a. =Structural Changes.=--The external genitals, besides showing the pudendal pilosity referred to above, are all greatly increased in size. The penis is increased in all of its dimensions, the testes become very much increased in size, the scrotum, probably because of the increased weight of the testes, is also lengthened. b. =Functional Changes.=--The testes and associated glandular bodies gradually develop the power of forming perfect semen, capable of fertilizing the human ovum. When these organs thus become capable of procreation, the period of puberty is complete. In this connection it is important to note that the development of the testes produces a profound effect upon both the physical and mental characteristics of the young man. This effect is produced through a substance formed in the testes and reabsorbed into the body, thus gaining access to the blood where it exerts its mysterious but profound influence. Just how this affects the mind and body will be discussed in detail in a subsequent chapter. 2. PSYCHICAL CHANGES. Play and Work. a. =Sports.=--Most of the higher animals, particularly man, and all races of men, devote a large part of the energies of the adolescent period to sports or games in which individuals contend with each other or teams of individuals contend with opposing teams in games that bring into play the various powers of the neuro-muscular system: such as alertness of all the senses, readiness and correctness of judgment, agility, speed and strength of movement. Sports might be criticised by some because they represent non-productive expenditure of energy. On the other hand, no energy ever expended by man is so highly productive of so precious a material as results from manly athletic sports. The products of these games are the substances consumed by them, paradoxical as that may at first appear. The use of brain, muscle and glands and the consumption of the cell substances of these tissues results in the development of the nerve, muscle and gland cells into a condition larger, better equipped and more responsive than before such use. Thus, athletic sports, while they make draughts upon the nerves, muscles and glands, develop all of these tissues to a high degree of efficiency. The plan of nature in this instinctive indulgence in sports must be evident. Nature is educating and developing the male animal (man) to the highest possible degree of efficiency, so that sports, instead of being non-productive, lead to the development of structures possessing a high degree of value, not only to the individual, but also to society. Furthermore, those qualities of mind that are encouraged on the athletic field between contestants in a game are the qualities that in the later serious struggles of life make most for success.