Two criteria that would identify the three muscle tissues are myofibers and nuclei.
There are three types of muscles:
1. Skeletal-Contains many nuclei along the length of the muscle cell. The nuclei are displaced peripherally. The skeletal fibers are arranged parallel. The myofibril is composed of bands.
2. Smooth-This tissue contain spindle-shaped cells with a single central nucleus. Smooth muscle fibers are shorter than skeletal.
3. Cardiac-The fibers of cardiac muscle are not simply parallel. Cardiac muscle fibers are long cylindrical cells with one or two nuclei.
I believe it is the "myelin sheath." White matter = myelinated (has myelin sheath), while gray matter = unmyelinated (no myelin sheath)!
