DEVELOPMENT AFTER BIRTH, ETC. 459 



eat and sleep. The natural food at this time is the milk of 

 the mother, and the digestive fluids do not, for some time, 

 possess the varied solvent properties that we find in the adult, 

 though observations upon the secretions of the infant are few 

 and rather unsatisfactory. The full activity of pulmonary 

 respiration is gradually and slowly established. Young ani- 

 mals appropriate a comparatively small quantity of oxygen, 

 and, just after birth, present a much greater power of resist- 

 ance to asphyxia than the adult. 1 The power of maintain- 

 ing the animal temperature is also much less in the newly- 

 born. 2 The process of ossification, development of the teeth, 

 etc., have already been considered. 3 The hairs are shed and 

 replaced by a new growth a short time after birth. 4 The 

 fontanelles gradually diminish in size after birth, and are 

 completely closed at the age of about four years. 



The period of life which dates from birth to the age of two 

 years is called infancy. At the age of two years, the transi- 

 tion takes place from infancy to childhood. The child is now 

 able to walk without assistance, the food is more varied, and 

 the digestive operations are more complex. The special 

 senses and the intelligence become more acute, and the being 

 begins to learn how to express ideas in language. The child 

 gradually develops, and the milk-teeth are replaced by the 

 permanent teeth. At puberty, which begins at from the 

 fourteenth to the seventeenth year, a little earlier in the fe- 

 male, th$ development of the generative organs is attended 

 with important physical and moral changes. 



The different ages recognized by the older writers were 

 as follows : Infancy, from birth to the age of five years ; ado- 

 lescence, or youth, to the twenty-fifth year ; adult age, to the 

 thirty-fifth year ; middle life, to the fiftieth year ; old age, to 

 the sixtieth year ; and then, extreme old age. A man may be 

 regarded at his maximum of intellectual and physical devel- 



1 See vol. i., Respiration, pp. 420, 490. 



8 See vol. Hi., Animal Heat, p. 405. 8 See pages 393, 416. 



4 KOLLIKER, Elements d'histologie humaine, Paris, 1868, p. 180. 



