Who had a heart attack?
Atticus Finch's wife died of a heart attack when Jem and Scout were small children. In fact, Scout who was only two years old does not remember her mother. On the other hand, Jem, who is four years older than Scout retains clear memories of his mother, and frequently misses her. Sometimes as the children play a game, Jem sighs and goes off to play by himself, but Scout "knows better" than to bother him when he is in this brown study.
The death of their mother explains the intimacy that the children have with their father as well as the friendship that they share as they must be both children and companions for their father. It also explains the stong protective instincts of Atticus and those that emerge in the children when their father is threatened by the mob at the jail and the group of men who appear in the front lawn.
Jem's sensitivity and poignant reaction to the verdict of the trial are reflective of his lack of security in a world that does not play by the rules. For, it is the mother of families who instills confidence into her children.