1086 SURGICAL AXD TOPOGRAPHICAL AXATOMY 



brane pass through the etlimoid and internal orbital foramina. The fact that the 

 nasal mucous membrane is loose and ill-su]>i)orted on the turl)inated bones allows 

 its vessels to give way readily, and thus forms a salutary safeguard to the brain, 

 warding off many an attack of apoplexy. 



2. Vein thn)Ugh the mastoid foramen, between the lateral sinus and the pos- 

 terior auricular and occipital veins. This is the largest, the most constant, and the 

 most superticial of the emissary veins. Hence the old rule of ai)i)]ying blisters or 

 leeches over it in cereljral congestion. 



3. Vein through the posterior superior angle of the parietal l)ctween the supe- 

 rior longitudinal sinus and the veins of the scalp. 



4. Vein through the posterior condyloid foramen between the lateral sinus and 

 the deep veins of the neck. 



5. Vein through the anterior condyloid foramen between the occipital sinus and 

 the deep veins of the neck. 



6. Ophthalmic veins comn:iunicating with the cavernous sinus and the angular 

 vein. These veins may be the source of fatal l)lood-poisoning, hy conveying out of 

 reach septic material, in acute periostitis of the orbit, or in osteitis, of dental origin, 

 of the jaws. 



7. Minute veins tl:rough the foramen ovale l)etween the cavernous sinus and the 

 pharyngeal and pterygoid veins. 



8. Communications between the frontal diploic and supraorl)ital veins, between 

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

 temjjoral and occipital diploic veins and the lateral sinus. 



The gravity of these emissary veins and their free conanunications with others 

 is shown by the readiness with which they l)ecome the seat of septic thrombosis, 

 and thus of blood-poisoning, in cranial injuries, erysipelas, suppuration of the scalp, 

 and necrosis of the skull. 



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

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

 uniformity, and, above all, elasticity. The inner is whiter, thinner, less regular-- 

 e.g. the dej^ressions for vessels, Pacchionian bodies, dura mater, and brain. Its 

 cliief characteristics are its fragilit}' (vitreous) and absolute inelasticity. 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. 



Results of the above varying elasticity. — A ])low on the head may fracture 

 the internal table only, the external one anil diploe escaping. This is difhcult to 

 diagnose, and thus it is impossible to judge of the severity of a fracture from the 

 state of the external table. This may be whole, or merely cracked, while the 

 internal shows many fragments, which may set up meningitis, or other mischief. 



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, like an egg-shell, is calculated to bear 

 relatively hard l>lows and also Id allow them to glide off. (3) The number of 

 bones tends to break u'p the force of a blow. (4) The sutures interrupt the 

 transmission of violence. (5) The internal membrane (remains of fo'tal peri- 

 osteum) acts, in early life, as a linear l)uffer. (6) The elasticity of the outer 

 table. (7) The overlapping of some bones, e.g. the parietal l)y 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 

 protulK'rance; (b) from the root of the nose to the zygoma; (c) the temporal ridge 

 from orbit to mastoid; (d) from mastoid to mastoid: (c) from the external occipital 

 protuberance to the foramen magnum. (9) Buttresses, e.g. malar and zygomatic 

 processes, and the greater wing of the sphenoid. (10) The mobility of the head 

 upon the spine. 



CRAXIO-CEREBRAL TOPOGRAPHY 



To make as clear as possible the points of practical importance which have, of 

 late years, been put on a definite basis, and which the surgeon may have to recall 

 and act upon at very short notice, cranio-cerebral topograi)liy will lie s})oken of 



