脊 is a bound morpheme - it does not stand alone in modern Chinese. Common compounds: 脊椎 (spine), 脊背 (back), 脊梁 (backbone), 屋脊 (roof ridge), 山脊 (mountain ridge).
Bottom is the meat radical, the body-tissue form of 肉 (not the moon 月 it looks identical to). It signals that 脊 names a body part, the spine or backbone. Same flesh radical groups 脊 with 肝 liver, 脑 brain, 腰 waist, 腿 leg, 背 back.
Top portion is a doubled set of vertebra marks arranged around a central spine - two human-like figures in mirror symmetry, picturing the stacked bones of the backbone. The whole upper graphic is one fused semantic unit, read here as a vertebra ideograph anchored on 人.
