I204 



HUMAN ANATOMY. 



magnum with the posterior part of the subarachnoid space of the cord. The arach- 

 noid passes from the back part of the under aspect of the cerebellum to the posterior 

 surface of the medulla and thus encloses a considerable space which at the sides of 

 the medulla is continuous with the upward prolongation of the anterior subdural 

 space of the cord. The lower part of the brain-stem is thus completely surrounded 

 by the subarachnoid cavity. The ventral surface of the pons is enveloped by the 

 upward extension of the anterior part of the spinal arachnoid, the cleft so enclosed 

 constituting the cisterna pontis, of which a median and two lateral subdivisions 

 may be recognized. From the upper ventral border of the pons the arachnoid 

 passes forward to the orbital surface of the frontal lobes, covering the corpora mam- 

 millaria, the infundibulum and the optic chiasm, and laterally to the adjacent project- 

 ing temporal lobes and thence, covering in the transverse stem of the Sylvian fissures, 



Fig. 1038. 



Olfactory tract 



Optic chiasm 



Internal carotid 

 artery 



Basilar artery 



Vertebral arteries 



Extension along 

 longitudinal fissure 



Extension ailong 

 Sylvian fissure 



Cisterna basalis 



Cisterna pontis 



Cisterna magna 



Inferior aspect of brain covered with pia and arachnoid, showing large subarachnoid spaces. 



to the frontal lobes. This large space, which includes the deep depression on the 

 basal surface of the brain, is the cisterna basalis. It is imperfectly subdivided by 

 incomplete septa of arachnoid tissue into secondary compartments, one of which lies 

 between the peduncles (cisterna interpeduncularis), another behind the optic commis- 

 sure (cisterna chiasmatis) and a third above and in front of the chiasm (cisterna 

 laminae terminalis). Anteriorly the cisterna basalis is continued over the convex 

 dorsal surface of the corpus callosum (cisterna corporis callosi), and on either side 

 along the stem of the Sylvian fissure (cisterna fissurae lateralis). Within the median 

 region of the cisterna basalis lie the large arterial trunks forming the circle of Willis. 

 These vessels are invested with delicate sheaths of arachnoid, which accompany the 

 smaller branches until they enter the vascular membrane to become pial vessels. 



The arachnoid also contributes sheaths to the cranial nerves as they pass from 

 their superficial origins to the points where they pierce the dura, these sheaths over- 

 lie those derived from the pia and, as do the latter, accompany the nerve-trunks for a 



