12 



The Bones of the Head 



being filled by the cribriform plate of the ethmoid. There, also, a 

 meningocele may protrude. At the line of articulation with the ethmoid 

 are the ethmoidal foramina, by which the ethmoidal arteries and the 

 nasal nerve enter the cranium. Fracture of this part of the skull may 

 cause sub-conjunctival haemorrhage from the ethmoidal arteries, and 

 when this is associated with severe bleeding from the nose, and with 

 escape of sub-arachnoid fluid (which must not be taken for the effect 

 of a nasal catarrh), there is ample evidence of fracture having taken 

 place. The upper surface of the plates is marked by the orbital con- 

 volutions of the frontal lobes. 



The frontal bone is developed in lateral halves. 

 The temporal. The squamous part lies in the temporal fossa, 

 and its bevelled margin overlaps the parietal bone, whilst its lower 

 and hinder part is bounded by the posterior root of the zygoma. The 

 anterior root of the zygoma is the eminentia articularis, which is 

 covered by cartilage, and receives the condyle of the jaw when the 

 mouth is widely opened. Behind the eminentia is the glenoid fossa, 

 cleft by the Glaserian fissure through which the chorda tympani leaves 

 the middle ear ; a branch of the internal maxillary artery also tra- 

 verses it. 



The front of the fossa, covered with cartilage, articulates with the 

 condyle of the jaw, and the hinder part lodges some of the parotid 

 gland ; a thin lamina of bone partitions it from the tympanic cavity 

 and the external auditory meatus. Thus < it is that a parotid abscess 

 may burst through the external ear. The inner surface of the 

 squamous portion is deeply grooved by the middle meningeal artery. 

 The mastoid portion scarcely .exists in childhood ; it is developed 

 with the petrous portion under the name of 

 petro-mastoid bone. But as puberty comes 

 on it is hollowed out into air-cells which 

 open into the back of the middle ear, their 

 mucous lining being covered with columnar 

 ciliated epithelium. It is by way of these 

 cells that tympanic suppuration sometimes 

 reaches the surface as a post-auricular 

 abscess, raising the skin and pushing the 

 pinna forward ; sometimes, however, the pus 

 finds its way into the external meatus, thus 

 the matter reaches the outer surface of the 

 membrana tympani without traversing it. 

 ' *e same way, in the case of fracture of 

 the base of the skull, blood may escape from 

 the external ear though the membrana, as shown by otoscopic exami- 

 nation, has not been damaged. In every case of suppuration in the 

 mastoid process the surgeon should be prompt in securing evacuation 

 by the gouge or trephine. 





