Or is it because, as a result of cooling, the heat collects in the interior region of the body, and the flesh, as the heat leaves it, contracts more and more, and, as it draws together, the hair grows more upright, just as when one fixes a twig or some other object in the ground and fills the space round it and collects the soil on every side, it is more likely to remain erect than if one leaves the soil loose round it?