At the start of the story, when the baby Coyotito is stung by a scorpion, his mother Juana instantly resolves to save him no matter what it takes.
Juana's strength of character has always surprised her husband Kino:
Kino had often wondered at the iron in his patient, fragile wife. She, who was obedient and respectful and cheerful and patient, could bear physical pain with hardly a cry. She could stand fatigue and hunger almost better than Kino himself. In the canoe, she was like a strong man. (chapter 1)
Juana, then, may appear quiet, dutiful and even submissive and 'fragile' on the surface, yet she possesses great reserves of strength, both physical and mental. This is the 'iron' in her. When her baby is threatened she instantly determines to call the doctor from town, irrespective of the fact that he never bothers to come into the heart of the Indian community which he despises. But Juana will not be baulked, and although she accepts that the doctor will not come to them, she straightaway goes to call on him herself. She simply does not hesitate, and Kino has to follow her.
This sense of Juana's strength is reinforced by the image of her as a 'lioness' (chapter 1). To save her child, she is prepared to do anything, face anything.