A condensed cytoplasmic structure that covers the nucleus of mammalian spermatozoa except for a narrow zone around the insertion of the tail. It shows two distinct regions, a subacrosomal layer and, continuing caudally beyond the acrosomic system, the postacrosomal sheath. The perinuclear theca has been considered a cytoskeletal scaffold responsible for maintaining the overall architecture of the mature sperm head; however, recent studies indicate that the bulk of its constituent proteins are not traditional cytoskeletal proteins but rather a variety of cytosolic proteins.