The slush in the sac? Or it's butt(or "cork", if you will), icicles? The sac most likely insulates the slushy mix, so as to further prevent drastic changes in temperature.
<awaychuck> alk your mom is so stupid that she thought the chain rule was used by antebellum mathematicians in the South
On Patrat: <Dracoyoshi8> i like to imagine its cheeks are full of semen