what term refers to the visible part of the tooth
Dental anatomy The term "crown" of a tooth can be used in two ways. The term "anatomic crown" of a tooth refers to the area above the cementoenamel junction (CEJ) or "neck" of the tooth.[9] It is completely covered in enamel. The term "clinical crown" often is convenient in referring to any part of the tooth visible in the mouth, but as a rule the unqualified term "crown" refers to the anatomic crown. The bulk of the crown is composed of dentin, with the pulp chamber within. The crown is enclosed within bone before the tooth erupts, but after eruption the crown is almost always visible in an anatomically normal and clinically healthy mouth.
Optic nerve Based on this anatomy, the optic nerve may be divided in the four parts as indicated in the image at the top of this section (this view is from above as if you were looking into the orbit after the top of the skull had been removed): 1. the optic head (which is where it begins in the eyeball (globe) with fibers from the retina; 2. orbital part (which is the part within the orbit). 3. intracanicular part (which is the part within a bony canal known as the optic canal); and, 4. cranial part (the part within the cranial cavity, which ends at the optic chiasm).[2]