158 CLINICAL APPLIED ANATOMY. 



in the first place by the blood stream, but it has been suggested 

 that the primary invasion may be from the nasopharynx along 

 the perineural and perivascular lymphatic sheaths or by way of 

 the Eustachian tube to the tympanum and thence to the meninges 

 by veins which traverse the petro-squamous suture if that be still 

 open. If this suture be closed there are still other efferent veins 

 from the tympanum which may carry infection. 



The subarachnoid space communicates directly with the tubular 

 investment of the auditory nerve and so with the perilymph 

 spaces of the internal ear, but it does not communicate with the 

 endolymph spaces or with the tympanum. Consequently it is 

 improbable that infection travels to the meninges from the middle 

 ear along the auditory nerve sheath. Infection, however, may 

 and does travel in the reverse direction, and in this way otitis 

 interna sometimes develops secondarily to the meningitis, but 

 the otitis media of posterior basic meningitis is more commonly 

 due to infection along the Eustachian tube from the nasopharynx 

 of a stuporous patient. 



In early cases of posterior basic meningitis the subarachnoid 

 cisterns are filled with a greenish exudation so that their 

 extent and connections become quite obvious. These cisterns are 

 formed by the wide separation of the pia and the arachnoid at 

 certain spots, whilst in other positions these membranes are 

 closely connected by a very small meshed areolar bond. 



At the base of the brain a sheet of arachnoid passes between 

 the under surfaces of the temporal lobes near their inner margins, 

 and by bridging over the interpeduncular space forms the 

 cisterna basalis. This basal cistern is prolonged laterally along 

 the Sylvian fissures and anteriorly round the front of the corpus 

 callosum into the great longitudinal fissure. Posteriorly the 

 cisterna basalis is continuous with the cisterna pontis, which at 

 its fore part has lateral prolongations around the crura cerebri 

 leading to the large angular interval which lies dor sally between 

 the front of the cerebellum and the corpora quadrigemina. The 

 cisterna pontis is Continued behind into the tubular investment 

 of the medulla. This investment when viewed from below is 



