The bones (ossa) form the passive elements of the locomotor system. The bones that make up the movement system of the body form a bony roof by connecting with each other as mobile and immobile in a certain order. This roof is called skeleton (sceleton). The skeleton or this bony framework forms the distinctive external appearance of mammals. Bones are hard and durable structures and prepare the ground for the attachment of muscles. It can move with the contraction of the muscles adhering to them. The bones that form the skeleton are histologically composed of bone tissue. Bone tissue is made of organic and inorganic substances. Organic substances comprise on average 1/3 of a bone and inorganic substances the remaining 2/3. Bones are divided into three groups according to their shape: long, short and flat. There are also irregular bones, sesame bones, organ bones and air bones, which are considered as a separate group.

The skeleton is divided into two main parts: the skeleton axiale, which includes the head, spine and thorax, and the skeleton appendiculare, which consists of the bones of the extremities. The bones of the skeleton articulate with each other in three different ways: movable, semi-movable or non-movable and can move in various directions.