The Limbs

The skeleton of the limbs is primarily formed by the long bones. The term ‘long bone’ was perfectly chosen by ancient anatomists. An upper arm or an upper leg is, however, hardly tubular in the true meaning of the word. It is clear that the ancient anatomists chose their nomenclature based on a natural feeling for the morphological dynamic. Long bones have a radial and parallel structure, are hollow and have, when cross-sectioned, a nearly circular shaft.  When compared with the cranium, the differences in morphological characteristics between the cranium and the long bones are immediately apparent.  The shape of the long bones displays a completely different dynamic than the shape of the cranium. The form of the cranium has the characteristic of a sphere; the long bones have a radial and parallel structure. Just as the cranium is not a perfect sphere, neither are the long bones perfectly radial. One must, therefore, use the term morphological characteristic. In those places where several bones in the arm or leg are positioned next to each other, there is a clear parallel aspect: the bones of the lower leg, the forearm, the metacarpal bones and metatarsal bones, the fingers, and the toes run practically parallel to each other.  The long bones are created by the ossification of cartilaginous skeleton, which functions as the precursor of the definitive skeleton. It is of mesodermal origin and the ossification occurs by the replacement of cartilage by bone. In enchondral ossification, the mesoderm first develops into cartilage and then into bone.  The ossification of the long bones occurs from two ossification centers: the epiphyseal ossification centers in the proximal and distal parts of the bone and the ossification from the periost of the diaphysis. Between the epiphysal centers and the diaphysis are ringshaped discs of cartilage that continue to produce cartilage for a long period of time.  The bone spicules that originate from enchondral ossification are parallel and are situated lengthwise in the direction of the bone. The bone increases in thickness because it is built up peripherally and is dissolved in the center of the shaft such that the bone marrow cavity is created.  This type of skeletal forming has a direct connection with the impact of gravity on the organism. The morphology and the degree of calcification of the long bones are, to a great degree, determined by the effect of gravity.  Research on the structure of the bone spicules in relation to the effect of gravity is unequivocal in this. Bone spicules are formed in accordance with the lines of gravity that are operative in the skeleton. In a weightless situation, the skeleton of the torso and the extremities is particularly threatened by decalcification because of the effect of insufficient gravity.  Enchondral ossification of the skeletal bones in the head also occurs in that portion of the cranium that develops from the branchial arches and the paraxial sclerotomes. The architecture of the bone spicules in this portion of the skeleton displays the same characteristics as the architecture of the extremities.  In this area, we also find the forces that are exerted on the skeleton by the chewing musculature. Most of the bone tissue disappears from those areas where these forces can no longer influence the skeleton.  This is particularly the case after the loss of teeth, which normally transfer these mechanical forces onto the skeleton.