40 L. BOLK 
It is a well established fact that the shape of a bone and es- 
pecially its internal structure, are the results of the mechanical 
and muscular forces acting upon it. In accordance with the 
mechanical principle of securing the maximum of strength with the 
minimum of material, the cancellous tissues of each bone is 
so arranged as best to withstand the strains and stresses to which 
the bone is usually subjected. So the internal architecture of 
each bone is quite in accordance with the fundamental laws of 
physics; systems of 'pressure lamellae' running in definite direc- 
tion are crossed by sets of 'tension lamellae.' A great number 
of investigators have tried with good results to analyze the struc- 
ture of the different bones of the human skeleton from this point 
of View. Only in regard to the skull in general, and particularly 
the cranial vault, are we without definite knowledge as to the 
structure of the bony framework of the different bones of the 
skull and the relation between the statical and dynamical 
external forces to which it is subjected. The whole of our knowl- 
edge is confined to the fact that the structure of the plate-like 
bones of the cranial vault exhibits the following appearance : the 
outer and inner surfaces are formed by two compact layers, 
having sandwiched between them a layer of cancellous tissue. 
Nevertheless concerning the cranial vault we find ourselves 
under relatively favorable circumstances, because the general 
conditions are so very simple here that the problem can be 
elucidated sufficiently from a mere theoretical standpoint. For 
the function of the cranial vault being principally a protective 
one, the number of mechanical stresses to which the frontal 
half of the skull is subjected is slight. There are but two factors 
to take in consideration, namely the weight of the facial cran- 
ium with the soft parts of the face as a constant working 
factor, and the pressure effectuated by the temporal muscle 
during its contraction. The weight of the facial cranium is 
transferred surely for the greatest part by means of the zygo- 
matic arches to the middle of the base of the cranium, and so 
there remains as the only important external force acting upon 
the anterior and lateral part of the skull, the pressure of the 
temporal muscles, when the jaws are firmly closed. Surely this 
