CLASSIFICATION OF ADULT MATERIAL
Four examples from the previous section have been selected to represent phases of normal pneumatization. The word phase is used in contradistinction to such a term as stage because it is intended to imply absence of strictly delimiting boundaries. Figure 4 B shows the picture at birth; representatives of this phase will henceforth be referred to as group I. Figure 5 B shows the normal development one finds at the approximate age of 2 years; representatives of this phase will be referred to as group II. Figure 5 F reveals the phase which has extensive pneumatization except in the apex of the pyramid, where there is none; representatives of this phase will hereafter be called group III. Complete pneumatization of the pyramid (cells in the apex) is shown in figure 6F; representatives of this phase show no arrest and henceforth will be group IV.As an