CHAP, in.] THE PHASES OF LIFE. 1147 



frequency of the heart-beat much greater, viz. about 130 or 140 

 per minute, falling to about 110 in the second year, and about 

 90 in the tenth year. Corresponding to the smaller bulk of the 

 body, the whole circuit of the blood system is traversed in a 

 shorter time than in the adult (12 seconds as against 22) ; and 

 consequently the renewal of the blood in the tissues is ex- 

 ceedingly rapid. Relatively to the body-weight there is also 

 considerably more blood in the babe than in the adult. The 

 respiration of the babe is quicker than that of the adult, being 

 at first about 35 per minute, falling to 28 in the second year, to 

 26 in the fifth year, and so onwards. The respiratory work, 

 while it increases absolutely as the body grows, is, relatively to 

 the body-weight, greatest in the earlier years. It is worthy of 

 notice, that the absorption of oxygen is said to be during these 

 earlier years relatively more active than the production of car- 

 bonic acid ; that is to say, there is a continued accumulation of 

 capital in the form of a store of oxygen-holding explosive com- 

 pounds (cf. 289). This, indeed, is the strikimg feature of 

 infant metabolism. It is a metabolism directed largely to con- 

 structive ends. The food taken represents, undoubtedly, so 

 much potential energy ; but before that energy can assume a 

 vital mode, the food must be converted into tissue ; and, in such 

 a conversion, morphological and molecular, a large amount of 

 energy must be expended. The metabolic activities of the 

 infant are more pronounced than those of the adult, for the 

 sake, not so much of energies which are spent on the world 

 without, as of energies which are for a while buried in the 

 rapidly increasing mass of flesh. Thus the infant requires over 

 and above the wants of the man, not only an income of energy 

 corresponding to the energy of the flesh actually laid on, but 

 also an income corresponding to the energy used up in making 

 that living sculptured flesh out of the dead amorphous proteids, 

 fats, carbohydrates and salts, which serve as food. Over and 

 above this, the infant needs a more rapid metabolism to keep up 

 the normal bodily temperature. This, which is no less, indeed 

 slightly ('30) higher, than that of the adult, requires a greater 

 expenditure, inasmuch as the infant with its relatively far larger 

 surface, and its extremely vascular skin, loses heat to a propor- 

 tionately much greater degree than does the grown-up man. It 

 is a matter of common experience that children are more affected 

 by cold than are adults. The bodily temperature is moreover 

 less stable in the infant than in the adult, and departures from 

 the normal temperature have not the grave significance they 

 have in the adult. 



This rapid metabolism is hoAvever not manifest immediate^ 

 upon birth. During the first few days, corresponding to the 

 loss of weight mentioned above, the respiratory activities of the 

 tissues are feeble; the embryonic habits seem as vet not to have 



