The _Spinal Column_ or backbone, when viewed from the front presents a perpendicular appearance, but a side view shows four distinct curves. The bones composing it are called _vertebræ_. The body part of a vertebra is light and spongy in texture, having seven projections called _processes_, four of which are the _articular_ processes, which furnish surfaces to join the different vertebræ of the spinal column. Two are called _transverse_, and the remaining one is termed the _spinous_. The transverse and spinous processes serve for the attachment of the muscles belonging to the back. All these processes are more compact than the body of the vertebra, and, when naturally connected, are so arranged as to form a tube which contains the _medulla spinalis_, or spinal cord. Between the vertebræ is a highly-elastic, cartilaginous and cushion-like substance, which freely admits of motion, and allows the spine to bend as occasion requires. The natural curvatures of the spinal column diminish the shock produced by falling, running or leaping, which would otherwise be more directly transmitted to the brain. The ribs at the sides, the sternum in front, and the twelve dorsal bones of the spinal column behind, bound the thoracic cavity, which contains the lungs, heart, and large blood-vessels. [Illustration: Fig. 15. A representation of the pelvic bones. _e_. The lumbo-sacral joint. 2. The sacrum. _3_. Coccyx. _1,1_. The innominata. _4,4_. Acetabula.] The _Pelvis_ is an open bony structure, consisting of the Os Innominata, one on either side, and the Sacrum and Coccyx behind. The _Sacrum_, during childhood, consists of five bones, which in later years unite to form one bone. It is light and spongy in texture, and the upper surface articulates with the lowest vertebra, while it is united at its inferior margin to the coccyx. The _Coccyx_ is the terminal bone of the spinal column. In infancy it is cartilaginous and composed of several pieces, but in the adult these unite and form one bone. The _Innominata_, or nameless bones, during youth, consist of three separate pieces on each side; but as age advances they coalesce and form one bone. A deep socket, called the _acetabulum_, is found near their junction, which serves for the reception of the head of the thigh-bone. [Illustration: Fig. 16. 1. Portions of the backbone. 2. Cranial bones. _4_. Breast-bone. _5_. Ribs. _7_. Collar-bone. _8_. Arm-bone (humerus). _9_. Shoulder-joint. _10, 11_. Bones of the fore-arm (ulna and radius). _12_. Elbow-joint. _13_. Wrist-joint. _14_. Bones of the hand. _15, 16_. Pelvic bones. _17_. Hip-joint. _18_. Femur. _19, 20_. Bones of the knee-joint. _21, 22_. Fibula and tibia. _23_. Ankle bone. _24_. Bones of the foot.]