why borders convert to transparent from inside

when i convert the solid mode to texture that’s being visible and transparent

the solid view

the texture view


Depending on what’s chosen on N-tab:
Shading - Multitexture or GLSL -> Backface Culling
and on Object Data tab (triangle icon):
Normals ->Double Sided checkbox,
faces normals of which point away from screen are shown either transparent or opaque.