THE BONY SINUSES 1335 



(lateral) sinus and the posterior auricular and occipital veins. This is the largest, the most 

 constant, and the most superficial of the emissary veins. Hence the old rule of applying 

 blisters or leeches over it in cerebral congestion. 3. Vein through the posterior superior angle 

 of the parietal between the superior sagittal sinus and the veins of the scalp. 4. Vein through 

 the condyloid foramen between the transverse (lateral) sinus and the deep veins of the neck, 

 5. Vein through the hypoglossal canal between the occipital sinus and the deep veins of the 

 neck. 6. Ophthalmic veins communicating with the cavernous sinus and the angular vein. 

 These veins may be the source of fatal blood-poisoning, by conveying out of reach septic 

 material, in acute periostitis of the orbit, or in osteitis, of dental origin, of the jaws. 7. Minute 

 veins through the foramen ovale between the cavernous sinus and the pharyngeal and pterj^goid 

 veins. 8. Communications between the frontal diploic and supraorbital veins, between the 

 anterior temporal diploic and deep temporal veins, and between the posterior temporal and 

 occipital diploic veins and the transverse sinus. In addition to the veins specially mentioned, 

 the scalp and sinuses communicate by numerous diploic veins, by those in the inter-sutural 

 membrane, and through sutures before their obliteration, as already explained. 



Structure of cranium. — Two layers and intervening cancellous tissue. Each 

 layer has special properties. The outer gives thickness, smoothness, and uni- 

 formity, and, above all, elasticity. The inner is whiter, thinner, less regular — 

 e. g. the depressions for vessels. Pacchionian bodies, dura mater, and brain. 

 The diploe, formed by absorption after the skull has attained a certain thickness, 

 reduces the weight of the skull without proportionately reducing its strength, and 

 provides a material which will prevent the transmission of vibrations. 



A blow on the head may fracture the internal layer only, the external one and diploe escap- 

 ing. This is difficult to diagnose, and thus it is impossible to judge of the severity of a fracture 

 from the state of the external layer. This may be whole, or merely cracked, while the internal 

 shows many fragments, which may set up meningitis or other mischief. It is usual to find more 

 extensive splintering of the inner than of the outer layer (table). 



The average thickness of the adult skull-cap is about 5 mm. (I in.). (Holden.) The 

 thickest part is at the external occipital protuberance, where the bone is often 1.8 cm. (f in.) 

 in thickness. The thinnest part of the skull vault is over the temporal part of the squamous. 

 The extreme fragility of the skull here is partly compensated for the by thickness of the soft 

 parts; these two facts are always to be remembered in the diagnosis of a fracture of the skull 

 here, after a slight injury. Other weak spots are the medial wall of the orbit, the cerebellar 

 fossae, and that part of the middle fossa corresponding to the glenoid cavity. 



Anatomical conditions tending to minimise the effects of violence inflicted upon the skull.— 

 (1) The density and mobility of the scalp. (2) The dome-like shape of the skull. This is cal- 

 culated to bear relatively hard blows and also to allow them to glide off. (3) The number of 

 bones tends to break up the force of a blow. (4) The sutures interrupt the transmission of 

 violence. (5) The inter-sutural membrane (remains of foetal periosteum) acts, in early life, 

 as a linear buffer. (6) The elasticity of the outer layer (table). (7) The overlapping of some 

 bones, e. g. the parietal by the squamous; and the alternate bevelling of adjacent bones, e. g. at 

 the coronal suture. (8) The presence of ribs, or groins, e. g. (a) from the crista galli to the 

 internal occipital protuberance; (6) from the root of the nose to the zygoma; (c) the temporal 

 ridge from orbit to mastoid; (d) from mastoid to mastoid; (e) from external occipital protu- 

 berance to the foramen magnum. (9) Buttresses, e. g. zygomatic processes and the greater 

 wing of the sphenoid. (10) The mobility of the head upon the spine. 



THE BONY SINUSES 



Frontal. — When well developed, the frontal sinuses may reach 5 cm. (2 in.) 

 upward and 3.7 cm. (1| in.) laterally, occupying the greater part of the vertical 

 portion of the frontal bone. When very small, they scarcely extend above the 

 nasal process. In any case, they are rarely symmetrical. The average dimen- 

 sions of an adult frontal sinus are 3.7 cm. (IJ in.) in height, 2.5 cm. (1 in.) in 

 breadth, and 1.8 cm. (f in.) in depth. (Logan Turner.) The sinuses are sep- 

 arated by a septum. The posterior wall is very thin. Each sinus narrows 

 downward into the infundibulum. This is 'deeply placed, at the back of the 

 cavity, behind the frontal (nasal) process of the maxilla and near the medial wall 

 of the orbit. Its termination in the middle meatus is about on a level with the 

 palpebral fissure.' (Thane and Godlee.) Its direction is backward. 



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

 persistence of polypi and ozsena, and the fact that a patient with a compound fracture opening 

 up the sinuses can blow out a flame held close by. 



To open the frontal sinus, while the incision which leaves the least scar is one along the 

 shaved eyebrow, superficial laterally so to avoid the supraorbital nerve and vessels, running a 

 little downward at the medial end, it is always to be remembered that, where the sinuses are 

 little developed, this or a median incision may open the cranial cavity. To avoid this compli- 

 cation the sinus should always be opened at a spot vertically above the medial angle. 



The development of these by the twentieth or twenty-fifth year may render a fracture 



