Q.Name the two principal divisions of the human skeleton, stating the number of bones in each.

Pneumatic bones are lightweight, hollow bones containing air spaces connected to the respiratory air-sac system, an adaptation found in birds (class Aves) that helps reduce body weight for flight.

Many bones of birds — such as the humerus, femur, and some skull and vertebral bones — are pneumatic, meaning they are hollow rather than filled with marrow, with the internal cavity often connected to the bird's air-sac respiratory system. Despite being hollow, these bones are reinforced internally with thin, criss-crossing bony struts (a trabecular network) that keeps them strong even though they are light. This combination of reduced weight with retai …