238 



HUMAN ANATOMY. 



The time-honored custom of bUstering or leeching behind the ear in intra- 

 cranial inflammations rests on the fact that the largest emissary vein is the mastoid, 

 traversing the mastoid foramen and connecting the lateral sinus with an occipital 

 vein or with the posterior auricular. (For further discussion of these channels of 

 communication, see the section on the Venous System. ) 



While the spinal dura mater has no intimate connection with the inner surfaces 

 of the vertebrae (being separated from the arches by adipose tissue and from the 

 bodies by the posterior ligament), the dura mater of ^ the cranium becomes closely 

 attached to the bones, especially at the base, where it adheres tightly to the man> 

 ridges and prominences and to the edges of the foramina which transmit the nerves 

 and vessels. To the sides and summit of the skull the dura is less closely attached ; 

 hence in fractures at the base the dura is generally torn, and the risk both of 

 serious hemorrhage and of infection is thereby increased, while in fracture of the 

 calvaria it much oftener escapes. 



Fractures of the Cranium. — That fractures in this region are not vastly 

 more frequent is due to various factors ; among them are the rounded shape of the 



calvaria, causing blows to glance off ; the 

 division" of the separate bones into inner and 

 outer tables, with the comparatively spongy 

 diploe intervening; and the curved thicken- 

 ings which, like buttresses, strengthen the 

 skull externally, and extend on each side 

 through the supra-orbital ridge and the 

 upper border of the temporal fossa to the 

 mastoid process and thence to the occipital 

 tuberosity. From this latter point on the 

 inner surface other ridges, like the groining 

 of a roof, run forward in the median line to the frontal bone, downward to the foramen 

 magnum, and laterally, on either side of the groove for the lateral sinus, extend to 

 the mastoid. In very young persons the dome of the skull is made up of three dis- 

 tinct arches composed of the occipital, the frontal, and the parietal bones. In child- 

 hood the centre (the most prominent portion) of each of these bones is, on account 

 of early ossification, thicker than the rest, while the edges are connected by mem- 

 brane and are comparatively movable. These mechanical conditions, together with 

 the elasticity of the individual bones in young persons, make fractures of the skull in 

 them comparatively rare. 



In the adult the membranous layer between the sutures becomes thinner or disap- 

 pears and the bones denser and less elastic ; they are, therefore, more easily fractured. 

 The two tables may be broken separately, although this is rare. In almost all 

 cases in which fracture is complete the inner table suffers more than the outer. 

 This is because (a) it is more brittle ; (d) the fibres on the side of greatest strain 

 suffer most (as in "green-stick" fracture) ; (c) the material carried inward from 

 without is greater at the level of the inner table than at the point of application of 

 the external force. 



Agnew explains this diagrammatically as follows : 



Section of frontal hone, natural size, showing rela- 

 tion of external and internal tables of compact bone to 

 •intervening diploe. 



AB represents a section of the arch of the skull. CD and EF represent the lines 

 of a vertical force applied about G. The effect is to flatten the curve so that it is 

 as HI, while at the same time the vertical lines diverge (JK and LM ) and the particles 

 of bone in the external table tend to be forced together at N and separated or burst 

 apart at O. 



