POSTEEIOE BASIC MENINGITIS. 157 



the skull or the sheaths which surround the cranial nerves may 

 serve as channels of invasion. 



Acute meningitis is often secondary to disease of the ear or 

 nose ; it may also follow disease of the air sinuses of the skull, 

 the orbit, globe of the eye or brain itself. 



From the ear there are several paths of invasion. The canal 

 of the facial nerve affords a direct channel from the tympanum, 

 when its bony casing is incomplete. In disease of the cochlea 

 the sheath of the auditory nerve may convey the infection. The 

 veins of the tympanum may infect the superior petrosal or lateral 

 sinuses, and in infants and young persons the petro-squamous 

 suture may be permeable. Defects in the tegmen tympani may 

 afford a direct path for infection of the overlying membranes, 

 but here a subdural or temporo-sphenoidal abscess occurs rather 

 than leptomeningitis, since the dura, arachnoid and pia all lie in 

 the closest contact in this region. 



The free communication of the perineural lymphatics of the 

 olfactory nerve filaments with those of the nasal mucous mem- 

 brane explains the occurrence of leptomeningitis in connection 

 with nasal disease. Caries of the ethmoid will afford a direct 

 path to the membranes or the bone may be perforated during 

 operations in this region. Disease of the frontal or ethmoidal 

 sinuses may spread directly to the meninges in the neighbourhood. 



Suppurative disease of the orbit may extend to the interior of 

 the skull along veins or lymphatics and suppuration in the globe 

 of the eye may spread back to the membranes along the sheath 

 of the optic nerve. 



Abscesses near the surface of the brain can infect the pia and 

 arachnoid directly, and a localised leptomeningitis also may be 

 found overlying superficial cerebral new growths. 



Posterior basic meningitis is due to infection of the walls of the 

 great subarachnoid cisterns which lie at the base of the brain 

 and around the pons and medulla. The inflammation extends 

 through the transverse fissure to the interior of the ventricles 

 and may also involve the membranes of the cord by con- 

 tinuity. The infective agent is probably carried to the meninges 



