THE BONES OF THE UPPER EXTREMITIES are sixty-four in number, and are classified as follows: The Scapula, Clavicle, Humerus, Ulna, Radius, Carpus, Metacarpus, and Phalanges. The _Scapula_, or shoulder-blade, is an irregular, thin, triangular bone, situated at the posterior part of the shoulder, and attached to the upper and back part of the chest. The _Clavicle_, or collar-bone, is located at the upper part of the chest, between the sternum and scapula, and connects with both. Its form resembles that of the italic letter _f_, and it prevents the arms from sliding forward. The _Humerus_, the first bone of the arm, is long, cylindrical, and situated between the scapula and fore-arm. The _Ulna_ is nearly parallel with the radius, and situated on the inner side of the fore-arm. It is the longer and larger of the two bones, and in its articulation with the humerus, forms a perfect hinge-joint. The _Radius_, so called from its resemblance to a spoke, is on the outer side of the fore-arm, and articulates with the bones of the wrist, forming a joint. The ulna and radius also articulate with each other at their extremities. The _Carpus_, or wrist, consists of eight bones, arranged in two rows. The _Metacarpus_, or palm of the hand, is composed of five bones situated between the carpus and fingers. The _Phalanges_, fourteen in number, are the bones of the fingers and thumb, the fingers each having three and the thumb two. THE BONES OF THE LOWER EXTREMITIES, sixty in number, are classed as follows: The Femur, Patella, Tibia, Fibula, Tarsus, Metatarsus, and Phalanges. The _Femur_, or thigh-bone, is the longest bone in the body. It has a large round head, which is received into the acetabulum, thus affording a good illustration of a ball and socket joint. The _Patella,_ or knee-pan, is the most complicated articulation of the body. It is of a round form, connects with the tibia by means of a strong ligament, and serves to protect the front of the joint, and to increase the leverage of the muscles attached to it, by causing them to act at a greater angle. The _Tibia_, or shin bone, is enlarged at each extremity and articulates with the femur above and the astragalus, the upper bone of the tarsus, below. The _Fibula_, the small bone of the leg, is situated on the outer side of the tibia, and is firmly bound to it at each extremity. The _Tarsus_, or instep, is composed of seven bones, and corresponds to the carpus of the upper extremities. The _Metatarsus_, the middle of the foot, bears a dose resemblance to the metacarpus, and consists of five bones situated between the tarsus and the phalanges. The tarsal and the metatarsal bones are so united as to give an arched appearance to the foot, thus imparting elasticity. The _Phalanges_, the toes, consist of fourteen bones, arranged in a manner similar to that of the fingers.