This LM image (HA18) shows two peripheral nerve bundles in longitudinal section surrounded by connective tissue. Peripheral nerves are identified using both positive and negative criteria. They do not have lumens, so they are not blood or lymphatic vessels or epithelium. They are very cellular and have an outer layer of perineurium, so they are not connective tissue or smooth muscle. They do not have striations so they are not skeletal muscle. So, by a process of elimination, they are nerve because they are not epithelium, connective tissue or muscle. The positive criteria are the nuclei of Schwann cells and the perineurium.