The sphenoidal sinus is involved in at least 40-50 per cent. of basic fractures, comminution of the sinus wall being often so excessive that a probe can be passed with the greatest ease from the middle fossa into the naso-pharynx. Blood is thus allowed to escape readily into the naso-pharynx, and a source is opened up for the possible development of meningeal infection. Reference to the various illustrations of fractures of the base will supply further evidence as to the special liability of the sinus region to injury. It will be seen that nearly all fractures that pass one middle fossa to the other, or from one middle fossa to the opposite anterior fossa, traverse this region. The _frontal sinuses_, also two in number, are separated from one another by a thin osseous septum. Up to the age of puberty these sinuses are either absent or represented by a small cell. Subsequently, they develop rapidly, often extending into the orbital roof. The upper and inner boundary--usually very fragile--assists in the formation of the anterior fossa of the skull. The outer boundary--the perpendicular plate of the frontal bone--is much more dense, and, consequently, a fracture of the outer wall is almost necessarily associated with a fracture involving the inner or orbital boundary, that is to say with a fracture of the anterior fossa. _The ethmoid cells._ The ethmoid bone consists of a collection of cells which communicate with the nasal cavity (middle and superior meati), and which are merely separated from the anterior fossa of the skull by the thin cribriform plate. This plate of bone is of so fragile a nature that splintering occurs in the great majority of anterior fossa fractures. The special dangers that arise from the possibility of meningeal infection are obvious. _The auditory region._ That part of the petrous bone which encloses the auditory apparatus, and which transmits the seventh and eighth pair of nerves, is proportionately weakened and correspondingly liable to fracture. The special details of these fractures are dealt with on p. 102. [Illustration: FIG. 31. TO ILLUSTRATE THE RELATION OF BASIC FRACTURES TO CRANIAL NERVES.]