Skeleton (Gr., a dried body, from σκέλ.-λειν, to desiccate), the bony and cartilaginous framework of animals, and the ligneous structure of the leaves of plants. In the higher animals the skeleton is internal (endo-skele-ton); in many of the lower it is external (exo-skeleton). When the bones are joined by natural ligaments, they form a natural skeleton; when they are joined by wires and straps, the skeleton is said to be artificial. The study of the skeletons of different animals belongs to the subject of comparative anatomy; the human skeleton only will be described here. Bones may be classified as long, round, flat, and short. (See Bone.) The human skeleton consists of 208 bones, exclusive of the teeth, which are in reality parts of the digestive apparatus, and are developed from the mucous membrane. For convenience the skeleton may be divided into four regions: 1, the skull; 2, the trunk; 3, the upper extremities; 4, the lower extremities. The skull contains 30 bones, in three divisions, cranium, ears, and' face. There are 8 cranial bones, viz.: 1 frontal, 2 parietal. 2 temporal, 1 occipital, 1 sphenoid, and 1 ethmoid. The frontal bone forms the forehead, upper part of the eye sockets, and front part of the floor of the cranial cavity.

Just above the inner angles of the eyebrows are two marked prominences called the superciliary ridges, at which points the two tables of the bone separate considerably, enclosing cavities called the frontal sinuses which communicate with the nasal passages. The parietal bones, occupying the upper part and sides of the skull, are separated from each other by the sagittal suture, and from the frontal bone by the coro-nal suture. A curved ridge traverses both frontal and parietal bones at each side, which marks the origin of the fibres of the temporal muscle, lying in a depression behind and below the ridge, called the temporal fossae. The temporal bones, situated at the sides and base of the skull, consist each of three portions: an upright or squamous portion, a posterior or mastoid portion, and an internal or petrous portion. The upright portion articulates with the parietal bone by the squamous suture. The mastoid portion has a projection, felt behind the ear, called the mastoid process, which has a cellular structure, communicating with the middle ear or tympanum; the cells are not developed till after puberty. The petrous (hard, stony) portion is in the form of a triangular pyramid, and lies upon one of its sides in the base of the skull, its apex pointing forward and inward.

One of the openings into it, the internal auditory canal, transmits the auditory and facial nerves, and it also contains the tympanum. The temporal bones are pierced externally by the external auditory canal, which transmits the sonorous pulsations to the membrane of the tympanum. The under surface of the bone articulates with the lower jaw bone to form the joint. Just in front of this, and a little above, a process called the zygomatic springs forward to meet another of the same name from the cheek bone, forming a horizontal arch, the zygomatic, under which the tendon of the temporal muscle passes. The occipital bone consists of an upright and a basilar portion; the latter contains a large orifice, the foramen magnum, through which the brain connects with the spinal cord. On each side of the foramen magnum there is a condyle having an articular surface which rests upon a corresponding condyle of the atlas, the upper bone of the vertebral column. The basilar portion articulates in front with the body of the sphenoid bone, fig. 4, which in turn articulates with the ethmoid, fig. 5, the latter being situated at the root of the nose and held in position by the frontal and several bones of the face.

There are 8 ear bones, 4 in each ear, situated in the tympanum; they are described in the article Ear. The 14 bones of the face arc 2 nasal, 2 upper jaw or superior maxillary, 2 lachrymal, 2 cheek or malar bones, 2 palate bones, 2 inferior turbinated (in the nose), 1 vomer (septum of the nose), and 1 lower jaw, or inferior maxillary bone. (See illustrations.) Each upper jaw bone contains a large cavity called the maxillary antrum, which communicates with the nasal passage. The lachrymal bones are small oval plates situated at the inner angles of the orbits of the eyes. The palate bones are situated at the posterior part of the nasal passages, and enter into the formation of the roof of the mouth or palate and the back part of the floor of the orbits of the eyes. The lower jaw bone consists of a horizontal semicircular portion, having an alveolar process into which the lower teeth are set, and of a perpendicular portion, the ramus, divided into two branches, one of which terminates in the condyle to form the joint, and the other is the coronoid process, into which are inserted the fibres of the temporal muscle and a portion of those of the masseter, the two principal muscles of the jaw.

The floor of the skull is divided into anterior, middle, and posterior fossae, the two first lodging the anterior and middle lobes of the cerebrum, and the posterior fossa) lodging the cerebellum. (See Brain.) The bones of the trunk are 54 in number, viz.: the 24 bones called vertebrae, constituting, with the sacrum upon which they rest, the spinal column, 24 ribs, 4 pelvic bones, 1 sternum or breast bone, and 1 tongue bone. The two hip bones are naturally classified with the lower extremities, but as they are joined to the sacrum by immovable sutures, and form with it an important piece of animal mechanism, the pelvis, they are here included in the bones of the trunk. (See Pelvis.) The spinal or vertebral column, or backbone, forms the axis of the trunk, supporting it and the skull. All of the vertebra) but one have their principal features in common; i. e., they have a body, a spinous process, a spinal foramen for transmitting the spinal cord, and four articular processes, two superior and two inferior for articulating with each other. The spinous processes which project posteriorly together form the " spine," which marks the course of the spinal column.

The uppermost vertebra, called the atlas, has no body, but its place is occupied by a tooth-like process of the bone next below, called the axis, around which the atlas turns. There are 7 cervical, 12 dorsal, and 5 lumbar vertebra). The seventh cervical is peculiar from having a longer and more prominent spinous process than the others, which may be felt at the base of the neck. Between the bodies of the vertebra) are placed the elastic intervertebral cartilages, which permit flexion of the spinal column and prevent concussion of the spinal cord in walking and leaping. The ribs, 24 in number, are long flat bones of a semicircular form, and have an oblique position, their posterior extremities being higher than their anterior. The middle part of the curve is also depressed, so that the contraction of the respiratory muscles expands the cavity of the chest. There are 7 true and 5 false ribs on each side, the true ribs articulating with the sternum, while the false ribs lap on to each other, except the last two, which are free, and are called floating ribs. The sternum is a kind of breastplate, composed of three pieces, to which the collar bones and the ribs are attached. The tongue bone supports the root of the tongue and gives attachment to muscles for moving it.

The upper extremities contain 64 bones, 32 on each side, in six divisions: 1, the shoulder; 2, the arm; 3, the forearm; 4, the wrist or carpus; 5, the palm or metacarpus; 6, the fingers or phalanges. The shoulder contains two bones, the scapula and clavicle. The scapula is a flat triangular bone situated at the upper and back part of the chest on each side. It is traversed on its posterior surface by a spine which terminates in the acromion process, the prominent point of the shoulder. Below the acromion process is the head of the scapula, containing a shallow cup called the glenoid cavity, which receives the head of the arm bone or humerus. The outer extremity of the collar bone or clavicle (Lat. clams, a key) articulates with the acromion process, forming a kind of brace. The scapula is held to the trunk by powerful muscles, which allow of sufficient motion to give a variety of positions to the shoulder joint. The arm contains one bone, the humerus, the lower end of which by its expanded articular surface forms with the two bones of the forearm, the radius and ulna, the elbow joint.

The wrist or carpus contains 8 bones (see fig. 6), the palm or metacarpus 5, and the fingers or phalanges 14, the first and second phalanx containing 5 each and the third 4. The apparatus of the forearm is a marvel of animal mechanism. The upper extremity of the ulna forms with the articular surface of the humerus a firm hinge joint, but the head of the radius forms with it a rotatory joint by which pronation and supination of the forearm and hand are effected with grace and facility. The lower extremities contain 60 bones, 30 in each limb, in six divisions: 1, the thigh bone or femur; 2, the knee pan or patella; 3, the two bones of the leg, the tibia and fibula; 4, the 7 bones of the ankle or tarsus; 5, the 5 bones of the metatarsus; and 6, the 14 bones of the toes or phalanges. The femur is the longest, largest, and strongest bone in the skeleton. Its upper extremity contains the head, which fits into the socket of the hip bone, and the neck, which joins the shaft of the bone at an angle of nearly 45°, the union being marked by two strong processes called the greater and lesser trochanters, to which are attached strong muscles, the chief office of which is to rotate the thigh, and also to move it outward and inward.

Its lower extremity is expanded like that of the humerus, and articulates with the head of the tibia, the principal bone of the leg. The tibia articulates at its lower extremity with the astragalus, the bone occupying the summit of the arch of the foot, and the latter rests upon the calcis or heel bone, into which the tendo Achil-lis, the tendon of the strong extensor muscles of the calf, is inserted.

Fig. 1. - Front View of Skeleton. 1. Frontal bone. 2. Parietal. 3. Temporal; 4, its mastoid process. 5. Malar or cheek bone. 6. Upper maxillary. 7. Orbit of the eye. 8. Lower maxillary; 9, its ramus. 12. The cervical vertebrae. 13. Clavicle. 14. Scapula. 15. Sternum. 1G. First rib. 17. Seventh rib. 18. Twelfth rib. 19. First lumbar vertebra. 20. Last lumbar vertebra. 21. Sacrum. 22. Ilium. (See Pelvis.) 24. Humerus; 25, its head; 26, its outer condyle; 27, its inner condyle. 28. Radius: 29, its head; 30, its lower extremity. 31. Ulna; 32. its head; 33, its lower extremity. 34. Carpus or wrist. 35. Metacarpus. 36. Phalanges. 37. Femur or thigh bone: 38, its head; 3!), its neck; 40, its greater trochanter; 41, its lesser trochanter; 42. its outer condyle; 43, its inner condyle. 44. Patella or knee pan. 45. Tibia; 46, its head; 47, its lower extremity: 48, inner malleolus. 49., Fibula; 50. its head; 51, its lower extremity, forming outer malleolus. 52. Tarsal bones'(7). 55. Metatarsal bones (5). 56. Phalanges.

Fig. 2. - Back View of Skull, Trunk, and Left Arm. 1. Frontal bone. 2. Parietal. 3. Occipital. 4. Temporal. 5. Lower maxillary. 18. Head of scapula at junction of clavicle. 19. Supra-spinous fossa. 20. Infra-spinous fossa. 21. Anterior border. 22. Posterior border. 23. Inferior angle. 24. Olecranon process of ulna.

Fig. 3. - Floor of Skull. 1, 1. Orbital plate of frontal bone, forming most of anterior fossa?. 2. Cribriform plate of ethmoid bone. 3. Crista galli process. 4.4. Lesser wings of sphenoid bone. 5. 5. Middle fossae of base of cranium. 6, 6. Greater wings of sphenoid. 7. Olivary process. Immediately in front of this process there is a transverse furrow called the optic groove, in which lies the commissure or crossing of the optic nerves. This groove terminates in the optic foramina. 4, -1, fig. 4. (See Brain, vol. iii., pp. 193, 194.) S. Sella turcica, upon which rests the pituitary gland. (See Brain, p. 191.) 9. 9. Petrous portion of temporal bone. 10. 10. Round foramina for superior maxillary nerve. 11, 11. Oval foramina for inferior maxillary nerve. Interior to these two holes is a large slit-like opening on each side, giving passage to the internal carotid artery and some important nerves. 13, 13. Posterior fossa? of the floor of cranium. 14. Foramen magnum, for the spinal cord. 15. Basilar process of occipital bone. 16. 16. Grooved channel for the lateral sinus. (See Brain, p. 18s.) 17, 17. Internal auditory meatus, transmitting the auditory and facial nerves.

Fig. 4. - Sphenoid Bone, seen from above. 1, 1, Its greater wings. 2, 2. Its lesser wings. 3. Sella turcica. 4, 4. Foramina for the optic nerves. 5. 5. Sphenoidal fissures, for third, fourth, sixth, and part of fifth pairs of cranial nerves. 6, 6. Round foramina. 7, 7. Oval foramina. 8. Part of basilar process of occipital bone. 9, 9. Internal pterygoid plates, terminating in muscular or hook-like processes, over which pass the tendons of the tensor muscles of the palate. 10,10. External pterygoid plates.

Fig. 5. - Ethmoid Bone, seen from behind. 1. Central lamella. 2. Cribriform plate. 3. Crista galli. 5, 6, 7. Lateral mass of left side.

Fig. 6. - Hyoid or Tongue Bone, seen in front. 1. Body. 2, 2. Greater cornua. 3, 3. Lesser cornua.

Fig. 7. - Palmar Surface of Right Carpus and Metacarpus. 1. Scaphoid bone. 2. Lunar. 3. Cuneiform. 4. Pisiform. 5. Trapezium. G. Trapezoid. 7. Magnum. 8. Unciform, a, b, c, d, e. The five metacarpal bones.

Fig. S. - Tarsus and Metatarsus, forming Instep. 1. Astragalus. 2. Os calcis. 3. Boat-shaped or scaphoid bone. 4. Cuboid. 5. Internal cuneiform. 6. Middle cuneiform. 7. External cuneiform, a, b, c, d, e. The five metatarsal bones.