Milk shares with honey the distinction of being the only dietetic agency which is originally intended for this purpose and no other, and is practically the only food containing all the alimentary principles in anything like the normal proportions.

Milk And Its Properties

Being, therefore, a typical natural food, it is used very freely alike among civilised and uncivilised peoples, and it possesses the additional features of being moderately cheap and easily digested. This last property is due to the fact that it is not dependent upon the "appetite juice" demanded by all other foods, and that it requires very little gastric and pancreatic juice for its complete disposal. It is also a very staying and economical food, because Pavlov has proved that twelve hours after its ingestion it has only parted with 15 per cent, of its nitrogen, whereas bread has given up 50 per cent. of its nitrogen, a circumstance which has suggested to him the initiation of dietetic experimentation with a view to classifying foods in accordance with their economical properties.

A proof of its extraordinary ease of assimilation is indicated by the fact that it can be utilised by the system after direct injection into the veins; but for all this it is by no means easily tolerated by every person. Probably this is in no small measure due to the common occurrence of hyperchlorhydria amongst a meat-eating community, because, if a glass of milk be administered an hour before meals in a normal stomach, which should then be empty and contain no acid, and the recumbent position be assumed, it is all absorbed before meal time. Finally, Rubner has shown that on an exclusive diet of milk - 2,438 grams in quantity - containing 84 grams protein and two-thirds of the energy requirement of the individual, no less than 6'7 grams of protein were added to the body daily. To supply 2,400 calories daily 3,410 grams of milk would be necessary, and this contains 140 grams of protein, which would be too much for a normal adult. For this very reason, however, milk constitutes an excellent diet for growing children or invalids convalescing from a wasting disease.

Consequently, in all ages efforts have been directed to the preparation of more suitable forms for its administration to those who have difficulty in digesting it, and in this way koumiss, which is a fermented preparation of mare's milk, and kephyr, an ally made from cow's or goat's milk, have been introduced. Both of these fluids contain lactic acid, butyric acid, carbonic acid, and alcohol, and have been largely used in the East and elsewhere in the treatment of inflammation of the colon alike in children and adults.

Probably in this country at least the kephyr and koumiss cures will speedily lapse into oblivion, on account of the introduction of the treatment of intestinal and other affections by "soured" milk, or lacto-bacilline. It is important to note that "soured," or as it is often called "curdled," milk is not simply sour milk, but a definite preparation of fresh milk, curdled by the activity of the Bulgarian or Massol's bacillus in the presence of its associate, the Bacillus acidi lactici aerogenes, produced in symbiosis. This last bacillus, first isolated by Lord Lister, and described by him under the name of Bacterium lactis, is always present in milk, is quite harmless, and improves the taste of the curd, whereas it is always necessary to add the former. Lister pointed out the interesting fact that the Bacterium lactis is not to be found in the milk as it comes from the cow, but obtains access in the dairy, and that whilst universal in dairies it is rarely to be found elsewhere.

Sour milk, or buttermilk, is the residue left after butter is made from cream, and contains, besides water, 2 1/2 per cent. protein, 1 per cent, fat, 3 1/2 per cent. sugar, with a considerable amount of lactic acid which has been formed from the sugar.

It is freely consumed by the peasantry of Scotland, Ireland, and Bulgaria - in the first-mentioned country being usually eaten with oatmeal-porridge, in Ireland with potatoes, whilst in both it is used as a beverage during harvesting and other agricultural operations. It is not so well known in England, but something akin to it was utilised by the middle and lower classes in the Middle Ages. Harvey, in his notes of the autopsy of Thomas Parr - " Old Parr," who is stated to have lived to the age of 153 years - mentions that the body was sound and "young," and he apparently thinks this rather remarkable, as "his ordinary diet consisted of sub-rancid cheese, and milk in every form, coarse and hard bread, and small drink, generally sour whey."

Soured Milk or "Yoghourt." - Whether this was the original preparation used in Bulgaria from which the present day curdled milk has been evolved it is difficult to say, but it is certain that yoghourt, or yoghurt, is now in the home of its birth made from a special ferment called "maya." This contains several yeasts and bacilli, besides the bacillus Bulgarian or bacillus of Massol, and the Bacillus acidi lactici aerogenes, amongst them being the Streptococcus lebenis, the Mycoderma lebenis, and the Saccharomyces lebenis. This specific substance has been used from time immemorial in Eastern Europe as an article of diet, and some thirty or forty years ago was much in vogue in France.

Although it is therefore the revival of an old treatment, it has been rescued from the haphazard methods in practice by the scientific acumen of Professor Metchnikoff, so well known for his work at the Pasteur Institute in Paris. He has demonstrated that the essential organism involved in its preparation is the Massol bacillus - first isolated by Grigoroff in Massol's laboratory in Geneva - in association with the Bacillus acidi lactici aerogenes, and that the others are not only unnecessary, but may in some cases be harmful.

When sweetened sterilised milk - or, as Metchnikoff advises, skimmed milk - at the correct temperature is inoculated with these two germs, lactic acid is slowly formed, and the caseinogen is altered to casein in the form of clots or curds. The longer the process is allowed to continue the more lactic acid is formed, but it can be inhibited by raising to boiling point or suspended by placing on ice. Thus prepared, the milk is of semi-solid consistency, and besides the alteration in the sugar - and any sugar is acted upon excepting saccharose and maltose - some of its constituents are partially digested; in particular, a goodly proportion of the casein and phosphates is rendered soluble. Metchnikoff says: "A large proportion (38 per cent.) of casein is rendered soluble during the fermentation, which shows that its albuminous matter is prepared for digestion. Of phosphate of lime, which is the chief mineral substance of milk, 68 per cent, is rendered soluble during the fermentation." For this reason it is more easily tolerated by many people than ordinary milk, and in any case is in effect a therapeutic agency, being virtually a pure culture of lactic acid bacilli, each pint of milk being estimated by Professor Tanner Hewlett to contain 400,000 millions of bacilli.

Now as lactic acid, like most acids, is antiseptic and germicidal, even when found in ordinary milk which has accidentally gone sour, and as to a certain extent it protects such milk from the invasion of putrefactive organisms, it is only necessary to introduce it in suitable form into the alimentary canal, there to combat the growth of the germs of putrefaction. This is the contention of Metchnikoff, whose attention was first directed to the subject when investigating the cause of old age. Impressed by the longevity of birds possessing no colon, and by the persistent use of curdled milk in Bulgaria by its many centenarian inhabitants, he could not refrain from associating these two facts. He had previously - whilst studying the etiology of cholera - discovered that the intestinal canal of the ordinary healthy man always contained a great number of varieties of bacteria. He was forced to the conclusion that many of the toxins known to abound in the alimentary canal were the direct results of the activities of these micro-organisms, and thus began that brilliant research which has served to make his name practically a household word amongst the nations.