242 HUMAN ANATOMY. 



The course of the longitudinal sinus is indicated by a Hne drawn from the 

 nasion (the junction of the nasal and frontal bones) to the inion. 



The lateral sinus is irregular in its course (page 234). According to Macewen, 

 it may be fairly indicated by the two following lines : ' ' The first from the asterion to 

 the superior margin of the external osseous meatus, of which line the posterior two- 

 thirds correspond to the upper part of the sigmoid groove, which is also the more 

 superficial. The second line from the parieto-squamo-mastoid junction to the tip of 

 the mastoid process corresponds in its upper two-thfrds to the vertical part of the 

 sigmoid groove. The knee of the sigmoid — its most anterior convexity — is variable 

 in its position, but is generally on a level with the upper part of the external osseous 

 meatus. The sigmoid groove is situated at a variable distance from the external 

 auditory meatus, the tympanum, and the exterior of the skull. The distance 

 between the external osseous meatus and the sigmoid groove varies from one or two 

 to thirteen millimetres." 



The frequency with which infective thrombosis of the lateral sinuses occurs as 

 a complication of middle ear disease renders the topographical anatomy of these 

 sinuses and the associated region of the skull of great practical importance. 



The suprameatal triangle is formed by the posterior root of the zygoma running 

 somewhat horizontally above, the portion of the descending plate of the squamous 

 which forms the arch of the osseous part of the external auditory meatus below, and 

 a base line uniting the two, dropped from the former on a level with the posterior 

 border of the external auditory meatus. At this point there is usually a depression 

 in the bone, though occasionally there is a slight prominence as if the antrum had 

 bulged at that point. The apex of this triangular depressed area points forward 

 (Macewen). The mastoid antrum may be reached through this triangle. 



(The relations of this antrum, the facial canal, and the lateral sinus to one 

 another, to the temporo-sphenoidal lobe, and to the surface of the skull will be 

 considered in connection with the general subject of Cranio- Cerebral Topography, 

 page 1 2 14.) 



The size and extent of the frontal sinuses vary, as described on page 234. 

 The communication of these sinuses with the nose accounts for the frontal headache 

 in ozaena, and the fact that a patient with a compound fracture opening up the 

 sinuses can blow out a flame held close by. The frontal sinuses may be occu- 

 pied by bony or other tumors ; emphysema may result from fracture of the sinus 

 wall ; insects may gain access to these cavities and give rise to infection or to 

 epistaxis ; infective inflammations of the nose and naso-pharynx may involve the 

 sinuses. 



The spheyioidal sinuses are less important surgically, but these points should be 

 remembered : (i) fracture through them may lead to bleeding from the nose, which 

 is thus brought into communication with the middle fossa ; (2) the communication 

 of their mucous membrane with that of the nose may explain the inveteracy of cer- 

 tain cases of ozaena ; (3) here and in the frontal sinuses very dense exostoses are 

 sometimes formed (Jacobson). 



The Face. — The nasal bones are so joined together as to form a strong arch 

 resting upon the nasal processes of the superior maxillary bones. They are sel- 

 dom dislocated, because this line of union is one in which there is an alternation 

 in the bevelling of the sutures (similar to that between the frontal and parietal 

 bones). Thus the lower portion of the nasal bones overlaps the maxillary, while 

 nearer the root of the nose the latter is external. The line between the bones 

 and the nasal cartilages can easily be felt. The skin is very tightly attached to 

 the cartilages. 



The upper or frontal portion of these bones is very strong, and will resist a 

 great degree of force without fracture. The lower portion is most frequently broken, 

 usually within a half-inch of the lower margin. 



The resulting deformity is usually lateral, but if the perpendicular plate of the 

 ethmoid is broken the nose will be depressed. The thinness and close application 

 of the mucous membrane to the bones render these fractures almost invariably com- 

 pound. Emfihysema of the cellular tissue of the face and forehead may follow such 

 an injury. The vascularity of the bones leads to very rapid union, and it is therefore 



