The writer is using similes to describe the interaction between Kino and his wife Juana in this scene. Kino, consumed by rage, has just struck and kicked Juana, and as she lies on the sand, he makes a threatening noise at her that is like the "hissing" of a snake. He is actually like a snake at this moment, deadly. Juana, on her part, does not fight back. She stares at her husband as he stands over her "with wide unfrightened eyes, like a sheep before the butcher". Having done her part to try and rid them of the evil she sees threatening them, she is completely submissive, and will accept whatever punishment Kino gives her without protest (Chapter 5).
Juana recognizes that the pearl that Kino has found will only bring them trouble in their lives. She had told him earlier, "this pearl is evil. Let us destroy it before it destroys us" (Chapter 4). Kino, however, is determined to secure the riches he envisions by selling the pearl, and feels that he is up to facing whatever obstacles might come in his way. Already possessed by the sins of pride and greed, his eyes glow "fiercely and his muscles (harden)...his will harden(s)", and he declares, "no one shall take our good fortune from us". His face grows "crafty" as he insists, "I am a man", believing that that fact in itself will be enough to protect him from what is to come.
Juana, in her wisdom, is still afraid. She knows that being "a man" may not be enough; Kino is not indestructible, and might well be killed. Juana makes one attempt to to save them from the forces she believes will be their undoing; in the dead of night, she takes the pearl and attempts to throw it back into the sea. Kino stops her however, and she submits meekly to the punishment he metes out. The pearl is already exerting its evil influence. Kino in his rage becomes like a beast, threatening and deadly like a snake, while Juana awaits like a sacrificial lamb that he would destroy in order to achieve material wealth (Chapter 5).