A similar food to the yoghourt is prepared in the Balkan mountains from sheep's milk under the name of "Urgoutnik."[38] The milk is poured into a goat-skin or sheepskin bag, and a little of the fermented milk added, and is then left for some hours in a warm place. The milk consumed is replaced by a fresh supply. In some of the Balkan countries, they are not content with the fermentation of the milk, they add a little alum, which, under the name of "typsa," is well known for this purpose. The milk attains such a solid consistency that it can be put into a cloth and carried to market.[39] The various forms of sour milk which have been described in the foregoing pages may be said to be of the traditional kind, and with the light of modern knowledge, it has been possible to determine exactly what constitutes the active principle in use in the milk consumed in these countries, and, as we shall see, this principle has been applied so that, at the present day, a pure fermented milk may be obtained in any country, and there is every reason to believe that should such be adopted as a general article of food, it would contribute to the prolongation of human existence. It is due to Metchnikoff, of the Pasteur Institute, that so much prominence has been given to the use of fermented milks. He gave it as his opinion[40] that senility was caused partly by auto-intoxication or by the poison derived from putrefactive micro-organisms which inhabit the digestive track. These organisms increase with age, and under certain unhealthy conditions multiply enormously, particularly in the large intestine. Having arrived at this knowledge, Metchnikoff set to work to devise some means of combating the influence of these harmful microbes, and set up the hypothesis that the tendency to longevity which is exhibited in Eastern countries is due to the consumption of lactic acid organisms in the shape of soured milk. These organisms are more powerful than those of a putrefactive character and inhibit their growth. "In the presence of such facts," says Metchnikoff, "it becomes exceedingly important to find some means of combating the intestinal putrefaction which constitutes so incontestable a source of danger. Such putrefaction is not only capable of producing diseases of the digestive tube—_enteritis_ and _colitis_—but even of becoming a source of intoxication of the organism in its most varied manifestations.