282 PHYSIOLOGY CHAP. 



These figures show that a kilogramme of living animal matter 

 consumes much the same amount of energy after the period of 

 growth, and since this is also the case during the period of growth, we 

 may fairly conclude that in the higher animals the total amount 

 of energy consumed is as nearly as possible the same throughout 

 life. Man alone stands above the other animals examined in the 

 capacity shown by his living substance for developing a quantity 

 of energy which is relatively much greater. The fact that man 

 grows more slowly than other mammals, lives longer and consumes 

 a greater amount of energy implies that his metabolism proceeds 

 in a different way from that of other animals, probably owing to 

 the higher cerebral development attained by him. 



Growth stamps the organism with a certain number of physio- 

 logical characteristics, especially with regard to the tissues and 

 organs. 



At birth the child has all its organs, some of which, the thymus, 

 for example, are fated to disappear, but they are imperfect in size, 

 shape, function, and structure. They attain the adult state by 

 means of a series of histological changes, such as hyperplasia or 

 cellular proliferation, hypertrophy or increase in cellular volume, 

 and differentiation, which transform the initial homogeneous 

 elements into cells highly specialised in the morphological, physico- 

 chemical, and physiological sense. 



The epithelial and lymphoid tissues always retain so Bizzozero 

 asserts the power of increasing and renewing themselves by 

 hyperplasia ; the connective, cartilaginous, and bony tissues, the 

 smooth muscular fibres, the tissue of the liver, pancreas, kidneys, 

 and salivary glands grow by hyperplasia during foetal life and the 

 first period of extra-uterine life, and by hypertrophy during the 

 later periods ; the striated muscular and the nervous tissues grow 

 by hyperplasia only at the beginning of embryonic life and lose 

 this power before birth ; the muscular fibres and nerve-cells, the 

 true permanent structures, increase during extra-uterine life by 

 hypertrophy only, and may undergo differentiation. 



One important fact which the study of development brings out 

 is that the growth in the different parts of the organism does not 

 proceed on parallel lines ; in the same individual the organs and 

 systems do not reach the adult stage at the same time. 



In order to follow the progress of construction as a whole in 

 the living individual, we must have recourse to the criteria 

 afforded by weight and height and in recent years by the radio- 

 graphic study of the bones. 



From birth to early maturity the human body grows to about 

 three and a half times its original height, while its weight increases 

 about twenty-fold. The height of a child increases by half as 

 much again during the first year of life ; it measures about 50 cms. 

 at birth and 70 to 80 at the end of the first year ; at six years old 



