ON THE EVOLUTION OF THE CRANIOCEREBRAL TOPOGRAPHY IN PRIMATES *

The primary problem of primatology today is that of hominization. Hominization refers to evolutionary development starting with lower primates and terminating with the genus Homo. This development is characterized by structural and functional changes in the hands and feet, brain, skull, and vertebral column, to name only a few. These changes took place during the different phases of the evolution of nonhuman primates, except for the last evolutionary steps of the development of the human brain. The brain reaches its highest point in development within the genus Homo. In connection with the evolution of the human brain, the skull has also undergone its last evolutionary steps within the genus Homo. The subject of my paper is the problem of the evolution of the craniocerebral topography, especially the kyphosis of the skull and of the brain, and their relationship. This problem is of interest mainly to anthropologists and comparative anatomists in the field of primatology. The problem, however, of the evolution and the ontogenetic development of the brain and the skull in mutual topographical dependence is also a problem for experimental and medical research; for instance, for stereotactic operations and transections of the hypophysial stalk. The changes in the topographical relations are only small ones, but they exist, and they are not unimportant. It is known that the skull base of man has a kyphosis which is situated in the basisphenoid (sellary kyphosis) . In nonhuman primates, the kyphosis is situated in the presphenoid (presellary kyphosis). It can easily be seen in very young animals, but in adults the kyphosis often can be discerned only in the cerebral surface of the skull base (Hofer & Spatz, 1963; Hofer, 1953, 1965). It is possible to determine the extent of the kyphosis by a modification of the method of Landzert ( 1 866). The methods used were critically described in previous papers (Hofer, 1957, 1960, 1965; Hofer & Spatz, 1963; Hofer & Tigges, 1963; Angst, 1967). An angle, called the “basis-angle,” is formed by two lines, of which one indicates the anterior or presellary skull base in its position relative to the postsellary skull base; the frontal opening of the angle is measured. Anatomically, the kyphosis is always located in the presphenoid or the basis-phenoid, but the vertex of the constructed angle need not strictly correspond to the anatomical situation. Its location depends, apparently, to some degree on the location of the points used to construct the straight lines forming the angle. Its width indicates the degree of the deflection of the skull base at its cerebral surface (FIGURE 1) . The characteristic aspect of the kyphosis of the human skull base is that this angle is wider than that in nonhuman primates. In man the vertex is located above the sella, while it is before the sella in nonhuman primates. In the chimpanzee the location of the vertex most closely approaches the location in man, but it is always found lying just frontal to the sella. In the orang the vertex is found much more frontally with regard to the sella than in the chimpanzee. As is well known, primates also have a kyphosis of the brain formed by the