The smaller shelf-widths which Mr. Day notes at Munich have come into occasional use also in other museums, American and foreign. In Boston the show-space tends also to be set higher.

The reduction in the cubic contents of museum cases here advocated, in harmony with Mr. Day's suggestion and newer practice, is the second radical improvement in these fixtures since public museums were instituted. The first is an improvement from the point of view of the museum; the second from the point of view of the visitor. The device known in Europe as the Reichenberger case (due to Dr. Gustav E. Pazaurek, Director at the time of the North Bohemian Museum of Industrial Art), and in America as the Boston case (independently invented with a different mechanism by Mr. W. W. MacLean of the Boston Museum), consists in opening a case by lifting its top with a windlass instead of unlocking its doors with a key. This was a proposal in the interest of the security of the contents from dust, damp and theft. The reduction of the size and particularly of the depth of cases is a proposal in the interest of the easy visibility of their contents. By making also this second advance in the construction of these necessary fixtures, the museum would be in a position to fulfill more perfectly both of its essential functions, first as guardian and then as expositor of the treasures committed to its charge.

The use of smaller cases has for a corollary a reduction in the number of objects shown simultaneously. It would be another step in the pathway which modern museums have already entered upon in dividing their contents into show and study series and in alternating objects between the two. The era of smaller and changing exhibits is also an era of better exhibition.

THE FUNCTION OF MILK IN THE SCHEME OF EVOLUTION

BY HENRY DWIGHT CHAPIN, M.D.

NEW YORK

GOETHE once remarked that blood is a very peculiar juice.

We

can say the same of milk. Modern physiological researches have shown that certain glands and secretions of the body have much larger functions than have hitherto been assigned to them. We need only refer to the so-called "internal secretions" of various glands, formerly unrecognized, that are now known to exert a marvellous influence not. only on physical life, but on mental development as well.

These facts have led us to give a closer scrutiny to the more familiar fluids of the body, of which milk is one of the best known, as it constitutes the universal food for the young of all mammalia. In serving this most important function it is recognized as a complete food, containing in itself all the elements required to support life. These include protein for growth and tissue repair, with mineral salts to aid in this function; carbohydrates and fats that produce heat and energy, and an abundance of water so necessary to carry on all the processes of life. From a nutritional standpoint, it is thus a perfect food, and all milks are alike in this respect. While each species of mammalian young is perfectly nourished by the milk of its own mother, the food elements are present in varying proportions in different species, this depending largely on the rapidity of growth of the offspring.

Another peculiarity common to all milks is that when collected from the mother they are always in fluid form, but as soon as taken into the stomach of the young they become more or less solid. This is due to a process of coagulation that takes place only in one of the ingredients— the protein-but which thus always alters the form of the ingested milk. While the carbohydrates and fats in their composition and reaction to the digestive secretions are a good deal alike in different milks, the proteins are essentially different. It is further to be noted that coagulation of the proteins of milk takes place in different degrees in the different species.

We are now led to two queries: (1) What is nature's object in presenting a fluid that always coagulates in the stomach that receives it, and (2) Why do the milks of different species coagulate in different ways? An answer will be found in studying the relation between the milk and the particular digestive tract that is destined to receive it. While a certain portion of the protein of all milks coagulates on coming in contact with rennin or rennin and acid, the manner and extent of

the coagulation will stand in a direct relation to the proper evolution of the digestive tract of the animal.

While there are many grades of coagulability in the milks of different animals, we may for practical purposes distinguish three of these grades and consider their significance. The protein may coagulate in a solid, gelatinous or flocculent manner. In the ruminant herbivorous animals, such as the cow, sheep or goat, the protein coagulates in solid, tough masses that can not readily escape from the stomach. In these animals, digestion is always largely gastric and the stomach forms seventy per cent. of the digestive tract. Later on, this stomach will be called upon largely to digest tough, stringy masses of hay and straw and the previous exercise on the tough curds of the milk develops it for this future work.

In the non-ruminant herbivora, such as the mare and ass, the protein coagulates in gelatinous masses that can easily leave the stomach. There is an object in thus passing the curds quickly along, as in this class of animals digestion is largely intestinal, and the intestines form about ninety per cent. of the digestive tract. Later on, grasses and grain must be largely digested in the intestinal portion of the tube, and hence the curd is here also especially adapted to develop a certain part of the intestinal tract for its future work.

In human milk the curd is thrown down in flocculent masses-a form intermediate between the solid and gelatinous types of curd previously noted. While digestion begins in the stomach, it is largely carried on and completed in the intestine, and the stomach forms only about twenty per cent. of the digestive tract. The curd is thus adapted to start the development and motility of the stomach, and finishes by instituting these functions in the bowel which is destined to play a predominant part in digestion. Here again the curd, as far as form is concerned, furnishes, to a certain extent, an analogue and precursor of the future food of the infant. The curd forms small, flocculent masses, and the future food must be separated later into small particles by chewing before digestion can take place to the best advantage.

We have thus seen that the milk of herbivorous animals, whose digestion is principally gastric, forms solid curds that can not easily leave the stomach; that the milk of herbivorous animals whose digestion is principally intestinal, forms gelatinous curds which easily leave the stomach and pass into the intestine; and that woman's milk, which is intended for a digestive system in which gastric digestion is more than that of the horse or ass, but not so great as the cow or goat, curds in flakes that stand between the other two types of curds. Hence it is a law that coagulation of the proteins of milk always takes place in such a way as to most readily adapt the digestive tract for its future work, as this function needs special preparation. It is thus seen that while a

certain amount of protein is present in the milk of all animals and is necessary for tissue building and growth, this protein must not only be coagulable, but must curd in a certain specific way in each species of animal for the proper evolution of their digestive tracts.

In studying the life history of animals it is observed that all commence life in an exceedingly simple form, and for a time their development proceeds along lines so nearly parallel that it is impossible to determine to what species the embryos belong. As development proceeds, a divergence of form and structure is noticeable. At birth this divergence is so great that there is no difficulty in distinguishing species, but the variation in the functions in nutrition at this time is not very great, especially in mammals.

The milks of different mammals at birth can be made interchangeable for many individuals of the young of various species, and, as far as nutritive value is concerned, they are often fairly satisfactory substitutes for each other. But at the end of the natural suckling period of many mammals, no such interchange of food would be possible. To realize what a divergence in the digestive functions has been taking place during the suckling period, imagine an infant, a kitten and a calf all being fed successfully on cow's milk. Here it is evident that at the very beginning of life the difference in their digestive processes is not very great; but wait a year until all three have passed the suckling period. The infant will be just beginning to eat soft food, the kitten will have developed so that it can eat flesh and bones, and the calf will be thriving on grass and hay. In one short year the divergence of their digestive tracts has been so great that the natural food of the calf is then wholly unsuited to the kitten or the infant, yet the chemist will find that the food of all three at this time contains the same basic nutritive elements as it did at birth. An important matter that seems to have been generally overlooked as far as milk is concerned is that this natural fluid is a food for a digestive tract that is rapidly changing its form and function, and the differences in the digestive properties of the milks of various species are for real and specific purposes.

As nutrition is the basis of all physical life, we see how important a function milk performs at the very beginning of existence in developing and preparing the digestive tract of each species so that it can digest and assimilate food that must nourish it in later life. We must thus emphasize the fact that milk through its protein has a developmental as well as a nutritive function to perform.

A directly practical point that can be deduced from this study is the importance of the mothers of every species suckling their own offspring, as they always do except in the highest species-man. The milks of different species are not readily interchangeable because the proteins have functions in helping to develop such radically different digestive

apparatuses. From a nutritional standpoint milks do not differ very markedly, but in developmental quality they are far apart. This forms a very good additional reason why every human mother should, if possible, nurse her own infant. The higher mortality following artificial feeding is thus not the only reason in favor of maternal nursing. In the former case by using the milk of another species-the cow—we put a hard curding milk into a stomach intended and adapted for a soft, flocculent curd. This is not only the cause of much indigestion, but such substitution fails to adequately carry out one of the functions that milk was intended to perform in the scheme of evolution,-namely, in each species to specially develop certain parts of the gastro-intestinal tract that must later perform most of the work of digestion.