MEMBRANES 373 



Anterior to the pons the arachnoid membrane crosses 

 between the projecting temporal lobes, and covers in the 

 deep hollow in that region of the base of the brain. The 

 space so enclosed is called the cisterna interpeduncularis, and 

 within it are placed the large arteries which take part in the 

 formation of circulus arteriosus. The cisterna interpedun- 

 cularis is continuous, anteriorly, with the cisterna chiasmatis, 

 which lies anterior to the optic chiasma and lodges the anterior 

 cerebral arteries (Fig. 141). 



All the subarachnoid cisterns communicate in the freest 

 manner with one another, and also with the narrow sub- 

 arachnoid intervals on the surface of the cerebrum. The 

 subarachnoid space does not communicate in any way with 

 the subdural space. In certain localities, however, it com- 

 municates with the ventricular system of the brain by small 

 apertures. Three such apertures are described in connection 

 with the fourth ventricle, whilst another slit, on each side, 

 is said to lead from the cisterna interpeduncularis into the 

 lower and anterior end of the corresponding inferior horn of 

 the lateral ventricle. 



Extending laterally from the cisterna interpeduncularis, 

 on each side, is the cisterna fossa lateralis cerebri, which 

 runs along the stem of the lateral fissure into the lateral fossa, 

 which is the recess in which the insula lies. In it lie the 

 middle cerebral vessels. Anteriorly, the cisterna chiasmatis 

 is continuous with a prolongation which extends into the 

 longitudinal fissure along the anterior cerebral vessels. A 

 dilatation of the subarachnoid space over the dorsum of 

 the mid-brain, around the great cerebral vein (O.T. vena 

 rnagna Galeni), is called, the cisterna vence magnce, cerebri. 



The cisterna venae magnae cerebri was opened when the 

 mid-brain was divided, during the removal of the brain. 

 The dissector should now examine the other cisternae by 

 carefully dividing the arachnoid, where that has not already 

 been done. 



Dissection. Divide the arachnoid, in the median plane, 

 along the anterior surfaces of the medulla oblongata and the 

 pons (if the division has not been made previously), and turn 

 the flaps to the sides. When that has been done the upper ends 

 of the vertebral arteries, and the basilar artery, which is formed 

 by their union, will be exposed, lying in the cisterna pontis. 



Carry an incision backwards and laterally, through each flap 

 of arachnoid, into the angle between the medulla oblongata and 

 in 24 & 



