MEMBRANES AND BLOOD VESSELS 



441 



of the spinal meninges into the posterior part of the cranium. It is formed 

 by the arachnoid membrane bridging over the wide interval between the 

 posterior part of the under surface of the cerebellum and the medulla 

 oblongata. 



The cisterna pontis is the name given to another of these recesses. It is 

 the continuation upwards, on the floor of the cranium, of the anterior part 

 of the subarachnoid space of the spinal meninges. In the region of the 

 medulla oblongata it is continuous on either side with the cerebello-medul- 

 lary cistern, so that this subdivision of the brain is completely surrounded 

 by a wide subarachnoid space. Within the cisterna pontis are the vertebral 

 and basilar arteries. 



Anterior to the pons the arachnoid membrane crosses between the pro- 



Lateral lacuna 



Arachnoideal 

 granulation 



Superior sagittal sinus' 



Blood vessels 

 Grey cortex 

 of a gyrus"^ 



Arachnoideal granulation 

 Lateral lacuna 



Dura mater 



ater 

 j,*Subaracrmoid space 



T*^2K ^Arachnoid 



Falx cerebri 



FIG. 176. Diagram of a frontal section through the middle portion of the 

 cranial vault and subjacent brain to show the membranes of the brain 

 and the arachnoideal granulations. 



jecting temporal lobes, and covers in the deep hollow in this region of the 

 base of the brain. This space is called the cisterna iuterpedtmcularis, 

 and within it are placed the large arteries which take part in the forma- 

 tion of circulus arteriosus. It is continuous anteriorly with the cisterna 

 chiasmatiS) which lies anterior to the optic chiasma and lodges the anterior 

 cerebral arteries. 



All the subarachnoid cisterns communicate in the freest manner with 

 one another, and also with the narrow intervals on the surface of the cere- 

 brum. The subarachnoid space does not communicate in any way with 

 the subdural space. In certain localities, however, it communicates with 

 the ventricular system of the brain. 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 end of the corre- 

 sponding inferior horn of the lateral ventricle. 



Extending laterally from the cisterna interpeduncularis, on each side, 

 is the cisterna fossae lateralis cerebri, which extends along the stem of the 

 lateral fissure into the lateral fossa, around the middle cerebral artery. 

 Anteriorly the cisterna chiasmatis is continuous with a prolongation which 



