The growth of epiphyseal cartilages of long bones, of spheno-occipital synchondrosis of the cranial base, of the cartilage of the nasal septum, of lateral cartilaginous masses of the ethmoid, of cartilage between body and greater wings of the sphenoid (all stemming from the primary cartilaginous skeleton of the organism), is subject to general extrinsic factors and, more specifically, to the growth hormone (STH) and somatomedin. In this case, orthopedic devices can alterate the direction but not the amount of growth. 2) The growth of condylar, coronoid and angular cartilages of the mandible, of the cartilage of the midpalatal suture, and of the cartilage in some cranial sutures (all of secondary formation during phylogenesis and ontogenesis) is subject to local extrinsic factors as well as to growth hormone and somatomedin. In this case, appropriate orthopedic devices may modulate both the direction and the amount of growth. 3) Our cybernetic models attempt to account for the mechanisms of facial growth. By intensifying the forward growth of the nasal septum cartilage, the STH and somatomedin stimulate the forward growth of the upper jaw, i.e. the forward positioning of the superior dental arch (the position of which is the "constantly changing reference input" of the servosystem). The "operation of confrontation" between the position of the upper and lower occlusal surfaces (the position of the lower dental arch is the "controlled variable" of the servosystem) gives then rise to a "deviation signal" (originating from detectors of occlusal adjustment) whose "reduction" is made possible by a supplementary postural activity of the lateral pterygoid muscle resulting, extemporaneously, in an appropriate forward positioning of the mandible and, with time, in a supplementary growth of the condylar cartilage. By intensifying the outward growth of lateral cartilaginous masses of the ethmoid and of the cartilage between the body and greater wings of the sphenoid, the STH and somatomedin bring about a lateralization of both the left and right sides of the upper jaw and, in this way, stimulate the growth of the secondary cartilage of the midpalatal suture.
