Juana and Kino are too poor to afford a doctor so they use an old-fashioned natural remedy to treat Coyotito's wound. Juana makes a poultice out of seaweed, which she then applies to her son's swollen shoulder. The treatment seems to work and the swelling goes down immediately after Kino discovers the valuable pearl. It's almost as if the pearl has brought Kino and his family good luck, at least for now.
The use of an old natural remedy to treat Coyotito's wound illustrates the significant cultural gap that separates Kino's family from the modern world. Natural remedies are an important part of indigenous culture, and Kino and Juana want to maintain the old traditions for as long as possible. Although they are prepared to resort to modern medicine if needs be, they're forced by the doctor's indifference to stick to what's tried and trusted. Besides, the doctor proves himself to be a devious, trustworthy individual, who as well as trying to cheat Kino out of his valuable pearl, cynically prolongs Coyotito's suffering for his own selfish gain.