instances are claimed to possess great remedial properties, as, for instance, in toxic diarrheas, constipation, intestinal putrefaction, arteriosclerosis, and even rheumatism, intestinal tuberculosis and typhoid fever. They have been advocated and used for sprays in diphtheria and other mouth and throat infections. The preparation of sour milk with the aid of these tablets or powders has also become a very common practice in recent years, and there can be no doubt but that in many instances the use of sour milk so prepared has been attended with success.

That the value of these Bacillus bulgaricus products has been grossly exaggerated can hardly be questioned, and particularly in so far as the remedial properties of the commercial powders and tablets are concerned. On the other hand, the consumption of sour milk, as advocated by Metchnikoff and his followers, has much to commend it. The value of the sour milk does not lie in the acids or the acid-producing bacteria, however, as will be shown in this paper, but in milk as such, irrespective of the character of the milk as to whether it is sweet, sour, whole, skimmed, raw or pasteurized milk.

In 1892 Rovighi showed that a kephir diet caused a great reduction in the ethereal sulphates (intestinal putrefaction products) in the urine, and of indol in the intestine. He believed that acids played an important rôle in suppressing intestinal putrefaction, but could not substantiate this by practical experiment. In the same year Winternitz demonstrated that milk strongly inhibits putrefaction, and held that this was due to the lactose, and not to the acids resulting from its decomposition by bacteria. The following year Schmits brought about a marked reduction in intestinal putrefaction products by feeding lactose. These observations have been repeatedly verified by other investigators.

The intestinal contents of the new-born infant are free from bacteria and other microscopic organisms. Very soon after birth, however, bacteria make their appearance and in the course of but relatively few hours become very abundant. The nature of these microorganisms is determined by the food of the infant, as was shown by Tissier and others. In the normal breast-fed child an organism first observed and described by this writer, and known as Bacillus bifidus, is present to the exclusion of all other forms. This organism readily attacks milk sugar without gas production, but has no putrefactive properties; in other words, it does not decompose albuminous substances with the formation of injurious products.

A change from breast- to bottle-feeding is accompanied by a change in the character of the intestinal flora. The simplified type is transformed into a mixed flora in which another organism, Bacillus acidophillus of Moro, assumes much prominence. When the diet becomes more varied by the addition of other foods, as, for example, egg and bread, the intestinal flora becomes still more complex and gradually assumes the character of that of the adult.

The bacterial flora of the intestine of man and animal may in a very large measure be determined by the diet. We have repeatedly shown that the bacterial contents of the intestine of the white rat may be transformed from the mixed to a simplified flora by the addition of milk to the regular diet. In the course of one to three days a flora is established which resembles that of an infant subsisting entirely on milk. When milk sugar is fed along with the milk, or alone in sufficient amount, the typical bifidus flora of the breast-fed child is obtained. The same transformation may be brought in the intestine of man, though relatively larger amounts of milk and of milk sugar are required. Torrey demonstrated that the feeding of a high caloric diet (milk and milk sugar) to typhoid patients tended to reduce the putrefying types of bacteria and to encourage the so-called "acidophilic" organisms which characterize the intestinal flora of infants subsisting entirely on milk.

The favorable influence of milk on the bacteria of the intestine is due largely to the lactose or milk sugar in the milk, which at times contains as much as six per cent. of the carbohydrate. The harmless and perhaps even beneficial bacteria of the Bacillus bifidus type readily attack milk sugar, and in the presence of this sugar find their optimum environment in the intestine, and develop so readily as to suppress or crowd out the harmful or putrefactive forms which find their most favorable cultural conditions in a medium rich in nitrogenous matter and poor in carbohydrate.

The preponderance of the sugar-loving type is not due to the acids that are produced in the decomposition of the sugars, nor can the reduction or disappearance of the putrefactive forms be ascribed to the presence of free acids in the intestine, for, except when they are introduced into the intestine in large amounts, acids rapidly disappear through absorption by the intestinal wall or by neutralization. This has been shown repeatedly. Metchnikoff's contention that the use of sour milk and milk-souring bacteria is of benefit to the consumer because of the action of the acids that are thereby introduced into the

intestine is not supported by direct observation. The suppression of putrefactive processes is brought about by influences other than acids.

It has likewise been shown again and again that the ingestion of even very large numbers of Bacillus bulgaricus with or without milk does not lead to an implantation and acclimatization of this organism in the intestine. This bacterium, which is of such wide distribution in nature, and especially in different kinds of sour milk, is unable under any and all conditions to establish itself in the intestine of man or animal. However, an organism (Bacillus acidophilus) which in many respects is closely allied to Bacillus bulgaricus is a common inhabitant of the intestine. Like Bacillus bifidus, which it closely resembles in appearance and physiological properties, Bacillus acidophilus is comparatively rare in the intestine during an ordinary mixed diet or one rich in protein. The addition of milk or of milk sugar to the diet encourages the development of this organism in the intestine, and at times to such an extent that all other forms are apparently excluded. This bacterium has often been mistaken for Bacillus bulgaricus.

That the so-called Bacillus acidophilus is not the same as the other may be shown readily by feeding milk which has been sterilized. The same transformation takes place in the intestine as when unheated sour milk is consumed. The ingestion of milk in any form, sweet, sour, raw or sterilized, leads to this same result. On the other hand, the feeding of even very large numbers of Bacillus bulgaricus in water, without milk or lactose, does not result in the implantation of the ingested organism, and the intestinal flora remains unchanged. It may be said, therefore, that diet is an important factor in determining the character of the intestinal flora, and that foreign bacteria find it difficult and as a rule impossible to establish themselves in the intestine. Bacteria which are of the strictly diseaseproducing type are, of course, not to be included in this category.

Aside from the significant fact that milk when taken in sufficient amount regulates the biochemical changes which take place in the lumen of the intestine, milk as such constitutes a most important article of diet. The value of milk as a food lies not only in its peculiar composition, that is in the proteins, fat, sugar, inorganic salts and other well-known ingredients, but also in certain demonstrable but as yet vaguely-defined substances known as "vitamins" or "accessories." These vitamins belong to a group of agents which are widely distributed

in nature, and which are now regarded as an essential factor in diet. Let us next consider some of the evidence which supports the assertion that milk possesses unique dietary properties.

Hopkins clearly demonstrated that the feeding of very small quantities of milk to rats which had been subsisting on a diet that was not conducive to normal growth brought about a rapid gain in the weight of the animals. Osborne and Mendel in their valuable experiments on the growth of animals have for several years been employing so-called "protein-free milk" as an indispensable ingredient of their basic diet to which certain isolated food substances (proteins, amino acids, etc.) are added. They state that "no artificial imitation of this natural mixture (milk from which all protein had been removed) has been devised to replace it satisfactorily for considerable periods of time." The weight and health of adult rats has been maintained for many months on a ration consisting of protein, starch, sugar, protein-free milk and lard. Young animals kept on this mixture decline after a definite period. If butter fat is substituted for the lard, however, growth is resumed. Hence, they consider the fat as one of the important food substances in milk. The fat is in all probability of much importance on account of the vitamines which it holds, and which are fat-soluble, as contrasted with the watersoluble accessories present in the protein-free milk. Ordinary skim milk contains both the fat-soluble and water-soluble accessories.

Numerous milk-feeding experiments have been conducted on young animals of different species, but few have been of much scientific value, owing to incomplete data and to a lack of adequate control of the experiments. The following investigation is one in which the writer was for several years engaged, and in which the results were a source of surprise even to those who were most intimately associated with the experiments.

THE INFLUENCE OF MILK ON GROWTH AND MORTALITY Numerous experiments were conducted on young chicks. When one to two days old the chicks were taken from the incubators, divided into uniform groups, usually six, each group containing from 25 to 60 chicks. Half of the lots, for example 1, 3 and 5, were supplied with milk in a shallow galvanized stew pan covered with a meshed wire, while the remaining lots, 2, 4 and 6, received no milk. Both the general groups of

chicks were constantly provisioned with a standard chick feed and dry mash. The milk was kept before the milk-fed chicks constantly. It was supplied as a rule as skimmed milk, rarely as whole. In some of the experiments sweet milk was employed, in others naturally soured, and in still others milk which had been soured by the Bulgaricus bacillus of Metchnikoff. In the preparation of the Bulgaricus product the milk was sterilized by live steam under pressure. Hence, both raw and sterilized milk was used in large amounts. Over five thousand chicks were employed in this investigation. The duration of the individual experiments was six to eight weeks.

In all of the experiments the milk-fed chicks were larger and in every respect appeared stronger and more vigorous than the corresponding control lots which received no milk. In a few instances the difference in the average weight of the two lots was almost 100 per cent. Furthermore, the combs of the milk-fed groups were redder, and the legs decidedly stronger, than those of the other lots. In one particular experiment, which was allowed to continue longer than the others, a marked difference was noted in the ages at which the males exhibited crowing propensities.

TABLE SHOWING THE INFLUENCE OF SOUR MILK FEEDING ON GROWTH (Combined results with 1,498 chicks)

That the growth-stimulating properties of milk are not

confined to sour milk is clearly mary of results on 375 chicks:

Combined averages:

Fed sour milk
Fed sweet milk

Given no milk

shown in the following sum

Gains per chick

0.48 pound per chick

0.44 pound per chick

0.39 pound per chick

The results obtained with naturally soured and with Bulgaricus milk were practically the same, hence no definite figures are presented here. It should be said, however, that a slight preference was shown for the naturally soured milk, as compared with the sweet and with the Bulgaricus product.