664 ANIMAL PHYSIOLOGY. 
in stature continues up to about the twenty-fifth year, though 
the increase is most rapid during infancy and puberty, when, 
in fact, the weight is also greatly augmented. 
Digestive System.— While it is now established that all of the 
digestive secreting mechanisms are active at or shortly after 
birth, it must be borne in mind that these, like the other organs, 
adapt gradually to the new conditions, This is a matter of 
practical importance in infant feeding. Thus, while it is true 
that the young infant’s saliva will act on starch, it is not to be 
supposed that its amylolytic powers are equal to those of the 
adult. 
Circulatory and Respiratory Systems.— The babe’s heart is 
larger than that of the adult relatively to its body-weight, and 
its action more rapid; hence the circulation is accomplished in 
a shorter space of time, an advantage when it is considered that 
the need for oxygen and tissue-food in the young organism is 
so great. 
The respirations are correspondingly rapid, and the actual 
amount of the respiratory interchanges is greater than in adult 
life. There appears, however, to be a storing up of oxygen— 
i. e., all of the oxygen used up does not shortly appear again as 
carbonic anhydride. 
The metabolism of the infant is very active, and is spent 
largely in construction; growth is in excess of waste; indeed, 
this feature is characteristic of the metabolism of all young 
animals. There is, in consequence of the excessive loss of heat, 
from a relatively larger surface than in the adult, the need for 
amore active metabolism; the young animal must eat more, 
to meet this waste. It is, moreover, in consequence of this fact 
that infants, when not protected better than adults, perish from 
a fall in the temperature, which their sensitive organizations 
can not endure. 
Immediately after birth the adaptation to the new environ- 
ment is less perfect than at a later period; respiration is feeble; 
the blood is imperfectly aérated ; the temperature is lower; the 
entire metabolism goes on but feebly: hence it is that newly 
born kittens, puppies, etc., can be immersed in water for a con- 
siderable period (twenty to thirty minutes) without drowning. 
The tissues do not demand much oxygen; they live on what 
they already have stored up, after that in the blood is ex- 
hausted—in a word, they behave much as they did during intra- 
uterine life. The excretions, as would be supposed from the 
rapid metabolism, are more abundant than in the adult. There 
