SKELETON, the more or less hard struc tures, mainly of bony nature, which form the in ternal axis of support of the soft parts in the higher or vertebrate animals. The term is used in comparative anatomy to designate support ing parts not only of internal, but also of ex ternal nature. Anatomists thus speak of an en« doskeleton as proper to the vertebrates, but both vertebrates and invertebrates may have certain hard parts developed on the exterior of their bodies (for example, shell of lobster, scales of fishes, etc.), and to the latter class of struc tures is applied the name exoskeleton. The parts of any endoskeleton may generally be grouped under the two heads of the spinal or axial skeleton and the appendicular parts. The former includes the skeleton of the head and trunk, the latter that of the limbs. The spinal skeleton involves the consideration of the skull (q.v.), spinal or vertebral column, com posed of its various vertebra and of the thorax (q.v.) or chest and pelvis (q.v.). The COM* position of the skull forms a subject of great intricacy and involves many highly technical considerations. The more general characters of the skull are noted in a separate article (see SKULL). The vertebra or component parts of the spine or backbone consist each of a solid piece or body, attached to which are various processes. The spinal arches spring from the posterior part of each vertebra and unite to form the spinous process; the spinal cord itself being protected within the canal formed by the apposition of the arches of the vertebra. In man seven cervical or neck vertebra, 12 dorsal and five lumbar vertebra exist as sep arate bones. The sacrum (q.v.) and coccyx, forming the terminal part of man's spine, are composed of united vertebra. Thirty-three vertebra are found in the human foetus normally, but in the adult five of these unite to form the sacrum; while the other four (more or less) form a short tail piece or coccyx, the latter being below the junction with the pelvis. The heavy, pelvic bone in man makes a base for the spine and head sockets for the femurs or leg-bones. The ribs in man correspond in
number with the dorsal vertebra. The limbs consist of homologous or corresponding parts and are attached to a series of bones constitut ing the larch* or support of the fore and hind limbs respectively. The scapula or shoulder blades and collar-bones or clavicles constitute the shoulder-girdle (see SHOULDER-JOINT), or arch supporting the fore or upper limb, while the lower limb is attached to the pelvic arch or pelvis. The upper limb or arm in man con tains 31 bones; the humerus or bone of the upper arm; the radius and ulna, or bones of the forearm; the carpal or wrist bones; the metacarpal bones, or those of the palm; and the phalanges, or bones of the fingers. The lower limb or leg in man contains 30 bones: the femur or thigh-bone; the tibia (shin) and fibula or bones of the leg; the tarsal bones or those of the ankle, corresponding to those of the wrist; the metatarsus or instep; and the phalanges or bones of the toes. (See the illus tration of the human skeleton under anatomy, Vol. I). The character and appearance of the bones changes materially with advancing years. The bones of a child at birth are soft and in complete. They grow, ossify and fuse together in a most interesting manner, so that the dis sector can judge approximately of the age of an individual by the condition of the bones.
Invertebrates.— Shell fish have exoskeletons or shells composed mainly of lime or flint, which is absorbed and utilized. The corals (q.v.) produce a formation that is their skeleton. The arthropods or snails have a horny little house or exoskeleton, the cuticle forming from the epidermis with an organic base of chitin on which carbonate of lime is deposited. In the lobster, scorpion and many crabs, the skeleton :., partly interior. Most mollusks leave shells in which the carbonate of lime occurs along with an organic base of conchiolin; in the cuttlefish there is a pronounced development of cartilage about the nerve centres of the head, suggesting a basis for a skull.