What is the primary feature of irregular bones?

The primary feature of irregular bones is that they possess an irregular shape. This characteristic is what distinguishes them from other categories of bones, such as long, short, or flat bones. Irregular bones, which include structures like the vertebrae and certain bones of the face, are characterized by their complex and varied shapes that do not fit into the standard classifications of bone. This irregularity allows them to play specialized roles in the body, such as providing support and protecting the spinal cord in the case of vertebrae.

The other choices describe features associated with different types of bones or structures. For example, the option regarding air spaces pertains more to the composition of certain bones rather than their shape. The shaft with two ends is a defining characteristic of long bones, while the description of being tiny and located along tendons relates to sesamoid bones, which have distinct functions and anatomical features. Thus, the defining factor for irregular bones is their unique and complex shapes.