in the lower animals whose teeth are comparatively free from caries, and that various species of human enamel, which are apparently especially liable to caries by reason of their pathological irregularities, may resist for years, whereas enamel, to all appearances sound, is often the seat of rapid decay. To such a statement all will agree, but it is an undoubted clinical fact that hypoplasic teeth do certainly undergo extremely rapid caries under certain conditions, the point of attack being almost invariably a pit or fissure on the enamel surface. Looking at the question from a broad general point of view it certainly by no means follows that because a tooth has a developmental defect therefore it must become carious, but we may say with a considerable measure of truth that exposed to those conditions which we have seen predispose to caries, the tooth with irregularities and deficiencies on its enamel surface is more liable to attack than the developmentally perfect one. Further, we are not sufficiently conversant with the microscopical defects or normal structure of the enamel surface of teeth generally to disregard microscopical defects as predisposing causes. Moreover the normal fissures of molar and bicuspid teeth are so frequently the starting points of caries that it is impossible to ignore the great importance of points of least resistance. Dental caries, although not a true disease, occurs in a cavity of the body bathed with secretions physiologically unstable, and containing living cells, all subject to various oscillations between disease and health, and we must therefore adopt in the study of caries many of the methods applicable to the etiology of disease in general. We have seen that the organisms of diphtheria and pneumonia may exist in normal individuals' mouths with no manifestation of disease, that the liberating cause of pathological energy, to wit the bacterium, is less than the resistance of the body cells, and that it is only in a limited number of instances that the balance appears depressed in the favour of the bacterium. A little careful consideration will show that dental caries has many points of similarity, for in some mouths no caries exists, notwithstanding the luxuriant flora present, whilst in others apparently similar in all respects caries is rampant.

Now the teeth themselves cannot be possessed of bactericidal action, whereas the fluids of the mouth are daily undergoing physiological variation, and it seems feasible that alterations in composi

tion, reaction or quantity of the buccal fluids may furnish many explanatory points in the etiology of tooth decay. Bacteria are notoriously sensitive to their environment, slight changes of medium, temperature, alkalinity and what not, favouring the development of one species to the exclusion of others; we may therefore briefly conclude that any circumstance or series of circumstances that favours the development of acid-forming bacteria and their adhesion and retention about the teeth, may be rightly considered as a predisposing cause of dental caries.

Food Stuff Chemistry. -Fermentation is of such supreme importance in dental caries that it will be as well to briefly mention some special points relating to fermentative changes.

Food stuffs are divisible into proteids, carbohydrates, and fats; of these carbohydrates are of the chief importance in caries.

Carbohydrates are classed in three main groups according to their chemical composition :

(1) Monosaccharides. (2) Disaccharides.

(3) Polysaccharides.

Many other carbohydrates exist, but they are physiologically unimportant.

(1) MONOSACCHARIDES (CH12O).-Dextrose, levulose (glucose). These carbohydrates are commonly found in nature, generally together, in fruits, seeds, roots and honey. Galactose, another carbohydrate of the group, is formed from the hydrolysis of lactose or milk sugar.

They are directly fermentable by yeast into alcohol and carbonic acid,

C6H12O6yeast + 2 C2H,. OH + 2 CO2,

or by several of the schizomycetes of the mouth into lactic acid.

This equation does not exactly express the entire change, as a certain amount of the sugar is used up by the growth of the organisms. To obtain the acid in pure form a fermentation is carried on in a large flask containing sugar (lactose or dextrose) with a layer of precipitated chalk at the bottom. As the acid is produced it combines with the calcium, forming calcium lactate. The lactate is filtered off when the action has ceased, and the acid recovered by distillation with sulphuric acid.

DISACCHARIDES (C12H22O11).-Cane sugar, milk sugar (lactose), malt sugar (maltose).

The disaccharides are regarded as condensation products of the monosaccharides with the elimination of a molecule of water.

The importance of this is seen in the fact that before fermentation of the higher sugars occurs they require hydrolising to the lower or monosaccharide form.

Cane sugar is not directly fermentable by yeast, but an invert ferment produced by the yeast changes the cane sugar to dextrose and lævulose, which is then fermentable. Some organisms occurring in the mouth are able to transform the sugar direct, but as a rule cane sugar takes much longer to ferment than the glucoses (dextrose and lævulose).

In the experiment, directly inversion of the cane sugar occurs, the solution which before produced no reduction of Fehling's solution now gives a marked reaction.

Maltose ferments readily with yeast and with the majority of mouth bacteria. It forms a typical osazone with phenylhydrazine. It is the chief sugar formed by the action of ptyalin upon starch in the mouth.

Lactose occurs only in milk. It is the most resistant sugar to the effects of yeast, but is fermentable by mouth bacteria, with the formation of lactic acid.

The genus B. lactis, first described by Lord Lister, is composed of a large number of different species. Organisms belonging to this class are invariably present in milk, and may be generally obtained from sour milk by cultural methods.

POLYSACCHARIDES (CH100g)n.-A large group of naturally occurring carbohydrates, the chief groups being the starch group, the cellulose, the gum group (dextrines, plant gums and mucilages).

Starch.-Not directly fermentable by yeast; is fermented by a few bacteria occurring in the mouth and intestine. Starch is also inverted to maltose by the action of the ptyalin of saliva, and by ferments produced and contained in certain bacteria, some of which are found in the mouth.