The Trunk has fifty-four bones, which are as follows: The Os Hyoides, the Sternum, twenty-four Ribs, twenty-four vertebræ or bones of the Spinal Column, the Sacrum, the Coccyx, and two Ossa Innominata. The Os Hyoides, situated at the base of the tongue, is the most isolated bone of the skeleton, and serves for the attachment of muscles. The Sternum, or breast-bone, in a child is composed of six pieces, in the adult of three, which in old age are consolidated into one bone. The Ribs are thin, curved bones, being convex externally. There are twelve on each side, and all are attached to the spinal column. The seven upper ribs, which are united in front of the sternum, are termed true ribs; the next three, which are not attached to the sternum, but to one another are called false ribs; and the last two, which are joined only to the vertebræ, are designated as floating ribs. The first rib is the shortest, and they increase in length as far as the eighth, after which this order is reversed.
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.
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.
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.
We are not less interested in tracing the formation of bone through its several stages, than in considering other parts of the human system. The formation of the Haversian canals for the passage of blood-vessels to nourish the bones, the earlier construction of bony tissue by a metamorphosis of cartilaginous substance, and also the commencement of ossification at distinct points, called centers of ossification, are all important subjects, requiring the student's careful attention. The bones are protected by an external membranous envelope, which, from its situation is called the periosteum. The bones are divided into four classes, long, short, flat and irregular, being thus adapted to subserve a variety of purposes.