290 PHYSIOLOGY CHAP. 



products is in proportion to the development of the muscles, the 

 solidification of the osseous system, the increased activity of the 

 exchange of material and energy, and the perfected thermal 

 economy. 



To these general characteristics of the youthful organism must 

 be added the important modifications brought about by the well- 

 known secondary sexual characteristics of the male and female. 

 The features become more marked and reflect the thought and 

 affections ; the sensations in youth acquire the variety and delicacy 

 of which the organs of sense are capable ; youth is the age of 

 sharp and easy perception, reliable memory and vivid imagination. 

 During this period the power of concentration is perfected, the 

 aesthetic taste and the ethical sentiment of social life are formed. 

 The youth, conscious of his powers, both present and to come, is 

 apt at times to be rash and thoughtless ; his lack of experience 

 frequently leads him into difficulties, so that we see the truth of 

 the French saying : " si jeunesse savait, si vieillesse pouvait." 



Youth merges into maturity (termed manhood in man), but 

 the transition from the first to the second stage is gradual and 

 not characterised by any special functional or morphological 

 change. Women are, however, commonly supposed to attain 

 maturity at the age of twenty-one, though the law looks upon 

 them as marriageable from the age of puberty. Men, on the 

 other hand, attain their full strength and will-power at the age of 

 twenty-five. It is during this period that the man usually founds 

 a family and the woman enters upon her maternal functions, 

 with all the attendant local and general phenomena which we 

 have discussed in the preceding chapter. As we have already 

 seen, fecundity in women lasts till about fifty years of age, and 

 in men till sixty and later, there being no absolute limits in this 

 respect. The cessation of growth (Quetelet), the full development 

 of the whole organism, together with its physical and moral 

 capacities, rendering man sufficient unto himself and free to 

 do as he chooses, the final constitution of his temperament, that 

 is to say, of the relation between his physical and moral nature, 

 fitness for generation and the higher development of the psychic 

 functions, are one and all characteristics of virility. The human 

 parabola has reached the culminating point depicted by Michel- 

 angelo in " The Dream of Life." 



The drawing of the perfect adult man and the measurements 

 for the proportionate development of the different parts of the 

 body (given by Leonardo da Vinci) for the assistance of painters 

 and sculptors may also be of interest to my readers (Fig. 122). 



The destiny of the individual and his functions in relation 

 to the society to which he belongs are for the most part decided 

 during this relatively permanent period of equilibrium in body 

 and virile energy. 



