Vertebrates.— In a primitive vertebrate, as the lancelet (Branchiostoina Lanceolatum), the body is supported by a median dorsal axis; and, disregarding slight bony supports for the mouth, the pharynx and the median fin, that is the active skeleton or framework. The median dorsal axis, which in the lancelet has not even the solidity of cartilage, is the notochord, a constant characteristic of all skeletons of vertebrates. This notochord may be likened to an elastic rod that in the embryo is formed under the neural canal, consisting of a longi tudinal axis for separating the dorsal nervous tube from the ventral alimentary passage. This notochord in the lancelet, the hog and the young lamprey is an unsegmented rod with a simple sheath. In the adult lamprey there form above the notochord, rudimentary arches of cartilage that maintain the spinal cord. Cartilaginous arches appear both above and below the notochord in the sturgeon, the chimera and the dipnoi, but there are no true vertebra. These are noted first in the elasmobranch fishes, in which the notochord is more or less constricted by the encroachment of its sheath, and divided into vertebrae. In the bony ganoid fishes, the vertebrae are ossified, as in higher vertebrates. The notochord, therefore, appears a provisional structure or scaffolding around which a backbone can be built. Each verte brae usually consists of the centrum or substan tial body, the neural arches that form a tube for the spinal cord and meet above in a neural spine, the transverse processes which project and are commonly connected with ribs and the articular processes which bind the vertebrae together into a strong but flexible backbone. Birds and mammals and many reptiles have a breast bone or sternum, to which the ribs unite ventrally.
The pectoral or shoulder-girdle consists of a dorsal shoulder-blade • or scapula, a ventral cora coid with the articulation for the arm or fore leg between them and of a forward growing col lar-bone or clavicle. The pelvic or hip girdle in each side consists of a dorsal ilium, a ventral ischium with the articulation of the leg be tween them, and of a pubic portion. From amphibians upward, the forelimb is a humerus articulating with the girdle, a lower arm com posed of radius and ulna side by side, a wrist or carpus of several elements, a hand with meta carpal bones in the palm, and with fingers corn posed of several joints or phalanges. The hind
limb consists of a femur articulating from the girdle, a lower leg of tibia and fibula side by side, an ankle region or tarsus of several ele ments, a foot with metatarsal bones in the sole, and with toes of severaljoints or phalanges. Digits in vertebrates tend to a regular sup pression toward the lower types. The pollex goes first, then the minimus index and annu laris, so that some animals, as the horse, have one digit (the hoof) remaining. In many of the vertebrates the phalanges are more numer ous than in man, as where the hand is modified to form a paddle, notably in the iehthyosauri and the cetacea.
In reptiles the ilium projects rearward, the ischia are often found meeting ventrally, a cartilage or bone projects rearward from the symphysis, supporting the anterior lip of the cloaca' orifice, forming a hypoischium. In birds the projection of the ilium is both for ward and rearward and it fuses with the vertebral column in the backbone. No union or symphysis is formed by the ischia and pubes in birds, with the exception of the ostrich and rhea. The ilium in mammals projects forward and symphysis of the ischia and pubes is com mon. the marsupials and monotremes a portion of the external oblique muscle ossifies to form marsupial bones.
The patella is first noted in the Reptilia, though not common to all. All mammals and birds have this bone excepting a few bats. In the marsupials it commonly remains in the cartilaginous stage. In birds the fibula is often absent or fuses with the tarsal bones, and no ankle joint remains. In the marsupials the enlarged upper end of the fibula may articulate with the femur. Consult any standard work on anatomy; Reynolds, 'The Vertebrate Skele ton' (1897) ; Dwight, Thomas, 'Clinical Atlas' (Philadelphia 1907) ; Fraser J. S. S.,