SKELETON 447 SKELETON 1 — Frontal bone 2 — Parietal bone 3 — Temporal bone 4 — -Occipital bone 5 — Malar bone 6 — Superior maxillary 7 — Inferior maxillary 8 — Cervical vertebrae 9 — Nasal bone 10 — Sternum 11 — Humerus 12— Ulna 13 — Radiu9 1-i — Lumbar vertebrje 15 — Innominate bones 16 — Sacrum 17 — Head of femur 18— Shaft of femur 19— Patella 20— Shaft of tibia 21— Fibula 22 — Greater trochanter of femur 23 — Condyles of femur 2-1 — Tuberosity of tibia 25 — Clavicle 26 — Condyles of humerus 27 — Head of radius 28 — Dorsal vertebrae 29 — Scapula 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 vertebrae, 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 cervi^als 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- 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 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.
