858 EMBRYOLOGY 



the right-handed and by left muscles in the left-handed. Still, while 

 this may satisfactorily explain dextral preeminence, it does not explain 

 the preeminence of the left side of the brain. 



DEVELOPMENT AFTER BIRTH AGES AND DEATH 



When the infant is born, the organs of special sense and the intelli- 

 gence are dull ; there is then very little muscular power ; and the new 

 being for several weeks does little more than eat and sleep. The natural 

 food at this time is the milk of the mother ; and the digestive secretions 

 do not for some time possess the varied solvent properties that are found 

 in the adult, though observations on the secretions of the infant are few 

 and rather unsatisfactory. The full activity of pulmonary respiration is 

 gradually and slowly established. Young animals appropriate a compara- 

 tively small quantity of oxygen, and just after birth they present a much 

 greater power of resistance to asphyxia than the adult. The power of 

 maintaining the animal temperature also is much less in the newly-born. 

 The processes of ossification, development of the teeth etc., have already 

 been described. The hairs are shed and replaced by a new growth a 

 short time after birth. 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 transition takes place from 

 infancy to childhood. The child is then 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 to express ideas in language. The child gradually develops, 

 and the milk-teeth are replaced by the permanent teeth. At puberty, 

 which begins between the fourteenth and the seventeenth years a 

 little earlier in the female the development of the generative organs 

 is attended with important physical and moral changes. 



The different ages recognized by physiologists are the following : 

 Infancy, from birth to the age of two years ; childhood, to the time of 

 puberty ; adolescence, 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 maxi- 

 mum of intellectual and physical development at about the age of thirty- 

 five, and he begins to decline after the sixtieth year, although as regards 

 intellectual vigor, this has many exceptions. 



As regards nutrition, it may be stated in general terms that the appro- 

 priation of new matter (anabolism) is a little superior to katabolism, up 

 to about .the age of twenty-five years ; between twenty-five and forty-five, 



