PRACTICAL CONSIDERATIONS : THE SKULL. 



239 



Fig. 264. 



Force applied to the vertex would tend to drive apart the lower borders of the 

 parietal bones, but the bases of the great arch formed by these bones are overlapped 

 by the squamous portions of the temporal, and thus this outward thrust is prevented. 

 If the force be applied to the frontal bone, as it overlaps the parietals at the middle 

 •of the coronal suture, it is transmitted to them and is resisted by the same mechanism. 

 The occipital bone and the bones at the sides of the skull (beneath the level of the 

 ridges that have been described) break more easily, as they are thinner, the diploe 

 is less developed, and the two tables are more closely united (Humphry); but 

 from their situation they -are less exposed to injury, and are protected by a thicker 

 covering of soft parts. 



Fractures of the base are usually due to indirect violence. They may result 

 from foreign bodies thrust through the nose, orbit, or pharynx ; or from a blow 

 upon the nose acting through the bony septum to produce fracture of the cribriform 

 plate of the ethmoid ; or through a blow or fall upon the point of the chin, driving 

 the condyles of the inferior maxilla into 

 the cranium. As a rule, however, the force 

 traverses the vault or, more rarely, the 

 spinal column (as in falls upon the feet or 

 buttocks). 



Fractures of the base are very frequent 

 for several reasons. The large expanse of 

 bone forming the vault is contracted at the 

 base into three comparatively narrow por- 

 tions, which descend in successively lower 

 planes from before backward, but which 

 all have relatively thin floors, on which the 

 force received at a distant portion of the cra- 

 nium is ultimately expended. This impact 

 reaches the base by the shortest route, so 

 that a blow of sufficient violence upon the 

 frontal bone will fracture the orbital plates in 

 the anterior cerebral fossa; upon the vertex, 

 the petrous portion of the temporal and 

 the floor of the middle fossa ; and upon 

 the occiput, the floor of the posterior or 

 cerebellar fossa. Furthermore, the base 

 is provided with a series of well-marked 

 ridges which aid in the transmission of 

 force and which fade away into the vault. 



The anterior ridges are gathered into 

 the lesser wing of the sphenoid and end at the sides of the anterior clinoid process. 



The middle group, collected into the petrous portion of the temporal bone, 

 passes to the centre of the base of the skull and terminates at the foramen lacerum 

 medium. 



The ridges of the posterior group, meeting at the torcular Herophili, continue 

 to the foramen magnum, at the posterior limit of which they divide and pass for- 

 ward to meet again in the basilar process, and end in the posterior clinoid process. 

 The region of the sella turcica is therefore the centre of resistance to the transmis- 

 sion of forces from the vault to the base. This is well surrounded by fluid, and the 

 vibrations which are concentrated here may thus become lost in the fluid without 

 injuring the brain-substance. 



The region of the middle fossa suffers, however, most frequently because : i. It 

 is connected (by the fronto-sphenoidal and petro-occipital sutures) with both the 

 other fossae, and hence often participates in their injuries. 2. It is intrinsically one 

 of the weakest parts of the skull, on account of the presence of the foramina lacera, 

 the carotid grooves, the hollows for the pituitary body, the depression for the sphe- 

 noidal sinus, the petro-sphenoidal suture, and the thin walls of the tympanum, of 

 the external auditory canal, and of the temporal fossa. Moreover, just in front of 

 this region the descending pterygoid processes and the lower jaw reinforce the 



Base of skull from above, showing lines of fractures. 



