It turns out that many materials can be solid at a higher temperature if the pressure is also higher. so even though it is hotter in the inner core the pressure in the core is also higher and you can have a solid iron nickel instead of liquid
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It is called the Rugae.
Rugae is a term used in anatomy to refer to a series of ridges produced by folding of the wall of an organ. For example the gastric rugae which allows for expansion of the stomach after the consumption of foods and drinks. The rugae of the vagina are the transverse ridges on the mucous membrane lining the vagina. They allow the vagina to stretch during childbirth.