Skeleton
a general term for the more or less hard parts of animals, whether forming an internal supporting framework-an endoskeleton, or an external 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. Almost all sponges are supported by loose or firmly fused spicules of lime or of flint, or have, as in the bath spone, an interwoven supporting skeleton of "horny" fibers. The spicules or fibers are formed by cells in the middle stratum of the sponge. Among cœlenterates various forms of skeleton, skeleton, both external and internal, both limy and "horny," are represented 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 sheltered, 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 exoskeletons, cuticles formed from the epidermis, consisting in great part of an organic basis of chitin, on which, in crustaceans and most myriopods, carbonate of lime is also deposited. As this cuticle is not always restricted to the outside of the animal, but sometimes extends inward, an apparent endoskeleton arises-e. g., in the lobster, the king crab, and the scorpion. Most mollusks have shells in which carbonate of lime occurs along with an SKELETON organic basis conchiolin, and in cuttlefish there is a remarkable development of cartilage around the nerve centers in the head-an analogue of the skull in vertebrate animals. 2 -1 39 4 5 6 8 -7 25 25 29 10 29 28 28 26 14 27 15 13 12- 17 22 13 15 12 17 22- 16 18 +18 19 23 19 24 20 20 21 21 SKELETON 1-Frontal bone 2-Parietal bone SKELETON 3-Temporal bone 4-Occipital bone 5-Malar bone 6-Superior maxillary 7-Inferior maxillary 8-Cervical vertebræ 9-Nasal bone 10-Sternum 11-Humerus 12-Ulna 13-Radius 14-Lumbar vertebræ 15-Innominate bones 16-Sacrum 17-Head of femur 18-Shaft of femur 19-Patella 20-Shaft of tibia 22-Greater trochanter 21-Fibula of femur 26-Condyles of humerus 27-Head of radius 25-Clavicle 28-Dorsal vertebræ 27-Head of radius 28-Dorsal vertebræ 29-Scapula See SKULL.
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 nal exoskeleton and the internal endoskeleton. The scales of fishes, the scales and scutes of reptiles, the scales, claws, and even feathers of birds, the remarkable bony armature of armadillos, the scales of pangolins, the claws of carnivores, the quills of porcupines, and even 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 epidermic; the scutes of crocodiles and the ganoid plates covering armadillos are dermic; the scales of elasmobranch and gan 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 a diagram of the entire skeleton. Altogether there are more than 200 bones, but some which are originally distinct become fused with their neighbors.
In the vertebral column there are originally 33 vertebræ, but in adult life the normal number is 26, for, while the first 24 remain distinct, five (the 25th to the nally 33 vertebræ, but in adult life the normal number is 26, for, while the first supporting the hip girdle, and the four Seven hindmost fuse more or less completely in a terminal tail piece or coccyx. cervicals 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 соссух.
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 vertebræ and with the transverse processes of the posterior one; ventrally the first seven pairs are connected with the vertebræ and with the transverse procing 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 SKELETON were more, and as life continues the number 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 girdigits, of which the four fingers have each three joints or phalanges, while the thumb which articulate with the humerus at the toral girdle is the shoulder blade or scapthe 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 pectoral girdle is the shoulder blade or scap-
SKELETON OF GIANT SLOTH ula. To this, at the shoulder joint, there is fused a small beak-like bone-the cora- coid-which is separate in birds and repis fused a small beak-like bone-the coracoid-which is separate in birds and reptiles, but reduced to a mere process of the scapula in all mammals except the monoor clavicle. tremes. Stretching from the breastbone to shoulder blade is the curved collar bone The skeleton of the leg also includes 30 bones in the thigh the femur, which articulates with the hip girdle; in the with the femur at the knee joint, where lower leg the shin bone or tibia and the splint bone or fibula, which articulate 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 fmgers. The pelvic girdle consists in early life of three paired bones-large dorsal the 25th year into a single haunch bone ilium, a posterior ischium, an anterior pubis on each side but these unite about with the socket of which the thigh articulates.
SKELLIGS