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The origin of formative forces is to be sought in genetically controlled growth processes. The forces generated by various system components are analyzed. The forces acting in mutual opposition can be illustrated by considering the brain as an intracranial force factor and the temporalis muscle as an external force factor. Apposition and resorption of bone is governed by the interaction between the stresses. The skull's primary growth centres such as the basicranial synchondroses must, like epiphyses of tubular bones, exert growth forces to overcome the restraining forces stemming from surrounding tissues. Growth can be suppressed by excessive counter-pressure, diverted by asymmetrically acting forces, and encouraged by the removal of suppressive forces.

The dimensions of the skull and the dry weight of the skull bones show definite sexual dimorphism. The dynamics of the growth in the skull dimensions and their vectorial presentation show that skull growth in Rattus norvegicus BERKENHOUT is most rapid between the 42nd d and the 4th month of life. This phase is followed by a phase of localized bone growth, which persists up to an age of 300 d. The faster growth of the viscerocranium leads to changes in the proportions of the skull. The relative positions of viscerocranium and mandible and the higher relative position of the dentures are similar to principle to those seen in man. Finally, the morphogenetic effect of the temporalis muscle together with that of the brain in the shaping of the skull are discussed.

Ligature of the right common carotid artery induces compensatory dilatation of the arteries of the head and neck, and particularly of the left common carotid artery. Functional mechanisms also caused dilatation of the arterioles. The enlargement of previously formed anastomoses and collaterals was also observed. Angiophysical effects and histochemical phenomena are implicated in the modification of previously existing anastomoses. The transport capacity of the collaterals increases as the area of the summated lumena increases and as the length of the vascular pathways decreases. The time factor, finally, also plays an important in role the formation of anastomoses, which continues even after an observation period of 12 months in the rat.

The cross sectional areas of arteries were calculated from their measured thicknesses. The monthly increase in size of the cross sectional area of the aorta is higher in both the controls and the operated animals between the 6th week and 4th month of life than during the subsequent period. The rate at which the cross sectional area of the aorta increased in operated animals was about twice that observed in the controls. The left common carotid artery increased its cross section in the operated animals most rapidly up to the fourth month. The cross sectional areas of all anastomoses increased five-fold by the 12th month of life. The quantities of blood flowing correlated closely with the angiometric values.

Cephalometric data show that the distances spanning several skull bones also have the highest growth rates. No sexual dimorphism can be observed in the skulls of animals that are six weeks old; such differences do not appear until after the fourth month of life. Allometric studies reveal a relative narrowing of the neurocranium and a relative broadening of the rostrum and zygomatic arch. The broadening of the zygomatic arch complex is related to the postnatal development of the masseter muscle. Vector analysis is used to demonstrate the growth activities of the different parts of the skull. Local changes in the bone during growth was observed in the regions of muscle origin and attachment.

The dry weight of the chewing muscles differ distinctly between male and female. In rats aged 8 to 12 months the dry weight of the chewing muscles after ligature of the carotid artery was lower on the side of the ligature. Necrotic fibres were occasionally found in the masseter muscle on the ligature side. Morphometric investigation of the masseter muscle fibres revealed no significant differences between the 2 sides in the operated rats. Since the rats were relatively young when the commune carotid artery was ligatured unilaterally, the consequences of the initially reduced blood supply to the chewing muscles on the right side could be compensated for rather well, mainly by the enlargement of previously existing anastomoses. Our quantitative and histological studies nevertheless show that muscular dysbalance of the masticatory apparatus was severe.

Unilateral and bilateral deformation of the temporomandibular joint was observed in young rats after unilateral ligature of the common carotid artery. They involved both the mandibular condyle and the mandibular fossa and were usually associated with a reduction in the dry weight of the masticatory muscles on the ligated side. This leads to the assumption that muscular atrophy is causally related to arthropathy of the temporomandibular joint. The methodological approach to induce changes in the temporomandibular joint may be used as a model of arthrosis of that joint.

The influence of the blood supply on craniofacial growth was investigated in 396 albino rats (Rattus norvegicus Berkenhout). This was done by blocking the right common carotid artery of animals aged 42 d. The rats were slaughtered at previously specified intervals after the operation. The ligature led to dilation of the other major arteries of the neck. Numerous collaterals and anastomoses were also formed. These occurred on the ligature side in the flow region between the proximal and distal stumps of the right common carotid artery, between the flow regions of the right external carotid artery and the right vertebral artery and between the flow regions of the right subclavian artery and the right external carotid artery. Further anastomoses arose between the arteries of the other side, i.e. the rostral thyroid artery, lingual artery, submental artery and labial mandibular artery.

After unilateral ligature of the common carotid artery in rats aged 42 d, the fresh weight of the brain does not deviate from that associated with normal growth in the course of subsequent postnatal development. This is believed to be due to the age at which the ligature was applied and to arterial compensatory mechanisms. The sensitive phases of cerebral development have already come to an end 42 d post partum. Ligature of the right common carotid artery can be fully compensated by circulus arteriosus (Willis), which provides hemodynamic pressure compensation.

The dry weight of the skull and jaw bone increases faster in operated rats than in the controls during the fourth and 12 months, but their rates of growth do not differ in the 8th month. The distances measured on the mandible reveal definite sexual dimorphism in the development of the different lengths and heights, these distance being greater in the male rats. Growth in the length of the processes predominated over the growth of the body of the mandible. Local changes in the bone were observed at the points of origin and attachment of muscles.

Growth is controlled by phylogenetic and ontogenetic factors. Regulation of bone growth can be regarded as taking place at 3 levels, a high, general level at which genetics and hormones represent the main regulators, a middle, regional level where the vascular influence predominates, and a low, local level dominated by mechanical factors. Stresses within the bone, which under physiological conditions are in a state of dynamic equilibrium, play a significant role in the shaping of bony structures. The apposition or resorption of bony material is accelerated if such stresses exceed or drop below certain limits. Biophysically, bone is comparable to technical materials consisting mainly of hydroxyl apatite and collagen. Deformation of the hydroxyl apatite is associated with piezoelectric effects which induce changes in bone structure.