SKELETON, a general term for the more or less hard parts of animals, whether forming an internal supporting framework—an endoskeleton, or an exter nal exoskeleton, often useful as armor.
Skeleton of Invertebrates.—Many of the Protozoa have shells of lime, or of flint, or of some organic substance, such as acanthin. These are formed by the living matter of the units, in the case of the lime and flint shells from materials absorbed from the surrounding water, but in what precise way we do not know. Al most all sponges are supported by loose or firmly fused spicules of lime or of flint, or have, as in the bath spone, an inter woven supporting skeleton of "horny" fi bers. The spicules or fibers are formed by cells in the middle stratum of the sponge. Among coelenterates various forms of skeleton, both external and in ternal, both limy and "horny," are repre sented by the different kinds of corals. With few exceptions these skeletons are produced by cells belonging to the outer layer or ectoderm of the animal. Worms have little that can be called a skeleton. The tubes, calcareous or otherwise, in which many sedentary worms are shel tered, have no vital connection with the animals which make and inhabit them. Echinoderms tend to be very calcareous; lime is deposited in the mesodermic tissue of the body in almost any part, though predominantly near the surface. Most arthropods have well-developed exoskele tons, cuticles formed from the epidermis, consisting in great part of an organic basis of chitin, on which, in crustaceans and most rnyriopods, carbonate of lime is also deposited. As this cuticle is not al ways restricted to the outside of the ani mal, but sometimes extends inward, an apparent endoskeleton arises—e. g., in the lobster, the king crab, and the scor pion. Most mollusks have shells in which carbonate of lime occurs along with an organic basis conchiolin, and in cuttlefish there is a remarkable development of car tilage around the nerve centers in the head—an analogue of the skull in verte brate animals.
Skeleton of Vertebrates.—Here we must distinguish first of all between the exter nal exoskeleton and the internal endo skeleton. The scales of fishes, the scales and scutes of reptiles, the scales, claws, and even feathers of birds, the remark able bony armature of armadillos, the scales of pangolins, the claws of carni vores, the quills of porcupines, and even the hair of ordinary mammals illustrate the variety of structures which may be included within the anatomical conception of an exoskeleton. All these structures are formed in the epidermis, or in the dermis, or in both combined. Tortoise shell and the scales of reptiles are epi dermic; the scutes of crocodiles and the plates covering armadillos are dermic; the scales of elasmobranch and ganoid fishes are due to both layers. But it is difficult to carry out any rigidly logical classification.
The Skeleton of Man.—As the bones of all the chief parts of the human body are described in separate articles, we need not do more than unify these by reference to a diagram of the entire skeleton. Alto gether there are more than 200 bones, but some which are originally distinct become fused with their neighbors.
In the vertebral column there are origi nally 33 vertebra, but in adult life the normal number is 26, for, while the first 24 remain distinct, five (the 25th to the 29th inclusive) unite to form the sacrum supporting the hip girdle, and the four hindmost fuse more or less completely in a terminal tail piece or coccyx. Seven cervitmls support the neck; 12 dorsals form the greater part of the back and bear ribs; five lumbars occur in the loins; these are followed by the sacrum and the coccyx.
The ribs, or elastic arches of bone which bound the breast, are normally 12 on each side. Most of them articulate dorsally with the bodies of two adjacent vertebra and with the transverse proc esses of the posterior one; ventrally the first seven pairs are connected with the median breastbone by means of interven ing cartilages, while the posterior five pairs are more or less free.
The skull consists in early adult life of 22 separate bones, but originally there were more, and as life continues the num ber may be further reduced by fusion. See SKULL.
The skeleton of the arm includes 30 bones—in the upper arm the humerus, which articulates with the shoulder gir dle; in the forearm the radius and ulna, which articulate with the humerus at the elbow; the wrist of eight carpal bones; the five metacarpals of the palm; the five digits, of which the four fingers have each three joints or phalanges, while the thumb has two. The important bone of the pec toral girdle is the shoulder blade or scap ula. To this, at the shoulder joint, there is fused a small beak-like bone—the cora coid—which is separate in birds and rep tiles, but reduced to a mere process of the scapula in all mammals except the mono tremes. Stretching from the breastbone to shoulder blade is the curved collar bone or clavicle.
The skeleton of the leg also includes 30 bones—in the thigh the femur, which articulates with the hip girdle; in the lower leg the shin bone or tibia and the splint bone or fibula, which articulate with the femur at the knee joint, where there lies a little "sesamoid" bone—the patella; in the ankle region seven bones, then five metatarsal bones forming the sole of the foot, and five toes with the same number of phalanges as in the fin gers. The pelvic girdle consists in early life of three paired bones—large dorsal ilium, a posterior ischium, an anterior pubis on each side—but these unite about the 25th year into a single haunch bone with the socket of which the thigh articu lates.