Development of the human body used to approximately judge a child's age based on physical development. Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testes in a boy. In response to the signals, the gonads produce hormones that stimulate libido and the growth, function, and transformation of the brain, bones, muscle, blood, skin, hair, breasts, and sex organs. Physical growth—height and weight—accelerates in the first half of puberty and is completed when an adult
Passions of the Soul in reality the actions of the body on the soul (art. 25). The soul suffers the influence of the body and is entirely subject to the influence of the passions. In the manner by which Descartes explains the human body, the animal spirits stimulate the pineal gland and cause many troubles (or strong emotions) in the soul. The passions attack the soul and force the body to commit inappropriate actions. It was therefore necessary for Descartes to study in the second part of his treatise the particular effects of each separate passion and its manners of manifestation. The study of