POSTEEIOE BASIC MENINGITIS. 159 



seen to be limited by fusion with the pia on the under surface of 

 the cerebellum some distance away from the sides of the medulla 

 and pons, whilst it becomes abruptly narrowed at the lower part 

 of the medulla where it is continued as the arachnoid sheath of 

 the spinal cord. The dorsal part of the arachnoid sheath of the 

 cord is continued over the dorsal part of the medulla as far as just 

 within the foramen magnum and then turns back rather abruptly 

 to fuse with the under surface of the cerebellum. It thus forms 

 the posterior boundary of the cisterna magna, which occupies the 

 angular interval between the medulla -and lower part of the cere- 

 bellum. The sheet of arachnoid just described as passing from 

 the medulla to the cerebellum is sometimes called the posterior 

 arachnoid bridge, and by careful removal of this the openings in 

 the pial covering of the fourth ventricle can be investigated. 

 This pial covering forms the anterior boundary of the cisterna 

 magna which, like the other cisterns, is a space between the pia 

 and arachnoid. (See Fig. 9.) 



The great blood-vessels of the brain penetrate the arachnoid and 

 then lie in the subarachnoid cisterns, whilst sheaths of arachnoid 

 tissue are prolonged along the issuing nerves. Irritation of the 

 first cervical nerve where it traverses the subarachnoid space just 

 below the medulla may account for the rigidity of the neck and 

 retraction of the head which are such a marked feature in posterior 

 basic meningitis. This nerve supplies the short muscles which pass 

 between the atlas and axis and the occiput, and also the overlying 

 complexus, whilst the next one or two cervical nerves supply the 

 splenius and the trachelo-mastoid. It is said that the rotatory 

 movements between the head and spine are usually retained, c . 

 at all events not resisted. It is also possible that inflammatory 

 irritation of the superior vermis of the cerebellum and the region 

 of the corpora quadrigemina accounts for the retraction and 

 opisthotonos which may occur. At the transverse fissure of the 

 brain, which lies immediately above the corpora quadrigemina, a 

 fold of pia mater with a basis of arachnoid tissue, forming the 

 velum interpositum, becomes invaginated ; this affords a track 

 by which inflammation spreads to the cavities of the ventricles. 



