The first, though incomplete. data were furnished by liischof and Fehling. Within recent years the writer and Soldner have analyzed the bodies of six normal newborn infants, according to a method pro posed by Camerer (the elder). Each body was frozen and then ground up. The whole material was extracted with alcohol and ether, and the resulting substance Toor in fat and water) was pulverized. The indi viduai analyses agreed very well with each other, and the following average values for one infant were obtained: In the two figures following the values which Volkmann found for the adult are added for comparison: The body of the newborn is relatively richer in water and fat (the latter constitutes nearly one-half of the dry residue) but poorer in nitrog enous material, and particularly in ash, than is that of the adult. The relative small amount of muscle and bone in the newborn will explain this fact. According to Vierordt, the muscles of the newborn represent 23 per cent. of the total body weight, while those of the adult represent 43 per cent. The skeleton represents in both cases 16 per cent., but the composition of the latter differs very considerably in the infant and in the adult. Neither the sex nor the absolute body weight of the infant has any influence on their composition: nevertheless, it appears probable that heavy newborn infants of well-to-do mothers are richer in fat.
Seeing how much the composition of the newborn differs from that of the adult, a comparison with that of the foetus at different ages is of great interest. On the basis of his investigations, Fehling gives the following data The most striking feature is the large proportion of water; the younger the foetus, the larger the percentage.
The conditions after birth are similar. As previously shown, the adult is relatively poorer in water and richer in ash than is the infant, and it may be assumed that in the aged the body contains more ash and less water than does that of the adult, although there are no exact investigations extant. From the early ftetal period until old age the relative water content of the body decreases, while its ash content increases. This process of drying ornt lielps to explain ?vhy the metab olism is more active in the early rota] period than later. since the higher the percentage of water, the easier are the processes of metab olism carried on.
The investigations of Cliacoso, de Lange, Michel, iihiner, and Hugounenq teach us the composition of the ash of the foetus and of the newborn.
The knowledge of the ash content of the newborn enables us to test the validity of Bunge's theory. lie found that in numerous fami lies of mammals, the percentage composition of the ash of the newborn and of the mother's milk was about the same, with the exception of iron, while the ash of the blood differed very much, as the following table shows: From these figures, Bunge concluded that the epithelial cells of the mammary glands collect selectively all the inorganic materials from the blood in exactly those proportions which the offspring requires in order to grow and to become equal to the organism of its parents, and that therefore the proportions of the inorganic substances in the total organism of the suckling are nearly identical with those in the mother's milk. He transferred his conclusions to the human species.
Siildner's analyses of the ash of the newborn and of human milk are given in Table 5. and a comparison of the figures shows that this hypothesis is not correct. In a more recent paper Bunge corrected his opinion. lie claims that the ash of the newborn of different species of mammals seems to have nearly the same composition. But the slower the suckling grows, the more the ash of the milk differs from that of the offspring. The milk ash becomes richer in alkalies combined with chlorine and relatively poorer in phosphates and sodium chloride. Runge ex plains this on teleological grounds. Ile claims that the ash of the milk has to serve a double purpose. It has to furnish material for the con struction of the tissues, and also for the excretions, particularly the urine. The more rapid the growth of the suckling, the more the first purpose preponderates; the slower the growth, the more the second. The per centage figures cited by Bunge seem to bear out this opinion.
The proportion of the individual constituents of the ash of the milk is one of the numerous factors which determine these processes of metab olism. But in reality the absolute amounts of the ash introduced must he taken into consideration (see Table 13, p. 3S2). A comparison of the ash constituents of the milk and of the body is not sufficient in the present state of our knowledge to explain satisfactorily the conditions of the inorganic metabolism. It may readily he that the combination of the metals and of the other elements with organic molecules, and their presence ill the form of inorganic compounds or ions in the milk differs very much from their arrangement in the body substance.
The digestive organs of the infant differ in many points from those of the adult. The peculiarities that are of most practical importance will be briefly considered.
The development of the salivary glands begins during the second month of the foetal period. This secretion has the power to decompose starch in the first days of life. The quantity of the secretion is small because chewing. the most potent physiological stimulus, is wanting. Therefore only little starch can be decomposed hut the secretion be comes more abundant when the teeth make their appearance with the gradual transition from liquid to solid food. The reaction in the care fully cleansed cavity of the mouth is neutral or weakly alkaline. The acid reaction which is frequently observed, is due to decomposed par ticles of food.