After initial injection, pigment is dispersed throughout a damaged layer down through the epidermis and upper dermis, in both of which the presence of foreign material activates the immune system's phagocytes to engulf the pigment particles. [wikipedia]
The buttocks are formed by the masses of the gluteal muscles or 'glutes' (the gluteus maximus muscle and the gluteus medius muscle) superimposed by a layer of fat.
