_The Ulna._—A long thin bone, but larger than the radius, and situated on the inside of the fore arm. _The Radius._—So-called because it is the rotary bone of the fore arm. It is situated on the outside of the fore arm and parallel with the ulna. _The hand_ is subdivided into the wrist or _carpus_ bones, the _metacarpus_ or the bones of the palm, and the _phalanges_ or the bones of the digits. There are twenty-seven bones in each hand. =The Bones of the Lower Extremities.=—The bones of the lower extremities consist of _the pelvic girdle_, _the thigh_, _the leg_ and _the foot_. _The pelvic girdle_ consists of three portions, the _ilium_, the _pubis_, and the _ischium_. _The Ilium._—The ilium is the superior, broad and expanded portion and forms the prominence of the hip. The top part is called _the crest_. _The Ischium._—The ischium is the lowest portion of the girdle, and is the portion which supports the body when in a sitting position. _The Pubis._—This bone forms the front of the pelvis, and supports the external organs of generation. _The thigh_ is that portion of the lower extremity which is situated between the pelvis and the knee. It consists of a single bone called the femur. _The Femur._—The femur is the largest, longest and strongest bone in the skeleton. It is almost perfectly cylindrical. It extends from the hip to the knee. The bones of the leg are three in number and are as follows: patella, tibia, and fibula. _The Patella._—This bone is often called the knee cap or the knee pan. It is a flat triangular bone, situated at the anterior part of the knee joint. _The Tibia._—The tibia is situated at the front and inner side of the leg, and is next to the femur in strength and size. It is sometimes called the shin bone. _The Fibula._—The fibula is sometimes called the calf bone. It is situated at the outer side of the leg, and is a quite slender bone. The foot is divided into the _tarsus_, _metatarsus_, and the _phalanges_. There are seven tarsus bones, five metatarsus bones, and fourteen phalanges bones, making a total of twenty-six bones for each foot. CHAPTER VI. ORGANOLOGY. The body itself is divided into the upper and the lower extremities and the trunk. The upper extremities consist of the head and arms. The lower extremities consist of the legs. The trunk is that part of the body remaining after the head, arms, and legs have been severed from the body. =The Cavities.=—The body has three principal cavities: namely, the cerebro-spinal, the thoracic, and the abdominal. =The Cerebro-Spinal Cavity.=—The cerebro-spinal cavity is formed by the cranial bones, and the vertebral column. The cerebro-spinal cavity is divided into the sub-cavities, called the cranial cavity and the spinal cavity.