=The influence of air-sinuses, &c., on the line of the fracture.= The _sphenoidal sinuses_, two in number, are usually separated from one another by a thin septum. This septum is, however, often deficient, and a single cavity exists. The sinuses make their appearance about the seventh or eighth year; they vary greatly in size but, when fully developed, occupy the greater part of the so-called body of the sphenoid, extending backwards almost as far as the junction of the basi-sphenoid and basi-occiput, and spreading outwards into the wings of the sphenoid and over the roof of the orbit.
The sinus is bounded on all sides by a thin lamella of bone; its roof forms part of the middle fossa of the skull, the sides are separated by a thin bony wall from the cavernous venous sinus, and the floor aids in the formation of the roof of the naso-pharynx. There exists, therefore, in the very centre of the base of the skull--in the region of the so-called buttress of connexion between the posterior and anterior segments of the skull--an exceedingly weak area, one which must be implicated in the great majority of basic fractures. The ‘weak line’ of the base of the skull--previously referred to--is now still more accentuated.
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.