Final answer:
Adjectives are not considered a main term by the textbook. Anatomists use directional anatomical terms to describe the locations of different body structures (Option d).
Step-by-step explanation:
The main term that is not considered by the textbook is adjectives. The textbook includes anatomic sites, diseases, and nouns as main terms. Adjectives are not typically used in anatomical descriptions. Instead, anatomists use directional anatomical terms to describe the locations of different body structures. Hence, d is the correct option.